International Workshop on Breakdown Science and High Gradient Technology (HG2021)

19 Apr 2021, 06:45 → 21 Apr 2021, 18:00 US/Pacific

Emilio Nanni (SLAC National Accelerator Laboratory), Evgenya Simakov (LANL), John Lewellen (Los Alamos National Laboratory), Sami Tantawi (SLAC)

We are pleased to (re)announce the 13th workshop on breakdown science and high gradient accelerator technology*, HG2021, will be held virtually on Zoom from April 19-21, 2021.

 

Clearly the identification and advancement of high gradient accelerator technologies for a linear collider have been the main goal since the inception of the High Gradient workshop series. Historically, the workshop has heavily concentrated on progress of X-band accelerator technologies, the area in which the most recent research results have been shared and discussed. The tight collaborations among the participants have pushed practical accelerator technologies to a level that has never been achieved before. Knowledge gained through the HG workshops in the past, like the current depth in understanding RF breakdown, the procedure of fabricating and conditioning high gradient accelerators, and the novel designs of high power rf components, etc., have benefits far beyond the X-band accelerator community.

 

Besides the intensive focus on X-band high gradient accelerator technologies, the workshop has always made efforts to broaden the spectrum of technologies discussed and attract more talent in related fields. In recent years, the workshop has successfully recruited theorists in material science and experts in accelerator applications, whose participation has significantly enriched the program and generated mutual benefits. HG2021 will continue this journey. The workshop will share the latest advancements in, but not limited to, breakdown science, high efficiency high power RF sources, low breakdown rate high-gradient accelerators, low cost accelerator fabrication technologies, novel accelerator designs, accelerator applications to light source, medical, and industrial technologies etc. While it will be virtual, the format of HG2021 follows the format of preceding workshops, i.e. oral presentations and discussions.

We look forward to seeing you.

Local Organizing Committee:

Emilio Nanni (SLAC)

Evgenya Simakov (LANL)

Sami Tantawi (SLAC)

John Lewellen (LANL)

 

International Organizing Committee:

Walter Wuensch (CERN)

Toshiyasu Higo (KEK)

Valery Dolgashev (SLAC)

Gerardo D’Auria (Elettra)

Jiaru Shi (Tsinghua University)

Chunguang Jing (ANL)

Angeles Faus-Golfe (LAL)

Wencheng Fang (SINAP)

 

*This workshop has been endorsed by the International Committee for Future Accelerators - Panel on Advanced and Novel Accelerators (ICFA-ANA)

 

Monday, 19 April

1 Welcome

Speaker: Dr Emilio Nanni (SLAC National Accelerator Laboratory)

HG2021Intro.pptx

Session 1: Facility Update

Conveners: Dr Emilio Nanni (SLAC National Accelerator Laboratory), Dr Evgenya Simakov (LANL), John Power (Argonne National Lab)

2 UCLA High Gradient Cryogenic RF Research Program

We present an overview of the various research thrusts embraced by the UCLA program in very high field cryogenic RF acceleration, beginning with roots in both X- and S-band, and extending through a vigorous program in developing C-band accelerating structures. These are employed in extremely high brightness electron sources for both linear colliders and ultra-compact X-ray free-electron lasers (UC-XFELs). They are also planned for use in high gradient linacs that enable 1 GeV beams for the UC-XFEL project. This initiative is described, and its critical demands on cryogenic RF technology are reviewed.

Speaker: James Rosenzweig (UCLA)

HG2021Rosenzweig.pdf

3 Design and high-power test of a short prototype of high gradient S-band accelerating structure for the FERMI free electron laser linac upgrade

Located on the site of the Elettra Sincrotrone in Trieste, Italy, the FERMI free-electron laser (FEL) is a user facility fed by a 1.5 GeV, 10 to 50 Hz, S-band radio-frequency linear accelerator (linac). In order to achieve shorter wavelengths and improved beam quality, a high gradient upgrade to the facility has been proposed which will extend the current capabilities of the system allowing the generation of beams up to 1.8 GeV while keeping breakdown rates low enough for high machine up-time
To demonstrate the reliability and feasibility of the upgrade plan, a short prototype was built in collaboration with Paul Scherrer Institute (PSI), Switzerland. Using a newly commissioned S-band cavity test facility, the short prototype was successfully conditioned to an accelerating gradient of 40 MV/m with a pulse length of 600 ns at a breakdown rate of 8×10−8 bpp. A comprehensive overview of the testing facility, its data processing tools and the conditioning of this short prototype will be illustrated. Concluding the paper is a visual inspection of the cells for signs of damage resulting from RF breakdown during the high power testing.

Speaker: Nuaman Shafqat

HG2021.pptx

4 Update on CERN X-band activities

After many years, the high gradient programme at CERN, mainly using X-band accelerating structures is now consolidated. The required high gradient and breakdown rate for the CLIC machine is now consistently achieved and our efforts turn to manufacturability, industrialization and the integration of a full module including its power source. CERN is collaborating hand in hand with partner institutions in what is now a global network. We give au update of the activities going on at CERN as well as the new initiatives to come.

