RF Linear Accelerator Research Articles

Analysis of energy stability in 6/4 MeV dual energy RF electron linac

RF linear accelerators are used to produce high energy electron beams up to several MeV for various industrial, medical and research applications. Accelerators with switchable energy are increasingly finding application in non-destructive testing systems with material discrimination capability. These systems are based on pulsed accelerators with duration 3 to 10 micro second with repetition rate up to 400 Hz. Magnetrons are used as microwave source in relatively low power accelerator systems. This paper examines various factors affecting the pulse to pulse energy variation in such radiation sources. Contributions from individual subsystems are accounted and the cumulative performance is evaluated. Dose stability improvement with remedial measures has been discussed.

Linear electron accelerator for energy 8-50 MeV with injection from an electron source based on cluster plasma systems

For many years, one of the key problems of modern accelerator physics has been an increase of the rate of the energy gain in RF linear electron accelerators. The physical limits of the accelerating field intensity for metallic accelerating structures have been practically reached; therefore, new acceleration schemes are being considered, primarily acceleration in plasma and wakefield acceleration. The second aim is the generation of ultrashort (100 fs and less) electron bunches, for which RF photoguns are traditionally used. In this case, for RF photoguns, a serious problem that limits the intensity of electrons in a bunch is the influence of the own space charge during emission and acceleration in the near-cathode region, where the beam is weakly relativistic and the influence of the space charge on its dynamics plays the determinative role. The possibility of using a plasma cathode source as an injector for RF accelerator will considered. In the future, this may make it possible to bypass the limitations inherent in RF photoguns (sufficient influence of the space charge on the beam dynamics in the near-cathode region) and acceleration in the laser-plasma channel (low electron capture coefficient in the acceleration mode, wide energy spectrum - 10% or more at energies of tens and hundreds of megaelectrons). It is proposed to develop a combined accelerator in which a bunch generated in a laser-plasma channel is injected into a traditional metal structure. It is supposed that could be possible to generate a short (from 0.1 to 1.0 ps) electron bunches with an energy of several hundred kiloelectrons, which will make it possible to consider such source as an alternative to the photocathode. Next, the beam must be captured in the acceleration mode in a normally conducting section and accelerated to an energy of 50 MeV with the possibility of energy tuning. The features of such accelerator, the features of the electron bunch capturing in the acceleration mode, and the possible values of the energy spectrum in such a system will considered.

Simulation studies of beam dynamics in 50 MeV linear accelerator with laser-plasma electron gun

Conventionally, electron guns with thermionic cathodes or field-emission cathodes are used for research or technological linear accelerators. RF-photoguns are used to provide the short electron bunches which could be used for FEL’s of compact research facilities to generate monochromatic photons. Low energy of emitted electrons is the key problem for photoguns due to high influence of Coulomb field and difficulties with the first accelerating cell simulation and construction. Contrary, plasma sources, based on the laser-plasma wakefield acceleration, have very high acceleration gradient but rather broad energy spectrum compared with conventional thermoguns or field-emission guns. The beam dynamics in the linear accelerator combines the laser-plasma electron source and conventional RF linear accelerator is discussed in this paper. Method to capture and re-accelerate the short picosecond bunch with extremely broad energy spread (up to 50 %) is presented. Numerical simulation shows that such bunches can be accelerated in RF linear accelerator to the energy of 50 MeV with output energy spread not higher than 1 % .

Two Design Options for Compact Linear Accelerators for High Flux Neutron Source

We describe and compare two optimized design options of RF linear accelerators with different resonant frequencies at 162.5 MHz (f0) and 325 MHz (2∙f0). The RFQ + DTL linacs have been designed to provide 13 MeV acceleration to a proton beam for achieving a fast neutron yield of not lower than 1013 n/s via 9Be(p, n)9B reaction in pulsed-mode operation. Our design studies show that none of the two options is better than the other, but that the choice of operating frequency will mainly be determined by the accelerator length and RF cost consideration. This study can serve as a basis for the design of an initial stage of a new high brilliance Compact Accelerator-driven Neutron Source (CANS), aiming to use neutron scattering techniques for studying material properties in fundamental physics, materials science, nuclear energy, as well as for industries and societal challenges.

