Current Pulsed Beam Research Articles

Design improvements to the SNS ion source and diagnostics

The U.S. Spallation Neutron Source (SNS) is a state-of-the-art neutron scattering facility delivering the world’s most intense pulsed-neutron beams to a wide array of instruments which are used to conduct investigations in many fields of science and engineering. The accelerator system is fed by an RF-driven, multicusp, H− ion source which nominally provides pulsed beam currents of 50-60 mA (1ms, 60Hz). This report provides a discussion of ongoing design improvements to the SNS ion source and Low Energy Beam Transport (LEBT) as well as diagnostic upgrades undertaken since the previous ICIS conference. These improvements include (i) simple mechanical modifications to the source outlet aperture which resulted in dramatically increased extracted beam current and comparable or lower emittance at similar beam currents, (ii) design improvements to the LEBT chopper target which will enable full power beam-dumping during physics studies, (iii) refinement of the SNS Allison emittance scanner that has enabled the first reliable LEBT beam measurements at full beam power (65kV, 50-100mA, 1ms, 60Hz) on the SNS ion source test stand and (iv) the implementation of a thermal imaging camera for the monitoring the LEBT electrode temperatures. (v) The design of an advanced Cs system, capable of more efficient Cs utilization with significantly lower Cs losses from the source is also presented. Mechanical details, computational simulations and experimental results are discussed within the context of these improvements.

Low intensity beam current measurement of the Associated Proton Beam Line at CSNS

The Associated Proton beam Experiment Platform (APEP) beamline is the first proton irradiation facility to use naturally-stripped protons which come from H-beams interacting with the residual gas in the linac beampipe at CSNS. The stripped beam current, which is in the order of 0.1% of the original H-beam and approximately 10 μA, should be measured precisely to provide the proton number for irradiation experiments. Therefore, a low-intensity beam current measurement system was developed with considerations to eliminate the external interferences. An anti-interference design is adopted in this system with an elaboration of probes, cables and electronic low-noise technology to minimize the impact of environmental noise and interferences. This improves the signal-to-noise ratio and enables a more precise measurement of the microampere level pulsed beam current. The system was installed and tested during the summer maintenance in 2021 and 2022. It shows a good agreement with the measurement of the Faraday cup.

Beam-generated plasma formation near a dielectric target irradiated by a pulsed electron beam in the forevacuum pressure range

We have investigated the formation of electron beam-generated (EBG) plasma near a dielectric (ceramic) target and an insulated metal target, and the compensation of the negative charge accumulated on the insulated metal target when the targets are irradiated by an intense pulsed electron beam in the forevacuum pressure range (4–15 Pa). It is shown that the density of the EBG plasma near the irradiated ceramic target (or the irradiated insulated metal target) is greater than the plasma density for a beam propagating freely in a vacuum chamber (or for the grounded metal target). The EBG plasma near the target is formed with a certain delay with respect to the electron beam current pulse, because of which the negative potential of the insulated target is also compensated by a delay. The delay time in the formation of the EBG plasma and in the compensation of the target negative potential decreases with increasing gas pressure. Expressions have been proposed for estimating this delay time.

Caesium balance of the ISIS H- Penning ion source in long pulse operation

We have developed a semi-empirical model predicting the equilibrium caesium coverage (in monolayers) and the resulting work function of the ISIS Penning ion source cathode (in eV) in long pulse operation. We use the caesium balance model to predict the temporal structure of the extracted H- beam current pulses and compare the model predictions to experimental data with 60 mA, 2 ms beam pulses at 50 Hz. The experimentally observed droop of the beam current is reproduced by the model. The results imply that extracting square beam pulses in the long pulse operation mode requires more efficient cathode cooling. We apply the model for 70 mA, 3 ms beam pulses at 14 Hz, relevant for ESSνSB project. We conclude that achieving these parameters requires reducing the cathode temperature by 200 °C, and ramping the discharge current to cancel the discharge voltage droop.

Influence of electron beam generation on the parameters and emission characteristics of a constricted arc discharge in a pulsed forevacuum plasma-cathode electron source

We have explored the influence of accelerating voltage, gas pressure, and accelerating gap length on the discharge parameters and emission characteristics of the plasma of a constricted arc discharge which forms the emission plasma in a forevacuum-pressure, plasma-cathode, broad-beam electron source. We find that the back-streaming ion flux from beam plasma, formed by electron beam in the accelerating gap and beam propagation region, leads to increase in the maximum current and pulse duration of the constricted arc. The rise-time of the emission current pulse (and electron beam current pulse) depends not only on the formation of the emission plasma but also on changes in arc current flow conditions in the constricting channel, which occur during the pulse duration due to the back-streaming ion flux. There is a significant influence of accelerating gap length on the emission characteristics. Varying the accelerating gap length provides control of the delay time and rise time of the emission current pulse (e-beam current pulse), as well as the electron emission efficiency.