Speaker: Nuria Catalan Lasheras (CERN)

HG2021_NCL.pptx

5 The CompactLight Design Study

Gerardo D’Auria - Elettra Sincrotrone Trieste - on behalf of the CompactLight Consortium
CompactLight (XLS) is a H2020 Design Study funded by the European Union under grant agreement 777431 and carried out by an International Collaboration of 26 partners and 5 third parties.
The project, started in January 2018 with a duration of 48 months, aims at designing an innovative, compact and cost-effective hard X-ray FEL facility, complemented by a soft X-ray source that can be operated up to 1 KHz pulse repetition rate, beyond today’s state of the art, using the latest concepts for high brightness electron photoinjectors, very high gradient accelerating structures, and novel short period undulators.
The presentation will give an overview of the state of the project.

Speaker: Gerardo D'Auria (Elettra - Sincrotrone Trieste)

GdA_20211904_HG2021.pdf

6 The EuPRAXIA@SPARC_LAB X band LINAC

EuPRAXIA@SPARC_LAB is the new project of the National Laboratories of the INFN in Frascati and it is a FEL Facility based on plasma beam driven acceleration. The high quality electron beam is generated by an S-band injector and accelerated up to 1 GeV by an X band LINAC. In the presentation the main structure of the linac is presented with details on the X band accelerating module, accelerating structure design and prototyping strategy for structure realization.

Speaker: David Alesini (INFN)

HG2021_alesini.pptx

09:30 Coffee Break

7 CERF-NM: The C-band Engineering Research Facility in New Mexico

The C-band Engineering Research Facility in New Mexico (CERF-NM) consists of the US's highest-power C-band RF source to date; a shielded enclosure rated for 5 MeV and 10 uA; and flexible control and data acquisition system. This talk presents an overview of the C-band research program at Los Alamos, details of the CERF-NM facility, and future plans for the program.

Speaker: John Lewellen (Los Alamos National Laboratory)

Lewellen - CERF-NM v2.pdf

8 Gyrotron-based High Gradient THz Accelerator Test Facility

Particle accelerators at sub-terahertz frequencies would require high-power sources in this frequency range for testing, conditioning and driving the accelerators. Gyrotrons have major advantages for the application to acceleration, especially high average power capability and high wall plug efficiency. Reliable testing and conditioning of high gradient accelerating structures requires power in the range of few hundred kilowatts in short pulses of a few nanoseconds or less. At MIT we have built a dedicated setup for testing high gradient accelerator structures at 110 GHz using a megawatt power pulsed gyrotron source. This gyrotron is capable of producing up to 1.2 MW in pulses of 3 microseconds. The microwave power from the gyrotron is coupled into a low loss overmoded corrugated waveguide to transmit the power to a separate test bench where the accelerator structure is tested. The microsecond long pulses from the gyrotron are shortened to generate nanosecond timescale pulses using a laser driven semiconductor switch. We recently tested a SLAC accelerating structure operating in a TM01 mode at 110 GHz demonstrating an acceleration gradient of 230 MV/m at 570 kW of microwave power in 10 ns long pulses from the gyrotron. For future applications, the gyrotron pulses could be produced with high efficiency either by using a pulse compressor to produce the nanosecond scale pulses or by forcing the gyrotron to produce a self-modulated train of nanosecond pulses with a rep rate of few tens of MHz.

Speaker: Sudheer Jawla (MIT)

SudheerJawla_MIT.pdf

9 RF activities concerning S-band high-gradient applications at PSI

The RF activities related to high-gradient applications at PSI are mainly focused on the completion of the soft X-ray Athos beamline in SwissFEL with the new X-band diagnostic system that is under installation and on the upgrade of the SLS pre-injector linac in the framework of the new SLS2 project. A new simplified injector concept for SLS2 linac based on new S-band structures has been developed as possible substitute of the existing one. Furthermore, PSI is part of an FCCee Injector collaboration and, in this context, is in charge of a development of a positron source that will make use of large acceptance standing wave structures under study for the capture linac. This talk gives an overview of the design of new passive RF components in X-band for the Athos diagnostic system, the concept of a simplified injector based on high gradient S-band structures and the design of high-gradient S-band standing wave cavities for the FCCee e+ capture linac and for the e+ production experiment at PSI.

Speaker: Riccardo Zennaro (PSI)

HG2021_RZ_19_04_21.pptx

10 Smart*Light: Current Activities and Future Concepts

Inverse Compton sources are becoming a popular concept as the future of lab-based x-ray sources. SmartLight is one such facility, currently under commissioning at TU/e, which is based on high gradient X-band technology originally designed for the Compact Linear Collider (CLIC). Here we present an overview of the high-power RF system including the newly arrived 50-cell accelerator. To conclude, the presentation will introduce a concept study of a high gradient TW RF photogun which could replace the injector on Smart*Light to ultimately provide a significant boost to the X-ray flux.

Speaker: Thomas Lucas

HG2021.pdf

11 High Gradient Research Activities at AWA

Speaker: John Power (Argonne National Lab)

JohnPower-AWA-HG21.pdf

12:15 Lunch Break

12 Update on High Gradient Research at SLAC

In this presentation we will highlight ongoing high gradient research at SLAC which is focused on understanding the physics of breakdown, understanding the role of materials, developing innovative designs and realizing these structures with advanced manufacturing. Ongoing research spans the microwave to THz range, room temperature to cryogenic, and ultrafast to high energy.