Electron bunchers for industrial RF linear accelerators: theory and design guide

The acceleration of electrons in resonant linear accelerators (linacs) typically consists of three main stages: (1) emission of the electrons from the cathode and their pre-acceleration with a DC field to the energy of tens of keV; (2) grouping the DC electron beam into bunches and their synchronization with the correct phase of high-frequency electromagnetic fields, and (3) accelerating the bunches of relativistic electrons to the required energies. Although many books describe the theoretical and practical aspects of electron linac design, most of them concentrate on beam physics in either the gun stage or in the relativistic regime, while leaving the description of the bunching process rather general. The physics of non-relativistic motion is described in the literature on ion accelerators, but in practice, it cannot be scaled to electron machines due to the significantly different particle mass and acceleration rate, beam velocity change, and frequencies. In this tutorial review paper, we will fill this gap with a detailed description of the bunching process and provide practical advice on the design of bunching sections in industrial-grade electron linacs.

Microwave radiation from interactions of modulated electron beams with solid matter

RF-modulated electron beams, such as those produced by an RF linear accelerator, propagating through vacuum, air, and solid matter are well known to drive signals in microwave cavities and waveguides via interactions with these structures. Past experiments with a microwave waveguide in a radiation-shielded vault indicated the presence of a multipath propagation phenomenon, hypothesized to be a result of reflections of RF-modulated x rays. In this work, we study the signals induced in a microwave coaxial cable from nearby beam interactions with materials commonly found in accelerator facilities in order to better understand RF production and propagation in these environments. Our results show that (1) when an RF-modulated electron beam is incident on a block of aluminum, lead, or concrete, the frequency content of the induced microwave signals is strongly dependent on the orientation of the block and the relative position of the detector, (2) at least some of the detected signals are consistent with reflections off of the blocks, and (3) beam interactions with the blocks can induce appreciable microwave signals in detectors located tens of cm from the block.

The Beam Plasma Interactions Experiment: An Active Experiment Using Pulsed Electron Beams

The 1970s and 1980s were heydays for using active electron beam experiments to probe some of the fundamental physical processes that occur throughout the heliosphere and in astrophysical contexts. Electron beam experiments were used to study spacecraft charging and spacecraft-plasma coupling; beam-plasma interaction physics; magnetic bounce and drift physics; auroral physics; wave generation; and military applications. While these experiments were enormously successful, they were also limited by the technologies that were available at that time. New advances in space instrumentation, data collection, and accelerator technologies enable a revolutionary new generation of active experiments using electron beams in space. In this paper we discuss such an experiment, the Beam Plasma Interactions Experiment (Beam PIE), a sounding rocket experiment designed to (a) advance HEMT-based RF linear accelerator electron accelerator technology for space applications and (b) study the production of whistler and X-mode waves by modulated electron beams.

Demonstration of efficient beam-wave interaction for a MW-level 48 GHz gyroklystron amplifier

The development of high-frequency RF linear accelerators (linacs) requires the consideration of several technological challenges, such as electron bunch linearization. Presented in this paper is the design of the interaction circuit for a 48 GHz MW-level three-cavity gyroklystron amplifier, appropriate for application as a millimeter wave power source in a fourth harmonic linearizing system for an X-band linac. The output cavity is operated at the cylindrical TE0,2,1 mode, while the input and buncher cavities are operated at the TE0,1,1 mode. The interaction circuit has been designed using a combination of analytical calculations and particle-in-cell simulations. The optimized gyroklystron is shown, through simulation, to deliver an output power of up to 2.3 MW with a gain of 36 dB and an efficiency of 44% at 48 GHz, when driven by a 140 kV, 37 A electron beam.

