High-current Accelerator Research Articles

Pumping Systems for Compton Free-Electron Lasers: Microwave Undulators and Powering Sources

The concept of Compton-type free-electron lasers (FELs) operating in short wavelength ranges with a high efficiency and power level is currently underway at the IAP RAS (Nizhny Novgorod). This concept is aimed at reducing the energy of a driving relativistic electron beam and thereby increasing the efficiency of the electron–wave interaction in FELs, as well as making the oscillator relatively compact. The basis of this concept is microwave undulators of a new type—the so-called “flying” undulators. This paper is devoted to the results of the current studies of these undulators, their simulation, and “cold” electrodynamic tests in the Ka band. For powering microwave undulators, a spatially extended narrow-band ˇ Cerenkov surface-wave oscillators (SWOs) are developed in the specified frequency range driven by Sinus-6, a high-current accelerator, with a particle energy of 0.5 MeV, a current of 5 kA, and a pulse duration of 25 ns. The required sub-gigawatt power level of output radiation combined with a high stability of the narrow-band oscillation regime is achieved under conditions of a strongly oversized oscillator by using two-dimensional distributed feedback provided in a 2D doubly-periodic slow-wave structure. The design parameters of a 32 GHz/0.5 GW SWO intended for powering microwave undulators are presented and the results of its simulation and reported.

Combined zero degree structure beam dynamics and applications

The combined zero degree structure (KONUS) is a quasiperiodic structure. It was developed for the low-energy part of multigap drift tube linacs with H-type cavities. Their rf efficiency depends very much on a low electrical capacity of the drift tube structure, while in E-type structures like the Alvarez-DTL this is a minor effect. Therefore, instead of having quadrupole singlets integrated in voluminous drift tubes, KONUS allows one to develop a separated function drift tube linac (DTL) with a large voltage gain between two lenses. Very low beam injection energies can be realized, as the drift tube lengths can range down to around 10 mm. One KONUS period consists of a triplet lens, a rebuncher with a few gaps at a synchronous phase around $\ensuremath{-}35\ifmmode^\circ\else\textdegree\fi{}$, and the main multigap acceleration designed for a hypothetical zero degree synchronous particle. The longitudinal beam dynamics along this main acceleration section and the layout of the quadrupole triplet channel are explained in detail. Two examples for pulsed high current proton and heavy ion acceleration are included.

High-Current Pulsed-Repetitive Electron Accelerator “Sinus-320”: Formation and Diagnostics of a Wide-Aperture Beam

A wide-aperture (16 cm × 27 cm) source of e-beams for sterilization of plastic packages based on a“SINUS- 320” high-current accelerator with 450 keV energy, 9 kA current, 12 ns pulse duration, and repetition rate up to 100 Hz is described. A metal-dielectric cathode forms e-beam by explosive emission during a few nanoseconds of the voltage pulse. A high degree of e-beam uniformity can be achieved because of a large number of triple points (~800) and of electron scattering in a foil and in a 3 cm air layer for characteristic electron free paths in air up to half a meter.

The use of an RF undulator in the design of an accelerating structure

The idea of accelerating a beam in the accelerating structures based on an RF undulator poses great ad-vantages in high current proton and ions accelerators. The accelerating structure based on an RF undulator uses a combinational wave that consists of the zeroth and the first harmonics for acceleration and focusing. This paper presents the development of this accelerating structure for acceleration of a beam. In particular, we show that this structure is an H-type resonator composed from five cou-pled sections.

Theoretical Optimization of Neutron and Physical Characteristics of a Multiplying Pulsed Neutron Source Based on a Proton Accelerator

The neutron source operating at the Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research (JINR, Dubna, Russia), based on an IBR-2M pulsed reactor with a flux density of thermal neutrons of ~1013 n/(cm2 s) will exhaust its resource and be shut down by 2034. Optimizing this reactor to provide higher neutron flux density is practically impossible. Therefore, it is proposed to consider using a subcritical pulsed neutron source driven by a proton accelerator as a substitute for the IBR-2M. At present, nonmultiplying neutron sources based on high-current proton accelerators are mostly used around the world, for example, SNS or ESS with a beam power of 3 and 5 MW, respectively. In this paper, we present a theoretical comparative analysis of different categories of pulsed neutron sources driven by high-current proton beams, which can help to find the optimum solutions. It is shown that the subcritical one-zone booster with a tungsten target and a plutonium dioxide core can be a promising option. This type of source can provide an average thermal neutron flux density of more than 5.0 × 1014 n/(cm2 s) at a proton-beam power of 0.1–0.2 MW.