Speakers: Dr Emilio Nanni (SLAC National Accelerator Laboratory), Sami Tantawi (SLAC)

SLACUpdatev_FINAL.pptx

13 High Gradient Research at KEK / Nextef

In KEK, we have been performing X-band (11.4 GHz) high-gradient research with normal-conducting structures.
In this presentation, we review past achievements, and report the recent status of our high gradient test stand: Nextef.
Moreover, we delineate an outlook for future studies from various aspects.

Speaker: Tetsuo Abe

HG2021_KEK_ABE.pdf

14 Updates from Tsinghua X-band High power Test Stand (TPOT)

The X-band high-power test stand at Tsinghua University is now capable to deliver 50MW rf pulse at 40Hz stably. This presentation will introduce the recent configuration of the test stand and discuss about experiment results and plans.

Speaker: Jiaru Shi

HG2021_Jiaru_Shi.pptx

15 High Gradient Research Activities at SSRF/SARI

Soft X-ray FEL test facility (SXFEL-TF) at SSRF/SARI has been tested and accepted by Chinese government last year, which is based on the C-band high gradient linac. This talk will introduce details of the C-band high gradient technology at SSRF/SARI, and meanwhile the current status of C-band linac upgrading for SXFEL user facility is also presented. Considering the future SXFEL development, two types of C-band photocathode gun are under development at SSRF, including a 3.6-cell NC C-band photocathode gun, and a 2.6-cell cryogenic C-band photocathode gun. Both types have been completed with fabrication and low power RF test, in particular the 3.6-cell gun has been tested by high power, already reaching 180 MV/m higher than the target of 150 MV/m.

Speaker: Wencheng Fang

HG activites at SSRF-SARI.pptx

16 Overview of High Gradient Research Activities at IHEP

For the CEPC and HEPS projects at IHEP, S-band 3 meters long accelerating structure has been developed. The operating frequencies are 2856 MHz and 2998.8 MHz respectively. The two designs adopted rounding shape cell and internal water cooling. The CEPC structure has finished high-power test in the case of using the energy doubler. The peak power into the structure is 155.4 MW. The design of HEPS coupler is improved. At present, the first accelerating structure has completed. The matching, cold test and high-power test have finished. With the support of the Key Laboratory Fund at IHEP, an X-band open type accelerating tube has been developed with 24 cells, two matching cells and two couplers.

Speaker: Jingru Zhang

HG2021-Jingru Zhang.pdf

Tuesday, 20 April

Session 3: HG Structure: Design & Tests

Conveners: Luigi Faillace (INFN/LNF), Dr Valery Dolgashev (SLAC National Accelerator Laboratory)

17 Design consideration and R&D towards high gradient RFQs

This talk will cover RFQ design activity at CERN both 750 MHz and 352 MHz: design considerations and scaling; material study ongoing at CERN in particular with the effect of H- irradiation.

Speaker: Alexej Grudiev (CERN)

20210420_HG2021_AG_RFQdevelopment.pptx

18 Development of brazeless accelerating cavities

Recently Euclid Techlabs has developed a few X-band accelerating structures using a novel brazeless fabrication approach. Structures are targeted to different applications. It includes a side coupled accelerating structures for medical linac with an improved 133 MOhm/m of shunt impedance, a short-pulse wakefield power extractor, and a low energy accelerator. These structures have been tested at SLAC, ANL, and Euclid respectively.  We will report the experimental results, in particular discussing breakdowns for each type of structure, either magnetic field or electric field induced.

Speaker: Chunguang Jing (Euclid Techlabs)

Brazeless_HG2021_v1_04192021.pdf

19 Welded Cavities

High-gradient linacs are sought for various applications, from high-energy physics, industry and medicine, and require novel accelerating structures which are compact, robust and cost-effective. Due to the superior performance of accelerating cavities made of hard copper alloys, we have investigated two cost-effective welding processes, Electron Beam Welding and Tungsten Inert Gas welding, in order to preserve the hardness of the metal. We present the design, fabrication and high-power tests of two braze-free X-band cavities. This study is the result of a continuous, decade-long collaboration involving the SLAC, INFN-LNF and KEK. The high-power tests demonstrated accelerating gradients beyond 100 MV/m at a breakdown rate of 10-3/pulse/meter using a shaped pulse with a 150 ns flat part. This is an important step to validate our approach of structure construction and building practical multi-cell structures made of hard copper alloys. We will also present the design and fabrication of a welded X-Band accelerating structures made of two halves.