Широтная и амплитудная модуляция тока пучка для управления его мощностью в течение импульса субмиллисекундной длительности

Methods for controlling the power electron beam in a source based on a low-pressure arc discharge with layer stabilization of the emission plasma boundary are described. The control is carried out by the method of amplitude and latitude modulation within the duration of the beam current pulse with a time resolution of 10 μs at a submillisecond pulse duration. Two ways to control the power of the electron beam are implemented: the first one is based on changing the concentration of the emission plasma by modulating the arc discharge current, the second method uses grid control (the mode of operation of the plasma triode) at the constant discharge current. Simplified diagrams of the power supply sources of the discharge and intergrid control voltage, typical oscillograms of the main currents of the discharge system, a graph of a quick-acting power change, and a control characteristic of a plasma triode are presented.

First demonstration of the FLASH effect with ultrahigh dose rate high-energy X-rays

PurposeThis study aimed to evaluate whether high-energy X-rays (HEXs) of the PARTER (platform for advanced radiotherapy research) platform built on CTFEL (Chengdu THz Free Electron Laser facility) can produce ultrahigh dose rate (FLASH) X-rays and trigger the FLASH effect. Materials and methodsEBT3 radiochromic film and fast current transformer (FCT) devices were used to measure absolute dose and pulsed beam current of HEXs. Subcutaneous tumor-bearing mice and healthy mice were treated with sham, FLASH, and conventional dose rate radiotherapy (CONV), respectively to observe the tumor control efficiency and normal tissue damage. ResultsThe maximum dose rate of HEXs of PARTER was up to over 1000 Gy/s. Tumor-bearing mice experiment showed a good result on tumor control (p < 0.0001) and significant difference in survival curves (p < 0.005) among the three groups. In the thorax-irradiated healthy mice experiment, there was a significant difference (p = 0.038) in survival among the three groups, with the risk of death decreased by 81% in the FLASH group compared to that in the CONV group. The survival time of healthy mice irradiated in the abdomen in the FLASH group was undoubtedly higher (62.5% of mice were still alive when we stopped observation) than that in the CONV group (7 days). ConclusionThis study confirmed that HEXs of the PARTER system can produce ultrahigh dose rate X-rays and trigger a FLASH effect, which provides a basis for future scientific research and clinical application of HEX in FLASH radiotherapy.

Energy density distribution of a modulated electron beam in a source with a plasma cathode based on a low pressure arc

The article presents the results of studies devoted to the study of the energy density distribution in the amplitude-modulated regime of electron beam generation. It is shown that in the first ≈ 50 μs of the duration of the beam current pulse, its spatial rearrangement occurs, due to the development of the arc discharge current. Thus, the rearrangement of the arc current, which develops from the axis of the system, leads to an axial diving of the emission current density and the beam current density on the target. With the development of the arc current, the energy density on the target on the axis of the system decreases and after ≈ 50 μs takes on a steady-state value, which can change only as a result of a change in the conditions for generating an electron beam or the transition to a modulated regime of electron beam generation. It has been experimentally shown using calorimetric measurements that the shape of the electron beam current pulse with its amplitude modulation with a pulse duration of more than 100 μs has little effect on the distribution of the beam energy density in the target region.

Time behavior of an electron beam current pulse in the axial and peripheral zones of an anode in vacuum and gas-filled diodes

The study of the time behavior of a current pulse of an electron beam generated during a high-voltage nanosecond discharge in gas-filled and vacuum diodes has been carried out. As follows from the experimental results, in both cases, the distribution of the beam current density in the plane of a grounded anode is non-uniform. The highest beam current density is recorded in the axial part of the anode. It was established that in the case of a gas-filled diode, ~ 2 ns after the onset of the beam current pulse, its shape in the axial anode zone changes relative to that in the peripheral one. It is assumed that the most probable reason for this is the effect of compensation of the charge of the beam electrons by the positive charge of ions arising in the ionization process in the paraxial zone.

Modes of runaway electron beams during formation of diffuse discharges in air and nitrogen

Investigations of the generation of runaway electron beams (REs) and the for-mation of diffuse discharges during breakdown of gaps with a cathode, which has a small radius of curvature, have been carried out. In air and nitrogen at elevated pressure, based on the registration and analysis of the characteristics of radiation from discharge, as well as parameters of the RE beam current and dynamic dis-placement current, it is shown that, depending on the conditions (E/N, gas type and its pressure, design and material of the cathode, amplitude and front of the voltage pulse), diffe-&#x0D; rent modes of generation of runaway electron beams are realized. It was found that the ratio of the velocity of the front of the ionization wave (streamer) and the runaway electrons, as well as the design of the cathode and the delay time before the explosion of cathode microinhomogeneities, significantly affect the generation of runaway electrons. The conditions for the realization of various modes are de-termined; oscillograms of the beam current pulses and photographs of the glow of the gap are presented.

