Valery Dolgashev

The development of advanced, high gradient accelerating structures is one of the leading activity of the particle accelerator community. In the technological research of new construction methods for these devices, high-power testing is a critical step for the verification of their viability. Recent experiments showed that accelerating cavities made out of hard copper, fabricated without high-temperature processes, can achieve better performance as compared with soft copper ones. Recently, we have built cavities using Tungsten Inert Gas welding and the high-power experiments confirmed that this joining process is a robust and low-cost alternative to brazing or diffusion bonding. This is a good solution for high-gradient operation, with a gradient of about 150 MV/m in X-band, at a breakdown rate of 1 0− 3/pulse/meter using a shaped RF pulse with a 150 ns flat part. We continue the design, construction and high power …

This article will review methods used at the SLAC National Accelerator Laboratory and other world accelerator laboratories to design high-gradient normal conducting accelerating structures. A quest for compact radio-frequency linacs fueled decades of studies toward a higher accelerating gradient. A major phenomena limiting the increase of the gradient is vacuum radio-frequency breakdown; therefore, this paper will address the breakdown physics and discuss approaches that reduce the breakdown probability. This discussion will cover both the electrical design and fabrication technology of the accelerating structures to achieve practical operating accelerating gradients in excess of 100 MV/m. Most of the data described here were obtained during the development of 11 GHz linacs for electron–positron linear colliders, so extrapolation of the results to other frequencies should be performed cautiously.

Efficiency is crucial in a robust medical or industrial RF linear accelerator to minimize RF power costs. A high shunt impedance is essential for RF-to-beam efficiency, with cavity iris radius being a key parameter. However, a tradeoff is needed to avoid beam breakup in high current applications. To reduce costs, we developed a split block brazeless accelerating structure that utilizes knife edges for vacuum sealing, similar to the industry-standard conflat technology. This approach allows for rapid prototyping and cost reduction. Our report highlights progress made in implementing this method for low-energy accelerating structures, as well as providing a brief description of two types of linacs we are developing. As an example, in the final section, a linac for medical applications is introduced.

The wide-ranging requirements for the photon properties from multiple beamlines in superconducting based free-electron lasers (FELs) demand more challenging beam manipulation techniques. Shot-by-shot control of electron beam bunch length and peak current at high repetition rate up to megahertz is highly desired. In this paper, we present a comprehensive study of a method based on a 2-m-long normal conducting radio-frequency cavity to achieve fast and flexible control of beam compression and realize the full potential of the facility, including theoretical analysis, beam dynamics simulations, and conceptual cavity design for the Linac Coherent Light Source II and its high-energy upgrade. We illustrate the physical mechanism of the chirping cavity on the control of the final beam compression and propose methods to lower the requirements for the cavity parameters. The application of this method will allow …

A photoinjector system containing modularly-structured waveguide-mode launcher, which is reversibly connected to the RF gun (containing a tubular construction formed with disattachably-affixed to one another structurally-complementary halves); and a solenoid magnet in operation enclosing such tubular structure in a central hollow. The resulting quality, power, and frequency rate of operation as well as cost of manufacturing and operation of the system are superior as compared with those of a related art system.

A new frequency converter, operating at significantly higher power and efficiency than previous devices, is described in this paper. The proposed device is implemented as a klystron structure where a new design principle is used. New analytical formulas and a specific design procedure are proposed. The klystron frequency multiplier can be suitable for telecommunications and non-lethal weapon, scientific and medical particle accelerators while the most interested exploitations are in the field of high gradient particle acceleration and FEL devices for which no performant sources exist. The advanced klystron multiplier can replace all the low level circuitry for frequency multiplication as a less expensive alternative. Efficiencies in the range of 60% in the K-band range with power levels of 30 MW are possible without phase noise, sideband generation, jitter or chirp effects. The presented design principle is applicable to other bands or power levels.

We investigate the design of a 2.856 GHz accelerator system to provide energy modulation and RF-based steering for rapid 3-D beam scanning for hadron therapy. Using General Particle Tracer, we simulate proton beam transport through the accelerator and deflector cavities. Field maps of the accelerating and deflecting fields in each section of the beam line are produced using ANSYS-HFSS models of the cavity geometries. Designs are optimized for the case of sub-relativistic protons with 230 MeV kinetic energy and cover an energy modulation range of+/-30 MeV. We present beam profile data after transport to different transverse positions, achieved using a combination of dynamic RF deflector cavities and static permanent magnet quadrupoles. We discuss the beam profile aberrations introduced for large transverse deflections.