Speaker: Luigi Faillace (INFN/LNF)

Faillace-HG2021.pdf

20 A Ka-Band accelerating structure as a linearizer for the Compact Light XLS project

There is a strong demand for accelerating structures able to achieve higher gradient and more compact dimensions for the next generation of linear accelerators for research, industrial and medical applications. A future European light source, called Compact Light, operating in X band, has been proposed to extend FEL operation into the X-ray region further than other competing light sources. In this project a Ka-Band normal conducting high gradient RF accelerating structure is foreseen for linearizing the bunch phase space in order to compensate the non-linear distortions introduced by the RF curvature of the main accelerating cavities. In this talk we discuss the RF design of a 35.982 GHz linearizer, operating at third harmonic of the Linac frequency of 11.994 GHz, optimized to work at 100-125 MV/m accelerating gradient with an extremely low probability of RF breakdown and by using a high RF power of the order of 10 - 12 MW. In addition, preliminary estimations of the wakefields effects are presented.

Speaker: Bruno Spataro (INFN/LNF)

Spataro_ HG2021_April_20_2021.pdf

21 High Gradient S-Band experiments at IFIC

The IFIC High-Gradient (HG) Radio Frequency (RF) laboratory is designed to host a high-power infrastructure for testing HG S-band normal-conducting RF accelerating structures. The main objective of the facility is to develop HG S-band accelerating structures and to contribute to the study of HG phenomena. A particular focus is RF structures for medical hadron therapy applications. The design of the laboratory has been made through collaboration between the IFIC and the CLIC RF group at CERN. The layout is inspired by the scheme of the Xbox-3 test facility at CERN, and it has been adapted to S-band frequency. Currently, one of the two new normal-conducting HG S-band (2.9985 GHz) Backward Travelling Wave (BTW) accelerating cavities (β=0.38) designed and constructed by the TERA Foundation in collaboration with the CERN RF team, is being tested at the IFIC HG-RF laboratory. The main goal of the tests is understanding what the maximum achievable accelerating gradient of this new design and characterize the dark current formation in the structure, which could limit the applicability of this technology for medical applications. In this talk we present the progress made on the operation of the laboratory, first results on the conditioning of the BTW S-band accelerating cavity and future plans.

Speaker: Nuria Fuster (IFIC)

HG2021.pdf

09:30 Coffee Break

22 Development and high power testing of C-band accelerator components

This talk will report on the design, fabrication, and high power conditioning of multiple C-band high gradient components, such as C-band TM01 mode launchers and several test accelerator cavities. At LANL we commissioned a test stand powered by a 50 MW, 5.712 GHz Canon klystron. The test is capable of conditioning single cell accelerating cavities for operation at surface electric fields up to 300 MV/m. The rf field is coupled into the cavity from a WR187 waveguide through a mode launcher that converts the fundamental mode of the rectangular waveguide into the TM01 mode of the circular waveguide for coupling into the cavity. Several designs for mode launchers were considered and the final design was chosen based on a compromise between the field enhancements on the surface, operational bandwidth, and the simplicity and cost of fabrication. Four mode launchers were fabricated and cold-tested. Two mode launchers with the best transmission characteristics were installed on the waveguide line and conditioned to high power. The test stand is being used to test high gradient operation of multiple accelerator cavities including a beta=0.5 proton accelerating cavities, and electron accelerator cavities. The current test status will be reported.

Speaker: Dr Evgenya Simakov (LANL)

HG2021 Simakov.pdf

23 Design, fabrication and cold-testing of DLA structures

It has been technically challenging to efficiently couple external radiofrequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This talk presents a design, fabrication and cold-testing of a matching section for coupling the RF power from a circular waveguide to an X-band DLA structure with a dielectric constant εr=16.66 and a loss tangent tanδ=3.43×10^(-5). It consists of a very compact dielectric disk with a width of 2.035mm and a tilt angle of 60°, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. A microscale vacuum gap, caused by metallic clamping between the thin coating and the outer thick copper jacket, is also studied in detail. A choke geometry is added with a TE10-TM01 mode converter to remove the contact issue and bonding joints for assembling two copper parts together. Based on simulation studies, the prototypes of the mode converter with a choke and the DLA structure are fabricated. The bench measurements using a network analyzer has been performed to compare with the simulation studies.

Speaker: Yelong Wei

HG2021_Yelong Wei.pdf

24 High power test of mm-wave accelerators

GeV/m acceleration may be achieved at mm-wave and sub-Terahertz (THz) frequencies, thanks to the high shunt impedance. One of the main goals of this research is to study the physics of high gradient acceleration and rf breakdown at mm-wave frequencies. Previously, the only measurements of high gradients and rf breakdowns at mm-wave frequencies were performed in beam-driven accelerating structures at SLAC's FACET. These beam-driven structures sustained low gradients and high surface electric fields, and experienced damage from the beam halo. In this talk, the measurements of high gradient and rf breakdown rates (BDRs) in a 110 GHz accelerating cavity externally-driven by an rf source are reported. The accelerating structure has a unique quasi-optical coupling scheme and was fabricated and assembled with a very high precision. The cavity is powered by 10 nanoseconds rf pulses, chopped from 3 microsecond megawatt pulses from a gyrotron using a laser-driven silicon switch. The highest achieved accelerating gradient was 230 MV/m at 570 kW of peak input power, corresponding to a peak surface electric field of 520 MV/m after rapid processing of the cavity with more than 105 pulses. Initial breakdown rates are also reported and the BDR is expected to further decrease after extended running time. Building on the success of this experimental demonstration, we are developing a high-gradient field emission electron gun toward significantly compact, bright, and efficient particle sources. These results open up many frontiers for applications not only limited to the next generation particle accelerators but also x-ray generation, probing material dynamics, and nonlinear light-matter interactions at mm-wave and THz frequency.