Compact S -band linear accelerator system for ultrafast, ultrahigh dose-rate radiotherapy

Radiation therapy is currently the most utilized technique for the treatment of tumors by means of ionizing radiation, such as electrons, protons and x/gamma rays, depending on the type, size and depth of the cancer mass. Radiation therapy has in general fulfilled the main requirement of targeting thus damaging the malignant cells and sparing the healthy tissues as best as possible. In this scenario, electron linear accelerators have been operated as viable tools for the delivery of both high-energetic electrons and x-ray beams, which are obtained via the bremsstrahlung process of the electrons hitting on a high-Z material. Recently, it has been experimentally demonstrated that ultrahigh dose-rate bursts of electrons and x-ray beams increase the differential response between healthy and tumor tissues. This beneficial response is referred to as the FLASH effect. For this purpose, we have developed the first dedicated compact $S$-band linear accelerator for FLASH radiotherapy. This linac is optimized for a nominal energy of 7 MeV and a pulsed electron beam current of 100 mA and above. The accelerator is mounted on a remote-controlled system for preclinical research studies in the FLASH regime. We will show the rf and beam dynamics design of the $S$-band linac as well as the commissioning and high-power rf tests. Furthermore, the results of the dosimetric measurements will be illustrated.

Generation of a Pulsed Electron Beam by a Forevacuum Plasma-Cathode Source Based on a Constricted Arc Discharge

We describe the features of electron emission from the plasma formed by a constricted arc discharge, and the generation of a pulsed (100–150 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> ) low-energy (up to 10 keV) electron beam by a plasma-cathode electron source based on this type of arc in the forevacuum pressure range (5–20 Pa). The electron beam current pulse is delayed relative to the arc discharge current pulse. Limitation of the discharge current to the cylindrical part of the anode, increasing the gas pressure, and increasing the beam accelerating voltage all serve to reduce this delay time and provide a sharper rise time of the electron beam current pulse. The emission of electrons from the plasma formed by the constricted arc discharge is accompanied by a decrease in arc voltage and a corresponding increase in arc current. These characteristics are due to the specifics of operating the constricted arc discharge in the forevacuum pressure range and also the influence of backstreaming ions penetrating into the discharge system from the downstream electron beam region.

Experimental plasma maser as a broadband noise amplifier. II. Short pulse

This paper presents an experimental plasma maser driven by a 2-ns long, high-current electron beam with a typical particles energy of 270 ± 10 keV, a pulsed power of 450 ± 30 MW, and a total energy of 0.85 ± 0.03 J. Tunable plasma characteristics define variations in the spectral maxima of excited high-power microwaves in the range from 3 to 25 GHz. The short beam current pulse has provided the device operation in the mode of a noise amplification with the energy efficiency of 26% ± 3%, mean microwave power over the beam current pulse of 117 ± 10 MW, and an instant (peak) power of up to 430 ± 30 MW.

Cherenkov radiation and cathodoluminescence in sapphire, quartz, and diamond under the excitation of an electron beam

The paper reports data on the emission of sapphire, KU-1 quartz, and synthetic diamond in the wavelength range of 200–800 nm excited by a pulsed electron beam with an energy of up to 350 keV. Broadband radiation, the intensity of which increases with decreasing wavelength in the range of 220–400 nm, was detected. Such dependence of the radiation intensity on the wavelength is characteristic of Cherenkov radiation (CR). Moreover, the duration of the radiation pulse coincides with that of the electron beam current pulse. For sapphire and quartz, the duration of radiation pulses at the spectral range 220–400 nm is close to the duration to electron beam current pulses. For synthetic diamonds, both cathodoluminescence and CR are detected in this spectral region. The intense band of free excitons with maximum at 235 nm is found at high beam current densities and electron energies 50–350 keV.

On the influence of a cathode shape on the parameters of current pulses of runaway electron beams in a gas discharge when applying voltage pulses with a rise time of 200 ns

The influence of the shape of a cathode on the duration and amplitude of a supershort avalanches electron beam (SAEB) current pulses was experimentally studied. Three cathodes were used in experiments: tube, needle, and ball. Voltage pulses with a rise time of 200 ns were applied across an 8.5 mm gap filled with air and nitrogen at various pressures (12.5–100 kPa). It was found that at an air pressure of 12.5 kPa and a rise time of voltage pulses 200 ns, the largest amplitudes of SAEB current pulses (90 mA) were observed with the tubular cathode. The SAEB current pulse duration was τ0.5 ~ 130 ps. Two pulses with different duration, amplitude and energy of electrons were observed with the needle cathode at same pressure. When using the ball cathode, the duration and amplitude of the beam current pulses were τ0.5 ~ 250 ps and ~ 4 mA, respectively.