New approach to determining radiative capture reaction rates at astrophysical energies

Radiative capture reactions play a crucial role in stellar nucleosynthesis but have proved challenging to determine experimentally. In particular, the large uncertainty ($\ensuremath{\approx}100%$) in the measured rate of the $^{12}\mathrm{C}(\ensuremath{\alpha},\ensuremath{\gamma})^{16}\mathrm{O}$ reaction is the largest source of uncertainty in any stellar evolution model. With development of high-current energy-recovery linear accelerators (ERLs) and high-density gas targets, measurement of the $^{16}\mathrm{O}(e,{e}^{\ensuremath{'}}\ensuremath{\alpha})^{12}\mathrm{C}$ reaction close to threshold using detailed balance allows a new approach to determine the $^{12}\mathrm{C}(\ensuremath{\alpha},\ensuremath{\gamma})^{16}\mathrm{O}$ reaction rate with significantly increased precision ($<20%$). We present the formalism to relate photo- and electrodisintegration reactions and consider the design of an optimal experiment to deliver increased precision. Once the new ERLs come online, an experiment to validate the approach we propose should be carried out. This approach has broad applicability to radiative capture reactions in astrophysics.

SMALL-SIZE DIRECT-ACTION ELECTRON ACCELERATOR WITH A HIGH-EFFICIENCY NANOSECOND PLASMA-CURRENT SWITCH

The paper presents the results of a study undertaken to determine operating modes for a small-size direct-action electron accelerator with a high-efficiency nanosecond plasma-current switch (PCS). The investigations have shown that using PCS as a base it is possible to develop small-size nanosecond pulsed high-current electron accelerators with a voltage pulse sharpening coefficient of about 12, beam (flow) electron energy of 300…400 keV and current of 100 kA for pulse duration of 30 ns. The ways for improving the PCS and accelerator operating parameter stability, increasing the switching current and maximum accessible switching frequency are proposed.

Plasma Injectors for Quasi-Stationary High-Power Plasmodynamic Systems

Pulsed plasma injectors, that is, an “input ionization chamber” (IIC), an “anode ionization chamber” (AIC), and a high-pressure plasma injector, are described. Plasma injectors are designed to generate primary plasma in quasi-stationary high-power plasmodynamic systems, in particular, in the KSPU Kh-50 quasi-stationary high-current plasma accelerator. The results of studies of the main characteristics of plasma pulsed injectors are presented. The following charge currents in plasma were obtained: 23.5 kA for the AIC injector at a voltage of 3 kV, 90 kA for the IIC injector at a voltage of 0.65 kV, and 96 kA for the high-pressure plasma injector at a voltage of 3 kV.

Transient beam loading and rf power evaluation for future circular colliders

Interaction of the beam with the fundamental impedance of the accelerating cavities can limit the performance of high-current accelerators. It can result in a significant variation of bunch-by-bunch parameters (bunch length, synchronous phase, etc.) if filling patterns contain gaps that are not negligible compared with the cavity filling time. In the present work, this limitation is analyzed using the steady-state time-domain approach for the high-current option (Z) of the future circular electron-positron collider and for the future circular hadron-hadron collider. Mitigation of transient beam loading by direct rf feedback is addressed with evaluation of additional required generator power.

Development and vertical tests of a 166.6 MHz proof-of-principle superconducting quarter-wave beta = 1 cavity.

A low-frequency superconducting cavity is needed in main accelerators for storage ring light sources with ultralow emittance. A compact 166.6 MHz superconducting proof-of-principle cavity was designed adopting a quarter-wave β = 1 geometry for a High Energy Photon Source (HEPS). It is a 6 GeV diffraction-limited synchrotron light source currently being developed at the Institute of High Energy Physics. The cavity is exceedingly compact in size yet possessing a low resonant frequency. The nearest higher order mode is largely separated from the fundamental, making the cavity an attractive geometry for effective damping of these modes in high current accelerators such as HEPS. The achieved accelerating voltage of 3.0 MV is well beyond the designed 1.5 MV and required 1.2 MV for HEPS operation. High surface electromagnetic fields were reached with excellent rf and mechanical performances, and multipacting barriers were easily processed. This constitutes the first demonstration of a compact low-frequency β = 1 superconducting cavity for HEPS. The design, fabrication, surface preparation, and cryogenic tests of the cavity are presented.