Speaker: Mohamed Othman (SLAC)

2021_HG_OTHMAN_THZ.pdf

25 C-band high gradient cryogenic photoinjector research at UCLA

There is a high demand for usage of the bright coherent X-ray. However, its availability is very limited because the facility becomes very large, such as LCLS. UCLA is proposing a compact light source termed the UC-XFEL where the facility is designed to be less than 40-m long. For its realization, a the creation of a beam with extremely high 6D brightness is critically important. We adopted the technology of the C-band cryogenic normal conducting cavities for reaching very high (250 MV/m peak) electric fields and attendant high brightness electron beam production. We are designing a high gradient RF gun to produce a high brightness beam, and are employing the distributed coupling scheme designed at first SLAC at cryogenic temperature. We have started preparing for the basic experiments on the cryogenic injector. A half-cell C-band gun is designed and being constructed for the study of the cathode properties at the cryogenic temperature. We review these activities in the light of ongoing general research in experimentally exploring cryogenic C-band RF properties.

Speaker: Atsushi Fukasawa (UCLA)

HG2021_AtsushiFukasawa.pdf

26 High gradient, short filling-time parallel-coupled structure

This talk presents a novel high gradient parallel-coupled structure. The distributed coupling system of this structure is a special design to make the structure over coupled and to dispatch the input power to each cell very quickly. An X-band, 16-cell structure is designed to have 10 ns input pulse length. Based on the empirical equation of estimating the breakdown rate from accelerating field and the pulse length, the structure with 10 ns pulse has the capability to work at the gradient of 200 MV/m.

Speaker: Hao Zha (Tsinghua)

High gradient, short filling-time parallel-coupled structure.pptx

27 Development of an X-band Field Emission RF Gun at Tsinghua University

An X-band field emission rf gun has been developed at Tsinghua university. It is composed of 3 cells and a pluggable cathode. The first cell is designed to work at TM02 mode to eliminate the rf power leaking. The high-power testing was processed on the uprated T-POT. After conditioning of around 107 pulses, the peak accelerating gradient of the gun achieved 200MV/m and a stable emission current of around 3mA was detected. The cathode after conditioning was observed by WLI and SEM. Both craters and bulges were observed on the cathode surface, and their distribution is closely related to the electric field intensity and surface fluctuation caused by machining. An electron image system based on this gun will be built in our future plans.

Speaker: Liuyuan Zhou (Tsinghua)

Development of an X-band Field Emission RF Gun at Tsinghua University.pptx

12:45 Lunch Break

Working lunch (Questions and Discussion)

Session 5: Industry

Convener: Alex Murokh (RadiaBeam Technologies, LLC.)

28 High gradient hadron linacs R&D for medical applications

Proton and carbon radiotherapy systems demonstrated significant advantages in clinical efficiency and reduced toxicity profiles for many types of cancers, however the high cost of equipment and facilities are presently the limiting factor preventing hadron therapy from becoming the standard of care for a wider range of patients. Developing a high gradient linear accelerator capable of producing carbon beams with variable energy in a small footprint would offer a promising approach to reduce the cost and improve the quality of the treatment. In this talk we will review the ongoing R&D activities towards practical high-gradient linacs for hadron radiotherapy, pursued by different labs, and provide the recent progress with high-gradient structure development for the Advanced Compact Carbon Ion Linac (ACCIL).

Speaker: Sergey Kutsaev

HG21 - Hadron Radiotherapy.pdf

29 High Efficiency, Low Cost, RF Sources for Accelerators and Colliders

Calabazas Creek Research, Inc. (CCR) is investigating technologies advancing the state of the art in RF source technology. These technologies are focused on increasing performance and reducing cost of sources from 300 MHz to X-Band at power levels from 100 kW to 10 MW. Traditional devices are being redesigned to add capabilities and performance previously not available. These include multiple beam triodes-based sources, ultra-high efficiency klystrons, multiple beam IOTs, and phase and frequency controlled magnetrons. the performance goals include efficiencies exceeding 80% and costs as low as $.50/Watt. This presentation will review progress on these developments, including available test results.

Speaker: R. Lawrence Ives (Calabazas Creek Research, Inc.)

High Efficiency RF Sources (1).pptx

30 Accelerator-driven radiotherapy methods

Speaker: Jim Lewandowski

Accelerator Driven Radiotherapy_JimLewandowski_HG2021.pdf

31 High does rate linacs for FlashRT

TibaRay is developing the PHASER radiation therapy system for delivery of FLASH treatment of cancer. The hardware is based on several key developments centered around the concept of distributed coupling linacs that are highly efficient, compact and well-suited for productization.