Background:Open Access CommunicationA Hard Copper Open X-Band RF Accelerating Structure Made by Two Halvesby

We report the high gradient testing results of two single-cell off-axis coupled standing wave accelerating structures. Two brazed standing wave off-axis coupled structures with the same geometry were tested: one made of pure copper (Cu) and one made of a copper–silver (CuAg) alloy with a silver concentration of 0.08%. A peak surface electric field of 450 MV/m was achieved in the CuAg structure for a klystron input power of 14.5 MW and a 1 μs pulse length, which was 25% higher than the peak surface electric field achieved in the Cu structure. The superb high gradient performance was achieved because of the two major optimizations in the cavity's geometry:(1) the shunt impedance of the cavity was maximized for a peak surface electric field to accelerating gradient ratio of∼ 2 for a fully relativistic particle, and (2) the peak magnetic field enhancement due to the input coupler was minimized to limit pulse heating …

Recent research on high-gradient radio frequency (RF) accelerating structures indicates that the use of hard copper alloys provides improvement in high gradient performance over annealed copper. Such structures are made by bonding individually manufactured parts. However, there are no well-established bonding techniques that preserve the hardness, surface finish and cleanliness required for high gradient operation. To preserve the copper hardness, RadiaBeam has developed a novel high-gradient split accelerating structure, based on electron beam welding joining technique. This technique provides efficient bonding with strong, clean welds and minimal thermal loading, while maintaining a clean inner RF environment. Our RF design and fabrication methodology limits the small heat affected zone to the outer cavity envelop, with virtually no distortions or thermal loading of critical RF surfaces. It also …

The development of high gradient accelerating structures is one of the leading activities of the accelerator community. In the technological research of new construction methods for these devices, high-power testing is a critical step for the verification of their viability. Recent experiments showed that accelerating cavities made from hard copper alloys, can achieve better performance as compared with soft copper ones. The results of experiments showed that welded, hard copper cavities have shown breakdown rate of /pulse/meter at a gradient of about 150 MV/m, in the X-band, a using a shaped pulse with a 150 ns flat part. We continue the design, construction, and higher power experimental tests of three cells standing wave (SW) 11.424 GHz accelerating cavities fabricated with hard CuAg alloy to study the RF breakdown physics. Our aim is to fabricate the accelerating structures with innovative technologies easier to handle and cheaper; easier for surfaces inspection; easier for data elaboration and validation of joining techniques. The choice of these new technological approaches and design methods provides also the possibility of allocating the parasitic Higher Order Mode dampers. This paper describes the design of an optimized cavity made with sectors which provides a high longitudinal shunt impedance of the operating mode. The cavity will be fabricated by using the Tungsten Inert Gas process to realize a hard CuAg structure. Two three-cells SW X-band accelerating cavities, to be operated in the -mode and made out of hard CuAg alloy, were already fabricated at INFN-LNF by means of clamping and welding by using the TIG …

We will present the physics design of a compact, highly efficient, energy-tunable 9.3 GHz linac to generate up to 500 W of 10 MeV electron beam power for medical and security applications. This linac will employ a patented travelling wave accelerating structure with outside power flow which combines the advantages of high efficiency with energy tunability of traveling wave cavities. Unlike standing wave structures, the proposed structure has little power reflected back to the RF source, eliminating the need for a heavy, lossy waveguide isolator. In contrast to the side-coupled cavity designs, the proposed structure is symmetrical and therefore it does not have deflecting axial fields that impair the beam transport. The high shunt impedance will allow the linac to achieve an output energy of up to 10 MeV when powered by a compact commercial 9.3 GHz 1.7 MW magnetron. For pulse-to-pulse tuning of the beam output energy we will change the beam-loaded gradient by varying the linac’s triode gun current.

This paper reports the results of high gradient testing of the two C-band (5.712 GHz) normal conducting ????????= 0.5 accelerating cavities. The first cavity was made of copper and the second was made of copper-silver alloy with 0.085% silver concentration. The tests were conducted at the C-Band Engineering Research Facility of New Mexico (CERF-NM) located at Los Alamos National Laboratory. Both cavities achieved gradients more than 200 MeV/m and surface electric fields more than 300 MV/m. The breakdown rates were mapped as functions of peak surface fields. The gradients and peak surface fields observed in the copper-silver cavity were about 20% higher than those in the pure copper cavity with the same breakdown rate. It was concluded that the dominant breakdown mechanism in these cavities was not the pulse heating but the breakdown due to very high surface electric fields.

Argonne National Laboratory and RadiaBeam have designed the Advanced Compact Carbon Ion Linac (ACCIL) for the acceleration of carbon an proton beams up to the energies of 450 MeV/u, required for image-guided hadron therapy. Recently, this project has been enhanced with the capability of fast tumour tracking and treatment through the 4D spot scanning technique. Such solution offers a promising approach to simultaneously reduce the cost and improve the quality of the treatment. In this paper, we report the design of an accelerating structure, capable of operating up to 1000 pulses per second. The linac utilizes an RF pulse compressor for use with commercially available klystrons, which will dramatically reduce the price of the system..