Exploring the production of new superheavy nuclei with proton and α -particle evaporation channels

Existing experimental facilities limit the possibilities for discovery of new nuclides to those synthesized with cross sections above 100 fb, but the perspectives for future high current accelerators could lower this limit by two orders of magnitude. Therefore, in the present work excitation functions for fusion-$xn$ evaporation reaction channels induced not only by $^{48}Ca$ but also by heavier projectiles (usually leading to smaller cross sections) on actinide targets were calculated in the framework of the fusion-by-diffusion (FBD) model. For the first time, in this approach, channels in which a proton ($pxn$) or alpha particle ($\alpha$$xn$) is evaporated have been included in the first step of the deexcitation cascade. To calculate the synthesis cross sections entry data such as fission barriers, ground-state masses, deformations and shell effects of the superheavy nuclei calculated in a consistent way within the Warsaw macroscopic-microscopic model were used. The only adjustable parameter of the FBD model is the injection point distance $s_{inj}$ and the value determined in our previous analysis of experimental cross sections for the synthesis of superheavy nuclei of Z=114-118 has been used. Excitation functions for the synthesis of selected (cross section above a few fb) new superheavies in the range of atomic numbers 112-120 are presented. Observation of 21 new heaviest isotopes is predicted. A realistic discussion of the FBD model uncertainties is presented for the first time.

Underground nuclear astrophysics experiment JUNA in China

AbstractUnderground Nuclear Astrophysics in China (JUNA) will take the advantage of the ultra-low background in Jinping underground lab. High current accelerator with an ECR source and detectors were commissioned. JUNA plans to study directly a number of nuclear reactions important to hydrostatic stellar evolution at their relevant stellar energies. At the first period, JUNA aims at the direct measurements of 25Mg(p,γ)26 Al, 19F(p,α) 16 O, 13C(α, n) 16O and 12C(α,γ) 16O near the Gamow window. The current progress of JUNA will be given.

SuperKEKB main ring vacuum system status until the end of Phase-2 commissioning

SuperKEKB is an electron–positron collider with asymmetric energies at KEK aiming at a high luminosity of 8.0 × 1035 cm–2 s–1. The main ring (MR) consists of two rings: a 4 GeV positron ring (low-energy ring, LER) and a 7 GeV electron ring (high-energy ring, HER). During the Phase-1 commissioning of the MR from February to June 2016 and the Phase-2 commissioning from March to July 2018, the vacuum system worked well overall. By the end of Phase-2, the total beam doses (integrated beam currents) were 1113 and 1002 Ah, and the maximum beam currents were approximately 1010 and 870 mA for the LER and HER, respectively. The increase in pressure per unit beam current decreased steadily. The photodesorption coefficients reached approximately 1 × 10−6 and 7 × 10−8 molecules photon−1 with photon doses of 5.88 × 1024 and 9.27 × 1024 photons m−1 for the LER and HER, respectively. The beam lifetime was limited by the Touschek effect in both rings. Various new vacuum components, including those installed after the Phase-1 commissioning, such as the beam pipes for the Belle II particle detector, beam collimators, and so on, have been functioning well so far. The electron cloud effect was observed in the LER during Phase-1, but it was suppressed by additional countermeasures applied before Phase-2. Regarding the pressure bursts accompanied with beam losses, which were frequently observed during Phase-1, the frequency of the pressure bursts drastically decreased during Phase-2, but the longer operation time with lower beam currents during Phase-2 than Phase-1 should be taken into account. Several new problems, such as the heating of stainless-steel beam pipes due to the synchrotron radiation emitted from the final focusing quadrupole magnets at the collision point, were found during Phase-2, and countermeasures are being prepared for the Phase-3 commissioning, which will commence in March 2019. SuperKEKB has been accumulating various data and information that will facilitate the design and operation of the vacuum systems in future high-current accelerators.