9-sector compression magnet design

A 30 MeV RF electron linac-based neutron source will be set up by APPD, BARC at IGCAR, Kalpakkam, India. This facility will include neutron activation of sodium by photo neutrons for FBR shielding studies and Time of Flight (n-TOF) facility for various research applications. The n-TOF facility consists of electron gun that will generate 10 ns electron beam. It will be accelerated to 30 MeV by RF linear accelerator. The electron pulse at the exit of the linac will have 40% energy spread. A compression magnet is designed to compress 10 ns, 10 A electron pulse to 1 ns, 100 A. A 3600, 5-sector, zero gradient magnet having rectangular yoke of size 3.2,m × 2.7 m × 0.4 m and weight of ∼24 Ton gives pulse compression ratio of 10:1 at a magnetic field of 0.1 T. In order to get the same compression ratio with reduced magnet size, a 7200, 9-sector magnet having zero magnetic field gradient is designed. A magnet will have five physical sectors through which an electron beam will rotate twice. The analytical solution and CST Particle Studio simulation results are presented. To get 10:1 compression ratio, ∼0.15 T magnetic field will be required and overall magnet size will be reduced to 2.1,m × 2.1 m × 0.4 m.

Propagation and detection of RF-modulated electron and X-ray beams in air

Electron beams produced in RF linear accelerators will naturally be modulated at the RF frequency. Here we report measurements of the RF harmonic content of a 21.6 MeV electron beam coasting in air, as well as the RF harmonic content of x-rays produced from that electron beam, and the effects of these modulated electron and x-ray beams on several fast detection systems. The RF fundamental and its higher harmonics were found to be impressed onto the x-rays generated from the electron beam, and the response of an RF waveguide to passage of the modulated x-ray signal indicated that this harmonic content was also impressed onto the secondary electrons produced by the passage of the x-rays through the waveguide. An unexpected, interference-like effect was observed, which was particularly prominent in the case of the waveguide when struck by the modulated x-rays. The participation of secondary electrons produced by passage of the x-rays through the x-ray converter upstream of the waveguide was ruled out as a significant contributor to this effect.

Study of the first Mexican RF linear accelerator

In this work, the preliminary design of an electron linear particle accelerator (eLINAC) capable ofdelivering a beam within an energy range of 5 to 100 MeV using the S-band acceleration frequencyis presented. At its peak energy, it can be used as an injector for a synchrotron light source,independent free electron laser, or to generate neutrons. The eLinac will be separated into threebeam lines to deliver electron beams at dierent energies permitting the possibility of industrialapplications such as food irradiation, medical use, and national security.

Electron beam modified zinc phthalocyanine thin films for radiation dosimeter application

Present study deals with the effect of electron beam irradiation on electrical and gas sensing properties of zinc phthalocyanine (ZnPc) thin film prepared on flexible Biaxially Oriented Polyethylene Terephthalate (BOPET) sheet. The electron beam irradiation of ZnPc films were carried out using a 10MeV RF Linear accelerator, under different radiation dose levels from 1kGy to 30kGy. The pristine and irradiated films were characterized by using X-ray photoelectron spectroscopy, Atomic Force microscopy, UV–vis spectroscopy and X-ray diffraction. Samples irradiated in the dose range of 1–18kGy exhibit linear enhancement of electrical conductance due to increasing content of adsorbed oxygen. Beyond 18kGy the sample exhibit saturation in the content of adsorbed oxygen as well as in electrical conductance. The pristine ZnPc films exhibit excellent chemi-resistive response towards H2S gas in 1–20ppm range at room temperature. The response of the irradiated films decreases monotonically with increasing electron beam dose due to the strong binding of oxygen at the Zn sites. A plausible mechanism of electron beam induced modification of ZnPc films and its implication on charge transport as well as chemi-resistive gas sensing behavior are discussed. This work highlight the utilization of ZnPc thin films as potential radiation dosimeter based upon linear rise in electrical conductance.

Fermilab proton accelerator complex status and improvement plans

Fermilab carries out an extensive program of accelerator-based high energy particle physics research at the Intensity Frontier that relies on the operation of 8 GeV and 120 GeV proton beamlines for a number of fixed target experiments. Routine operation with a world-record 700 kW of average 120 GeV beam power on the neutrino target was achieved in 2017 as a result of the Proton Improvement Plan (PIP) upgrade. There are plans to further increase the power from 900–1000 kW. The next major upgrade of the FNAL accelerator complex, called PIP-II, is under development. It aims at 1.2 MW beam power on target at the start of the LBNF/DUNE experiment in the middle of the next decade and assumes replacement of the existing 40 years old 400 MeV normal-conducting Linac with a modern 800 MeV superconducting RF linear accelerator. There are several concepts to further double the beam power to [Formula: see text] 2.4 MW after replacement of the existing 8 GeV Booster synchrotron. In this review, we discuss current performance of the Fermilab proton accelerator complex, the upgrade plans for the next two decades and the accelerator R&D program to address cost and performance risks for these upgrades.