On the Influence of Electron Energy on Characteristics of the Cherenkov Radiation and Cathodoluminescence

A review of the results of investigations of the Cherenkov radiation (CR) and pulsed cathodoluminescence during irradiation of different crystals and polymethylmethacrylate (PMMA) with subnanosecond and nanosecond beams is made. The calculations demonstrating the influence of the electron energy and refractive index of a material on the CR intensity and spectrum, as well as its spatial characteristics are performed. The experimental data on the CR observation at the electron energy E up to 400 keV are presented. It is shown that an increase in E allows detecting the CR in diamond, leucosapphire, and quartz KU-1 crystals using a standard spectrometer. It is found out that is more difficult to register the CR in PMMA because of the radiation absorption in the region of wavelengths shorter than 300–350 nm and an internal breakdown of the specimens due to the accumulation of electrons in them at high beam current densities or (and) pulse durations, even with a monochromator and a PMT. Some recommendations are given for designing detectors of runaway electrons in tokomak-type facilities.

Cumulation of a High-Current Electron Beam During a Nanosecond High-Voltage Discharge in a Low-Pressure Diode

The results of an experimental investigation of the effect of cumulation of a beam of runaway electrons formed in a high-voltage nanosecond discharge at a reduced air pressure are presented. The optimal conditions of this effect in a discharge gap in a tubular cathode – grounded planar anode geometry were achieved at an air pressure of ≈5 Pa and an interelectrode gap of 2.75 mm. An electron-beam current pulse is recorded with a high time resolution (up to about 80 ps) behind the flat foil anode. It is found out that due to this effect a through hole is formed in a 20 μm-thick aluminum foil after 2–3 discharge pulses. The results obtained suggest that the electron energy in the second part of the beam current pulse is lower than that in its first part.

Filamentation and Self-Focusing of Electron Beams in Vacuum and Gas Diodes

In this paper, we experimentally studied pulsed electron beams with a high local density. The conditions in which the energy density cumulation is observed during the interaction of electrons with the anode are shown to develop in vacuum and gas diodes at nanosecond and subnanosecond durations of a beam current pulse and a decrease in the interelectrode gap. The average electron energy in filamentation and self-focusing of an electron beam in a vacuum diode of an accelerator at a current of ~2 kA and a no-load voltage of ~400 kV was established to be 50–100 keV while the energy density was 109–1010 J/cm3. It is confirmed that the beam current density in a gas diode can exceed 1 kA/cm2. It is hypothesized that superdense electron beams in vacuum and gas diodes are formed as a result of avalanche multiplication of runaway electrons in the cathode–anode gap plasma.

Разработка источника питания управляющего электрода технологической электронной пушки

The article deals with developing the model of a power supply voltage modulator for the electron-beam gun control electrode within the framework of the project aimed at elaborating scientific and technical fundamentals of controlling the electron beam additive form shaping process. Generally, this technology involves the use of high-power electron beams for sequentially fusing materials in the form of a metal wire. The electron beam electric current is controlled by applying the beam impulse on the fused metal wire. Despite the fact that active efforts are being taken by Russian and foreign companies for developing the technology, clear criteria for choosing the electron beam pulsed modulation parameters have not been determined and depend on the material being processed. The current technical solutions used for energizing the gun’s electron beam and controlling its power are briefly described. The electron beam current pulse modulation system is analyzed with a view to determine the electric voltage modulator optimum parameters and operating modes. A modulator model has been developed and proposed, using which it is possible to evaluate the impulse voltage parameters, to estimate the transients at the front and trailing edges of the modulator output voltage pulses with different duty cycles, and to select the proper components. The electrical circuit of the power supply voltage modulator for the electron-beam gun control electrode was simulated using the NI Multisim 11.0 simulation package.

Филаментация и самофокусировка электронных пучков в вакуумных и газовых диодах

AbstractIn this paper, we experimentally studied pulsed electron beams with a high local density. The conditions in which the energy density cumulation is observed during the interaction of electrons with the anode are shown to develop in vacuum and gas diodes at nanosecond and subnanosecond durations of a beam current pulse and a decrease in the interelectrode gap. The average electron energy in filamentation and self-focusing of an electron beam in a vacuum diode of an accelerator at a current of ~2 kA and a no-load voltage of ~400 kV was established to be 50–100 keV while the energy density was 10^9–10^10 J/cm^3. It is confirmed that the beam current density in a gas diode can exceed 1 kA/cm^2. It is hypothesized that superdense electron beams in vacuum and gas diodes are formed as a result of avalanche multiplication of runaway electrons in the cathode–anode gap plasma.