Intense Electron Beam Accelerator Research Articles

强流电子束加速器运行过程电磁辐射的实验研究

强流电子束加速器运行过程电磁辐射的实验研究

Improving the output voltage waveform of an intense electron-beam accelerator based on helical type Blumlein pulse forming line

The Blumlein pulse forming line (BPFL) consisting of an inner coaxial pulse forming line (PFL) and an outer coaxial PFL is widely used in the field of pulsed power, especially for intense electron-beam accelerators (IEBA). The output voltage waveform determines the quality and characteristics of the output beam current of the IEBA. Comparing with the conventional BPFL, an IEBA based on a helical type BPFL can increase the duration of the output voltage in the same geometrical volume. However, for the helical type BPFL, the voltage waveform on a matched load may be distorted which influences the electron-beam quality. In this paper, an IEBA based on helical type BPFL is studied theoretically. Based on telegrapher equations of the BPFL, a formula for the output voltage of IEBA is obtained when the transition section is taken into account, where the transition section is between the middle cylinder of BPFL and the load. From the theoretical analysis, it is found that the wave impedance and transit time of the transition section influence considerably the main pulse voltage waveform at the load, a step is formed in front of the main pulse, and a sharp spike is also formed at the end of the main pulse. In order to get a well-shaped square waveform at the load and to improve the electron-beam quality of such an accelerator, the wave impedance of the transition section should be equal to that of the inner PFL of helical type BPFL and the transit time of the transition section should be designed as short as possible. Experiments performed on an IEBA with the helical type BPFL show reasonable agreement with theoretical analysis.

A Compact 100-PPS High-Voltage Trigger Pulse Generator

Innovative design of a compact 100-pulses-per-second high-voltage trigger pulse generator, based on an incorporated structure with a curled parallel-plate capacitor and a spiral-strip pulse transformer, is presented. The repetitive trigger pulse generator was developed to trigger a trigatron, which has been used to pulse a 500-kV intense electron beam accelerator (IEBA). The trigger pulse generator consists of a spiral-strip transformer, a curled parallel-plate capacitor, two sharpening spark gaps, a repetitive controlling module, and a charging module. The generator system was designed based on the circuit theory and the electromagnetic theory. Experimental results show that the trigger pulse generator can deliver a 100-kV pulse with 80-ns rise time and 200-ns duration of full-width at half-maximum at a maximum repetition rate of 100 Hz. Due to its good stability, compact structure, and low cost, the high-voltage trigger pulse generator is an excellent candidate to trigger the IEBA system.

Research of a High-Current Repetitive Triggered Spark-Gap Switch and its Application

This paper discusses the development and testing of a low-jitter, high-voltage, high-current, and triggered spark-gap switch, designed as part of an intense electron-beam accelerator (IEBA). The voltage/pressure (V/p) curve of the switch was measured for N2 when the gas pressure is 0.10-0.25 MPa with the main gap spacing of 10 mm. In addition, the operating ranges, delay time, and jitter of the switch were measured during the single trigger experiment when the gas pressures were 0.10, 0.15, 0.20, and 0.25 MPa and the trigger voltage was 80 kV. The experimental results showed that the operating ranges of the switch were 25%-94%, 38%-86%, 40%-88%, and 44%-88% of the switch's self-breakdown voltage, and these results also identified a suitable operating condition for the application of the switch on the IEBA. When the gas pressure was 0.25 MPa, the operating voltage was 40 kV, which was about 88% of the self-breakdown voltage, the delay time was 50 ns, and jitter was 1.28 ns. At last, the repetition rate of the switch was measured when the switch was used as the trigger switch of the IEBA. The pulse repetition rate (PRR) is 10 Hz at the operating voltage of 20 kV and peak currents of 40 kA. The PRR is 5 Hz at the operating voltage of up to 40 kV and the peak currents of up to 80 kA. The switch works steadily and reliably.

Effect of a transition section between the Blumlein line and a load on the output voltage of gigawatt intense electron-beam accelerators

The flat voltage of the main pulse on the load of intense electron-beam accelerators (IEBA) is one of the important factors which affect the quality and characteristic of the output beam current of IEBA. In this paper, the effect of the transition section between the Blumlein pulse forming line (BPFL) and load on the output voltage of IEBA is analyzed in theory. Based on the telegraph equations of the BPFL, a formula which is used to calculate the output voltage of IEBA is obtained, and the factor which affects the output voltage is analyzed. It is found from the theoretical analysis that the wave impedance and transmission time of the transition section influence considerably the fluctuation of the main pulse voltage waveform at the load. In order to get a well-shaped square waveform at load and to improve the electron-beam quality of such an accelerator, there should be a proper wave impedance of the transition section, and the transmission time of the transition section should be designed as short as possible. At last, some experiments are performed on IEBA, and the experimental results show reasonable agreement with theoretical analysis.