A new RF window designed for high-power operation in an S-band LINAC RF system

A new RF window is designed for high-power operation at the Pohang Light Source-II (PLSII) S-band linear accelerator (LINAC) RF system. In order to reduce the strength of the electric field component perpendicular to the ceramic disk, which is commonly known as the main cause of most discharge breakdowns in ceramic disk, we replace the pill-box type cavity in the conventional RF window with an overmoded cavity. The overmoded cavity is coupled with input and output waveguides through dual side-wall coupling irises to reduce the electric field strength at the iris and the number of possible mode competitions. The finite-difference time-domain (FDTD) simulation, CST MWS, was used in the design process. The simulated maximum electric field component perpendicular to the ceramic for the new RF window is reduced by an order of magnitude compared with taht for the conventional RF window, which holds promise for stable high-power operation.

SU‐F‐T‐554: Dark Current Effect On CyberKnife Beam Dosimetry

Purpose:All RF linear accelerators produce dark current to varying degrees when an accelerating voltage and RF input is applied in the absence of electron gun injection. This study is to evaluate how dark current from the linear accelerator of CyberKnife affect the dose in the reference dosimetry.Methods:The G4 CyberKnife system with 6MV photon beam was used in this study. Using the ion chamber and the diode detector, the dose was measured in water with varying time delay between acquiring charges and staring beam‐on after applying high‐voltage into the linear accelerator. The dose was measured after the time delay with over the range of 0 to 120 seconds in the accelerating high‐voltage mode without beam‐on, applying 0, 10, 50, 100, and 200 MUs. For the measurements, the collimator of 60 mm was used and the detectors were placed at the depths of 10 cm with the source‐to‐surface distance of 80 cm.Results:The dark current was constant over time regardless of MU. The dose due to the dark current increased over time linearly with the R‐squared value of 0.9983 up to 4.4 cGy for the time 120 seconds. In the dose rate setting of 720 MU/min, the relative dose when applying the accelerating voltage without beam‐on was increased over time up to 0.6% but it was less than the leakage radiation resulted from the accelerated head. As the reference dosimetry condition, when 100 MU was delivered after 10 seconds time delay, the relative dose increased by 0.7% but 6.7% for the low MU (10 MU).Conclusion:In the dosimetry using CyberKnife system, the constant dark current affected to the dose. Although the time delay in the accelerating high‐voltage mode without beam‐on is within 10 seconds, the dose less than 100 cGy can be overestimated more than 1%.

Potential applications of the dielectric wakefield accelerators in the SINBAD facility at DESY

Short, high-brightness relativistic electron bunches can drive ultra-high wakefields in the dielectric wakefield accelerators (DWFAs). This effect can be used to generate high power THz coherent Cherenkov radiation, accelerate a witness bunch with gradient two or three orders of magnitude larger than that in the conventional RF linear accelerators, introduce energy modulation within the driving bunch itself, etc. The paper studies potential applications of the DWFAs in the SINBAD facility at DESY. The simulations show that the ultra-short relativistic bunches from the SINBAD injector ARES can excite accelerating wakefields with peak amplitudes as high as GV/m at THz frequencies in proper DWFA structures. In addition, it illustrates that the DWFA structure can serve as a dechirper to compensate the correlated energy spread of the bunches accelerated by the laser plasma wakefield accelerator.

Single pass, THz spectral range free-electron laser driven by a photocathode hybrid rf linear accelerator

A single pass, THz spectral range free-electron laser (FEL) driven by a photocathode hybrid rf-LINAC is considered, taking the Israeli THz FEL project developed in Ariel University as an example. Two possible configurations of such FEL are discussed: an enhanced coherent spontaneous emission FEL, and a prebunched FEL utilizing periodically modulated short electron beam pulses. A general study of the FEL configurations is carried out in the framework of a space-frequency approach, realized in WB3D numerical code. The configurations are studied and compared based on preliminary parameters of a drive hybrid rf-LINAC gun under development in University of California, Los Angeles.