Speaker: Arun Ganguly

2021-04-20_TibaRayPresentation_HG2021.pdf

32 Dielectric Based Compact Accelerator for Industrial Applications

With this talk, Euclid presents a portable lightweight and cost-effective ~MeV range accelerator that is compact enough to operate as a module for an easy stack-up to increase the deliverable radiation dose. We focus on the technology to replace the conventional copper linac with a significantly lighter and more compact new type of dielectric accelerator. The use of high permittivity ceramics reduces the transverse size of the accelerator significantly, making the thickness of the accelerating structure comparable to that of an ordinary pencil. This allows not only in a sizeable weight reduction of the structure itself, but even more important, a substantial reduction in the weight of the lead shield needed to enclose the structure.

Speaker: Alexei Kanareykin

Dielectric accelerator HG Workshop 2021.pdf

33 Sources for high gradient accelerators

Speaker: Mikael Lindholm (ScandiNova Systems)

HG2021, Sources for High Gradient accelerators, v1.pdf

Wednesday, 21 April

Session 6: Theory & Materials

Convener: John Lewellen (Los Alamos National Laboratory)

34 Atomistic approach in understanding of mechanisms leading to vacuum arcing

In my presentation, I will give the overview of the model, which we develop at the University of Helsinki. I will describe the motivation for the hypothesis, which gave a new angle in studies of material response to high electric field effects. We analyze the behavior of surface atoms under applied electric field to understand the macroscopic changes of properties of material surfaces. To approach the problem of vacuum arcing we divide the process in several stages, for each stage using a specific simulation tool. The results obtained this far for each stage will also be discussed.

Speaker: Prof. Flyura Djurabekova (University of Helsinki)

Djurabekova_HG2021.pdf

35 Model and observations linking plastic activity to arc nucleation

High Gradient breakdown is commonly assumed to trigger due to the formation of localized plasma enabling breakdown initiation. In this talk I will describe a model in which the initiating process is caused by stochastic fluctuations of the mobile dislocation population in the cathode. In this model, the mobile dislocation density normally fluctuates, with a finite probability to undergo a critical transition due to the effects of the external field. The model is fitted to experimental data of the dependence of the breakdown rate on the applied electric field's strength and temperature. The model assumptions are consistent with findings of a metastable array of mobile dislocations in copper cathodes. Finally, I will describe recent results of pre-breakdown fluctuations, which are consistent with the fluctuations predicted within the model.

Speaker: Dr Yinon Ashkenazy (Racah Institute of Physics- Hebrew University of Jerusalem)

ashkenazy_hg2021_v1.pdf

36 Atomistic modeling of the coupling between electric fields and bulk plastic deformation in RF structures

A notable bottleneck in achieving high-gradient RF technology is dictated by the onset of RF breakdown. While the bulk mechanical properties are known to significantly affect the breakdown propensity, the underlying mechanisms coupling EM fields to bulk plastic deformation in experimentally relevant thermal and electrical loading conditions remain to be identified at the atomic scale. Here, we present the results of large-scale molecular dynamics simulations (MD) to investigate possible modes of coupling. Specifically, we consider the activation of Frank-Read
sources, which leads to dislocation multiplication, under the action of bi-axial thermal stresses and surface electric-field. With the help of a charge-equilibration formalism incorporated in a classical MD model, we show that a surface electric field acting on an either preexisting or dislocation-induced surface step, can generate a long-range resolved shear stress field inside the bulk of the sample. We investigate the feedback between step growth following dislocation emission and
subsequent activations of Frank-Read sources and discuss the conditions where such a mechanism could promote breakdown precursor formation.

Speaker: Dr Danny Perez (Los Alamos National Laboratory)

Perez-HG-2021.pdf

37 Ab initio alloy design for C-band accelerators

The development of novel copper alloys with increased breakdown limits is crucial for the design of higher energy and more compact accelerators that operate at lower cost. Adding solute atoms in copper is a promising strategy to improve the breakdown limits, as solute strengthening can improve mechanical properties by limiting the plastic deformation under thermal loading. However, thermal stresses induced by RF dissipation are also increased by adding solute atoms, which is the driving force for plastic deformation. In this talk, I will present our progresses in ab-initio copper alloy design for C-band accelerators. We are using a figure of merit (FOM) based on the ratio of these two factors to screen dilute copper alloys on the periodic table. After computing FOMs of all the alloys, we will select the best copper alloys and determine the solute concentrations that can improve breakdown limits of copper. These results are important to guide the design of C-band accelerators.

Speaker: Dr Gaoxue Wang (Los Alamos National Laboratory)

HG2021 slides 04.21.pptx

09:00 Coffee Break

38 Developing Field Emission Models Employing Nanoscale Surface Characterization

A popular approach for modeling field emission in particle-in-cell (PIC) simulations is to employ a calibrated Fowler-Nordheim emission model. In this approach, the calibrated geometric enhancement factor, β, is often tuned to extremely large values (10-1000) to reproduce experimentally observed
currents. It is an open question if such high-β features actually exist, and thus whether this approach has an actual scientific basis or if the artificially high β is compensating for incomplete physics. We are pursuing an approach that will model field emission with a distribution of β, as well as the work function φ, where these distributions are taken from direct material surface measurements. A step in this analysis is to simulate fields in a domain with directly measured nm-sized surfaces from microscopy to produce actual β field enhancement factors. PIC simulations of mm-sized electrodes cannot resolve atomic-scale (nm) surface features and therefore we generate micron-scale models using probability distributions for effective “local” β, φ, and emission areas. We compare simulated nm-scale Fowler-Nordheim field emission currents with the currents generated using the micron-scale model on a coarse mesh with a perfectly flat model surface.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Speaker: Matthew Hopkins (Sandia National Laboratories)