Сильноточный имульсно-периодический ускоритель электронов «SINUS-320»: формирование и диагностика широкоапертурного пучка

Сильноточный имульсно-периодический ускоритель электронов «SINUS-320»: формирование и диагностика широкоапертурного пучка

The Ion-Hose Instability in a High-Current Multipulse Linear Induction Accelerator

The ion-hose instability is considered to be a potential danger for long-pulse, high-current electron linear induction accelerators (LIAs). As is shown in this paper, it is also a concern for multipulse LIAs. This instability is a direct result of beam-electron ionization of residual background gas, and the growth rate is proportional to the ion density. In a typical accelerator vacuum, there is little ion density decay between beam pulses, and the instability will grow throughout the pulse train. We have simulated this effect, taking into account ion density decay due to recombination and channel expansion. These simulations were done for the new 2-kA, 20-MeV Scorpius multipulse LIA presently being developed. The simulations have shown that the magnetic focusing field designed for Scorpius will be strong enough to inhibit ion-hose instability if the background pressure is kept below a value that is readily attainable with the present designs of induction cells and other accelerator components.

Investigation of the level of DNA double-strand breaks and mechanisms of cell death under irradiation of lung cancer and melanoma cells with ultra-high dose rate photon radiation

Research into the effects of radiation delivered at ultrahigh dose rates > 1 × 107 Gy/min to biological objects is a new promising area of radiobiology. The unique characteristics of the high-current nanosecond electron accelerator Mir-M enable its use in medical and biological research, specifically in the experiments aimed at investigating the effect of therapeutic doses at a dose rate up to 100 MGy/s. In this work we study the effects of ultrahigh dose rate photon radiation on human lung carcinoma (A549) and melanoma (MelMtp-x) cells lines and compare them with those of the therapeutic gamma unit Rokus-AM. We show that ultrahigh dose rates induce more significant damage in the studied cell lines at doses between 2 and 7 Gy, radioresistant melanoma being more sensitive to photon radiation delivered at ultrahigh dose rates.

Study of Biological Effects by Pulsed Bremsstrahlung Radiation of Ultrahigh Dose Rate at the Angara-5-1 Facility

Radiological methods of treatment of malignant tumors are widespread in medical practice. In addition to the type of radiation and dose and fractionation of the radiation regime, the dose rate is one of the factors that affects the effectiveness of treatment. The therapeutic dose rate lies in the range of tens of mGy/s. At modern high-current accelerators of relativistic electron beams, a significant increase in the dose rate to hundreds of MGy/s is achievable, which is more than 108 times greater than the therapeutic dose rate. It is difficult to predict the nature of processes in tissues and its cells at such radiation intensities. To determine the effect of extreme dose rate on the radiosensitivity of tissues at the Angara-5-1 facility, experiments were conducted to determine the lethal dose (LD50/30) for laboratory mice. Our result on the study of the LD50/30 dose (~100 MGy/s) allows us to make a conclusion about a possible higher lethal dose than the power range of the doses used for medical purposes.

Registration of over-accelerated electrons in a high-current picoseconds accelerator

A high-voltage (U = 280 keV) high-current (I = 5 kA) electron accelerator with a beam duration of 200 ps was investigated. The luminescent method showed that the beam diameter in the uniform pinching mode is about a micrometer, therewith the beam contains electrons with an energy exceeding 400 keV.

A high-current nanosecond electron accelerator MIR-M for biomedical research

A high-current electron accelerator MIR-M for biomedical studies of the reaction of healthy and tumor tissues, as well as cell cultures, exposed to X-ray radiation with high dose rate – up to 100 MGy/s at absorbed doses of 0.5 to 20 Gy in the single pulse, was developed and constructed. The main parameters of the accelerator are as follows: accelerating voltage – up to 800 kV, peak current – up to 50 kA, e-beam pulse duration – ≈80 ns. The design and parameters of the main systems of the accelerator together with the methods of measurements and first results of a comparative analysis of the effects of irradiation in vivo and in vitro using therapeutic devices and accelerator MIR-M are presented and discussed.

Magnetic Sweep of a Relativistic Electron Beam into a Rectangular Raster

The design of a magnetic sweep unit that provides a combination of two linear (with respect to time) sweeps of the relativistic electron beam and the radiation field of the accelerated electron beam swept into a rectangular raster and scattered by an air layer when approaching the target have been considered. Expressions for the distribution of absorbed dose over the whole exposed surface have been obtained taking into account the real distribution of beam current density. The considered expressions for the distribution function of absorbed dose can be used as a foundation for the calculations of magnetic sweep units for radiochemical equipment with high-current industrial electron accelerators, in particular, with escape of a focused beam of accelerated electrons into the atmosphere.