An X-band Transverse deflector CAVity (XTCAV) has been successfully developed for femtosecond electron and X-ray pulse temporal diagnostic at the Linear Coherent Light Source (LCLS). The working frequency for the deflector is 11.424 GHz. New free electron laser LCLS-II has two undulator beamlines, one Soft-X-Ray (SXR) and another Hard-X-Ray (HXR). The HXR line deflector is made of two one-meter long XTCAVs. We have designed, built, installed and commissioned another, 1.5 meter long X-band deflector in the Soft-X-Ray beam line. Both HXR and SXR deflectors share one klystron. RF power is transmitted from a 50 MW klystron to a tunnel in an overmoded circular waveguide and then directed to either of the deflectors using a remotely controlled variable RF power splitter. The power split ratio can be changed arbitrarily, and both deflectors can work simultaneously. The system is successfully commissioned and operational. In this article, we provide details on the development and commissioning of the new deflector.

High-charge proton beams with 200+ MeV and small energy spread are important for applications such as therapy of deep-seated tumors. We use large 2D and 3D particle-in-cell (PIC) simulations to explore the development of a hybrid accelerator that combines the advantages of laser-driven (compact, high-charge, 10s MeV) proton beams with high-gradient RF acceleration (controllable beam energy and energy spread) in a meter-scale compact system, eliminating the need for large and expensive RFQ to perform bunching. We use an adaptive mesh technique to do fully-kinetic modeling of the system self-consistently, from the laser-solid interaction, transport, to the meter-scale acceleration in the RF. We find that space-charge field is minimized in transport due to the screening of accelerated electrons, but must be controlled in the RF stage for effective acceleration. We show that by tuning the distance of laser …

Emerging Concepts for Medical Accelerator Technology Page 1 Rapid RF-driven 3D Pencil Beam Scanning for Proton Therapy Emma Snively, SLAC National Accelerator Laboratory IPAC’22: Applications of Accelerators, Technology Transfer and Industrial Relations June 13th, 2022 Page 2 Improving efficiency 2 Improving radiation therapy with new accelerator technology Collimators/masks/filters Mechanical motion X-ray conversion More compact structures Higher rep rate, higher current Image courtesy of Varian Medical Systems, Inc. All rights reserved. Pencil beam scanning RF phase control Direct electron therapy Chang DS, Lasley FD, Das IJ, Mendonca MS, Dynlacht JR (2014) Therapeutic Ratio. In: Basic Radiotherapy Physics and Biology. Minimize dose to healthy tissue Avoid sensitive organs Shape cross section Multiple entry angles Increasing the therapeutic window Page 3 FLASH Therapy Motion …

Side coupled accelerating structures are popular in the industrial realizations of linacs due to their high shunt impedance and ease of tuning. We designed and fabricated a side-coupled X-band accelerating structure that achieved 133 MOhm/m shut impedance. This structure was fabricated out of two halves using a novel brazeless approach. The two copper halves are joined together using a stainless steel joining piece with knife edges that bite into copper. This structure had been tested at high power at SLAC National Accelerator Laboratory. The performance of the structure had been limited by magnetic breakdowns on the side-coupling cells. In this paper we will present results of the high gradient tests and after-test analysis. Scanning electron microscopy images show a typical magnetic-field induced breakdown.

Pencil beam scanning of charged particle beams is a key technology enabling high dose rate cancer therapy. The potential benefits of high speed dose delivery include not only a reduction in total treatment time and improvements to motion management during treatment, but also the possibility of enhanced healthy tissue sparing through the FLASH effect, a promising new treatment modality. We present here the design of an RF deflector operating at 2.856 GHz for the rapid steering of 150 MeV proton beams. The design utilizes as TE11-like mode supported by two posts protruding into a pillbox geometry to form an RF dipole. This configuration provides a significant enhancement to the efficiency of the structure, characterized by a transverse shunt impedance of 68 MΩ/m, as compared to a conventional TM11 deflector. We discuss simulations of the structure performance for several operating configurations including the addition of a permanent magnet quadrupole to amplify the RF-driven deflection. In addition to simulation studies, we will present preliminary results from a 3-cell prototype fabricated using four copper slabs to accommodate the non-axially symmetric cell geometry.

There is a strong demand for accelerating structures able to achieve higher gradients and more compact dimensions for the next generation of linear accelerators for research, industrial and medical applications. Notably innovative technologies will permit compact and affordable advanced accelerators as the linear collider and X-ray free-electron lasers (XFELs) with accelerating gradients over twice the value achieved with current technologies. In particular XFELs are able to produce coherent X-ray pulses with peak brightness 10 orders of magnitude greater than preceding approaches, which has revolutionized numerous research fields through imaging of the nanoscopic world at the time and length scale of atom-based systems, that is of femtosecond and Angstrom. There is a strong interest for combining these two fields, to form a proper tool with the goal of producing a very compact XFEL in order to investigate …