First acceleration of a proton beam in a side coupled drift tube linac

We report the first experiment aimed at the demonstration of low-energy protons acceleration by a high-efficiency S-band RF linear accelerator. The proton beam has been accelerated from 7 to 11.6 MeV by a 1 meter long SCDTL (Side Coupled Drift Tube Linac) module powered with 1.3 MW. The experiment has been done in the framework of the Italian TOP-IMPLART (Oncological Therapy with Protons-Intensity Modulated Proton Therapy Linear Accelerator for Radio-Therapy) project devoted to the realization of a proton therapy centre based on a proton linear accelerator for intensity modulated cancer treatments to be installed at IRE-IFO, the largest oncological hospital in Rome. It is the first proton therapy facility employing a full linear accelerator scheme based on high-frequency technology.

Development of an automatic frequency measurement system for RF linear accelerator magnetrons

An X-band [9300 MHz] magnetron frequency measurement system was developed for the electron linear accelerators at the Korean Atomic Energy Research Institute (KAERI). The measurement and the display of the RF frequency during the accelerator operation time is a crucial factor for continuous operation for two key reasons. Firstly, if the RF frequency of the magnetron is not known, then the amount of frequency tuning cannot be known, and the appropriate RF power cannot be supplied to the accelerating-structure. Second, values including the accelerating-structure’s coolingwater temperature setting, the solenoid-magnet’s cooling-water temperature setting, and the tuning of the source’s (magnetron’s) frequency can be undertaken because the RF frequency is used as the reference. A key component of the accelerator is the accelerating-structure. The volume of the accelerating-structure changes according to the environment’s temperature; there, the resonance frequency of the accelerating-structure varies. When the resonance frequency of the accelerator is changed, the output becomes unstable, and a low beam energy is obtained. Accordingly, was developed a magnetron frequency-measuring device in order to stabilize the accelerator’s operation. The results of the test demonstrate that the measurement’s accurate up to 100 kHz, which enables the provision of an accurate RF power to the accelerating -structure. In this paper, we discuss the RF frequency measurement system for the magnetron to enable a more stable accelerator operation in a linac.

Auto-tuning systems for J-PARC LINAC RF cavities

The 400-MeV proton linear accelerator (LINAC) at the Japan Proton Accelerator Research Complex (J-PARC) consists of 324-MHz low-β and 972-MHz high-β accelerator sections. From October 2006 to May 2013, only the 324-MHz low-β accelerator section was in operation. From the summer of 2013 the J-PARC LINAC was upgraded by installing the 972-MHz high-β accelerator section, and the proton beam was successfully accelerated to 400MeV in January 2014. Auto-tuning systems for the J-PARC LINAC RF cavities have been successfully developed. A first generation design, an auto-tuning system using a mechanical tuner controller, was developed and operated for the first 3 years. Then the second-generation auto-tuning system was developed using a new approach to the RF cavity warm-up process, and this was applied to the accelerator operation for the subsequent 4 years. During the RF cavity warm-up process in this system, the mechanical tuner is constantly fixed and the input RF frequency is automatically tuned to the cavity resonance frequency using the FPGA (Field-Programmable Gate Array) of the digital feedback RF control system. After the input power level reaches the required value, input RF frequency tuning is stopped and it is switched to the operation frequency. Then, the mechanical tuner control begins operation. This second-generation auto-tuning system was extremely effective for the 324-MHz cavity operation. However, if we apply this approach to the 972-MHz RF cavities, an interlock due to the RF cavity reflection amplitude occasionally occurs at the end of the warm-up process. In order to solve this problem a third generation novel auto-tuning system was successfully developed in December 2013 and applied to the operation of the J-PARC LINAC, including the 972-MHz ACS RF cavities. During the warm-up process both the mechanical tuner controller and the input RF frequency tuning are in operation, and good matching between the input RF frequency and the RF cavity is obtained during the entire warm-up process. As a result of their excellent performance, 972-MHz ACS cavity operation and the J-PARC LINAC 400MeV upgrade have been successfully implemented. In this paper, the development of the auto-tuning systems and their applications will be described in detail.