Hopkins, Developing Field Emission Models Employing Nanoscale Surface Characterization for upload.pdf

39 Diamond at High Gradients

Diamond possesses extraordinary mechanical, thermal, and electronic properties – the name “diamond” is derived from “adamas,” meaning “invincible” – and has been deemed the ultimate frontier and engineering material of the 21st century. Indeed, diamond has the potential to reshape or revolutionize a wealth of applications across quantum computing, power grid infrastructure, implantable bio-electronics, opto-electronics and, of special interest, across pulsed power and accelerator R&D. In this presentation, we will review three important applications that can greatly benefit from the use of diamond devices. In all these applications, diamond needs to be operated between 0.1 and 10 GV/m.

First, we will present our recent results outlining the feasibility of creating an ampere-class high-brightness field emitter cathode technology for high frequency and high gradient injector developments. Our cathode material of choice is ultra-nano-crystalline diamond (UNCD). UNCD possesses exceptional emission efficiencies and low intrinsic emittance. Emission originates from grain boundaries, an important characteristic that allows for a simple planar geometry with ~10 nm roughness thus bypassing the need for traditional high-aspect-ratio geometries which limit cathode operation to 20-40 MV/m. As a result, operation at high gradients at or above 100 MV/m is feasible. We will present and review survivability and conditioning of UNCD up to 100 MV/m and corresponding output charge relations.

Secondly, we will review diamond as an ultrafast high-power device called diode avalanche shaper (DAS). When a diamond DAS is exposed to a nanosecond kV pulse, resulting in a field of ~1 GV/m, it functions as an ultrafast closing switch. This closing takes place on a sub-nano-second scale caused by the formation of the streamer traversing the diode at ~108 cm/s. For reference, streamers causing lightning in air are known to move as fast. Modelling results will be presented to illustrate how many MW of power can be switched over ~10 ps. One of the most prominent applications of the DAS is in ultrawide band (UWB) radio/radar. The use of diamond over Si greatly enhances peak output power and shortens pulse lengths, thus greatly improving the detection resolution and range, e.g., important for vehicular radar.

Lastly, we will review diamond as a waveguide material that could solve current limitations in efficient wakefield generation of optical fields on the order of 1 GV/m in the THz range. Such gradients are of paramount importance for beam-driven wakefield accelerators for TeV-class colliders and for most basic sciences where electric fields of up to 1 V/nm can be used for time-resolved THz-driven chemical reactions and materials phase transformation/transitions. We will review recently published 1 GV/m UCLA experiments with quartz waveguides at FACET-II and outline a thermal hypothesis to show how using diamond could potentially tackle ≥1 GV/m and/or increase repetition rate at FACET-II.

Speaker: Prof. Sergey Baryshev (Michigan State University)

40 Local power coupling as a predictor of high-gradient breakdown performance

A novel quantity for predicting the ultimate performance of high-gradient radiofrequency accelerating structures will be presented and compared with earlier quantities that it builds on such as the modified Poynting vector, Sc. This new method models a nascent RF breakdown as a current-carrying antenna and calculates the coupling of the antenna to the RF power source. With the help of a simple electron emission model to describe a nascent breakdown, the antenna model describes how a breakdown modifies the local surface electric field before it fully develops in any given structure geometry. For the structure geometries that this method was applied to, it was found that the calculated breakdown-loaded electric field was well-correlated with spatial breakdown distributions, and gave consistent values for the maximum breakdown-limited accelerating gradient between different structures.

Speaker: Prof. Jan Paszkiewicz (CERN)

Breakdown_Limit_HG2021_Jan_Paszkiewicz.pptx

41 Multi-scale multi-physics simulations of vacuum breakdown phenomena

Although vacuum arcs have been studied intensively for more than a century, the physical mechanisms involved in their ignition have not been fully understood yet. This is mainly due to the extreme nature and high complexity of the physical processes involved. Extreme phase changes occur in a sub-nanosecond timespan, involving various physical processes that occur at an atomic level and scale up to macroscopic sizes. In order to investigate such complex phenomena, multi-scale and multi-physics simulations that concurrently capture the various processes are necessary.

In this talk, an overview of recent advancements in multi-scale multi-physics simulations of vacuum breakdown phenomena shall be presented. Such simulations include molecular dynamics to model the movement of atoms under heating and electromagnetic field stress, concurrently coupled with particle in cell plasma simulations and finite element analysis for electrostatics, electron emission and heat transfer. These simulations capture the processes that occur when a nanoscopic field emitting metal protrusions enter thermal runaway, evaporate and ignite vacuum arc plasma, with the purpose of understanding the plasma ignition mechanisms, along with their limitations that can be exploited to minimize breakdown occurrence. Finally, a short overview of recent advancements of density functional theory calculations and kinetic Monte Carlo simulations for metal surface diffusion shall be given, showing that surface diffusion is a strong candidate mechanism for explaining protrusion growth on metal surfaces exposed to high electric field.

Speaker: Prof. Andreas Kyritsakis (University of Tartu)

Kyritsakis_HG2021_MPsim_VBD.pdf

Session 4: Snowmass Discussion

Conveners: Dr Emilio Nanni (SLAC National Accelerator Laboratory), Hans Weise (DESY), Sergey Belomestnykh (Fermilab)

HG2021_AF7RF_HW_SAB_EN.pptx

Session 7: RF Sources and Pulsed Power

Convener: Sami Tantawi (SLAC)

42 Development of X-band High Power High Efficiency Klystron

50MW X-band High Efficiency Klystron has been developed in CERN for CLIC Klystron driven scheme, as essential upgrade of its commercial counterpart. In this presentation, both the home-made simulation tool and specific design procedure will be elaborated. By taking the advantage of the updated coupling-cell module and optimization module implemented in KlyC, the retrofit Klystron could deliver 50MW power with efficiency of 67% and circuit length of 316mm. The original beam optics system could be reused with the revamped collect design, which is confirmed by home-made 2D code CGUN. Instabilities issues brought by multi-cell 2nd harmonic cavity are also well addressed and solved with comprehensive theoretical and numerical analysis. The prototype is now considered to be built with the collaboration of CERN’s industry partners.

Speaker: Jinchi Cai (CERN)

HG2021_Development of X-band 50MW High efficiency Klystron.pdf

43 A 3MW, 36GHz gyro-klystron for driving a harmonic lineariser for an X-ray FEL

Speaker: Adrian Cross (University of Strathclyde)

HG2021_F1_GyroKlystron_Strath.pptx

44 The LANL C-Band Engineering Research Facility (CERF-NM) Test Stand Installation, Operation and Initial Conditioning.

Abstract: We provide a system description of the LANL CERF-NM test stand, and discuss the operation and initial conditioning efforts. The specified klystron peak power of 50MW, operating with a 1µs pulse-width at a pulse repetition rate of 100 Hz (5kW average power) was achieved working into a matched water-cooled waveguide load.

Author List: M.E. Middendorf, J.T. Bradley III, C.E. Buechler, R.L. Fleming, E.G. Geros, D.V. Gorelov, H.J. Guas III, M.K. Kirshner, F.L. Krawczyk, J.W. Lewellen, L.N. Merrill, R.C. Moore, M.E. Schneider, E.I. Simakov, T Tajima.

Speaker: Mark Middendorf (LANL)

The LANL C-band Engineering Research Facility Test Stand Installation, Operation and Initial Conditioning.pdf

45 Development of Compact, Low-Voltage RF Power System Prototypes at SLAC

The RF power chain – including modulators and RF amplifiers themselves – is a major driver of capital and operational costs for any next-generation accelerator facility. For the current state of the art, RF power costs are likely prohibitive, and an order of magnitude improvement (in terms of $/peak kW) is needed. To this end, SLAC is developing a widely scalable RF source and modulator topology suitable for a range of commercial and scientific applications, to develop a diverse customer base and eventually leverage mass production of these devices to reduce costs. This has yielded multiple R&D programs in compact, integrated linac systems based on a modular, low voltage klystron topology. In this presentation, updates will be provided on SLAC’s prototype RF sources for these programs, and future research directions in this area will be discussed.

Speaker: Brandon Weatherford

HG2021_Final.pptx

46 Generation of 510 MW of power at X-Band using a metamaterial structure at the Argonne Wakefield Accelerator Facility

We present recent experimental results generating 510 MW of power at 11.7 GHz using a metallic metamaterial-based power extractor for use in structure-based wakefield acceleration (SWFA). SWFA is a novel acceleration scheme in which high-charge electron bunches are passed through a power extractor structure to produce a high-intensity wakefield. The resulting wakefield can either be used to accelerate a witness bunch in the same beamline or passed through a waveguide to a secondary acceleration beamline. Our approach uses a specifically-tailored metamaterial for the power extractor structure. The properties of the metamaterial, including an all-metal construction and simultaneously high group velocity and shunt impedance, allow us to overcome some of the difficulties encountered by other SWFA techniques.

Here we present the Stage 3 experimental design and results. The Stage 3 experiment builds on the success of the earlier Stage 1 and Stage 2 experiments, which generated 80 MW and 380 MW RF pulses, respectively, with several-nanosecond duration using the 65 MeV electron beam at the Argonne Wakefield Accelerator facility. The Stage 3 experiment implemented significant design improvements, including an all-copper structure, a fully-symmetric output coupler design, and treatment to reduce breakdown risk. These improvements led to the successful generation of 510 MW at 11.7 GHz, which is currently the highest power generated by an extractor for SWFA. This talk will discuss the background of the metamaterial-based design, the advancements that enabled our Stage 3 results, and the potential for increased power generation in the future.

Speaker: Julian Picard (MIT)

Julian Picard_HG2021.pdf