Effect of the change in the load resistance on the high voltage pulse transformer of the intense electron-beam accelerators

A high voltage pulse transformer (HVPT) is usually used as a charging device for the pulse forming line (PFL) of intense electron-beam accelerators (IEBAs). Insulation of the HVPT is one of the important factors that restrict the development of the HVPT. Until now, considerable effort has been focused on minimizing high field regions to avoid insulation breakdown between windings. Characteristics of the HVPT have been widely discussed to achieve these goals, but the effects of the PFL and load resistance on HVPT are usually neglected. In this paper, a HVPT is used as a charging device for the PFL of an IEBA and the effect of the change in the load resistance on the HVPT of the IEBA is presented. When the load resistance does not match the wave impedance of the PFL, a high-frequency bipolar oscillating voltage will occur, and the amplitude of the oscillating voltage will increase with the decrease in the load resistance. The load resistance approximates to zero and the amplitude of the oscillating voltage is much higher. This makes it easier for surface flashover along the insulation materials to form and decrease the lifetime of the HVPT.

Development of a Repetitive Wave Erection Marx Generator

A repetitive ten-stage wave erection Marx generator is developed to investigate the electrical characteristics of such compact devices and potentially provide an economical approach to realize the miniaturization of intense electron beam accelerators. Compact design has been made for the generator in order to achieve a proper stray capacitance of the spark gap electrode with respect to the ground in each stage because these proper grounded stray capacitances are critical for obtaining a good wave erection process. This generator is initially resistively isolated for single-shot tests and then changed to inductively isolated for repetitive operation. In single-shot experiments, the generator is tested to be able to deliver a high-voltage pulse of 210 kV and a rise time of about 5 ns on a 90-Omega dummy load at a charging voltage of 40 kV. This result agrees basically with that of the PSpice circuit simulation, which adopts a self-breakdown spark gap model. The preliminary experimental results of repetitive operation show that at a charging voltage of 30 kV, the generator can operate at 8.5 Hz without gas blow-off from the internal spark gaps, producing an output pulse of 150 kV and a rise time of less than 20 ns. Differences in the output pulse waveforms between resistively and inductively isolated configurations are analyzed.

Electrical explosion process and amorphous structure of carbon fibers under high-density current pulse igniting intense electron-beam accelerator

AbstractThe development of pulsed power technology, particularly for inductive energy storage, promotes the extensive discussions of electrical explosion process in high energy density. This paper presents the electrical-explosion behavior of carbon fibers subjected to about 20 kA, ~5 µs high-density current pulse igniting an intense electron beam accelerator. After electrical explosion, and surface rupture, submicron particles, fibrillar and strip-shaped structures were observed, experimentally supporting the microstructure model (skin-core heterogeneity) of carbon fiber. Interestingly, the start and turn-off of the current were followed by radiation pulses with different intensities. It was found that the radiation was focused on the explosion stage which was characterized by an oscillating current. The instabilities of plasma produced during the explosion process play an important role in the microstructure changes of carbon fibers and the radiation generation.

Effect of transition section between the main switch and middle cylinder of Blumlein pulse forming line on the diode voltage of intense electron-beam accelerators

AbstractThe rise-time of the pulse on the diode of the intense electron-beam accelerators (IEBA) is one of the important factors that affect the quality and characteristic of the output beam current of the IEBA. In this paper, effect of the transition section between the main switch and middle cylinder of strip spiral Blumlein line (SSBL) on the diode voltage of IEBA is analyzed in theory, based on the theoretical analysis of the wave propagation along the Blumlein pulse forming line (BPFL), the traveling time of the transition section has a great effect on the rise-time, the plateau-time, and the fall-time of the output voltage at the matching load. Furthermore, the operation of the whole accelerator consisting of the primary energy-storage capacitor, the Tesla transformer, a main switch, the transition section, BPFL, and a resistive load was simulated using a circuit-simulation code called PSpice, and the dependence of the output voltage on the inductance of the main switch and the transition section was obtained. It was found from the wave propagation theory and the circuit simulation using computer results that the wave traveling time of the transition section between the main switch and the middle cylinder of the SSBL influences considerably the rise-time, plateau-time, and fall-time of the voltage waveform at the matching load. In order to get an ideal square pulse voltage waveform at the matching load and to improve the electron beam quality of such an accelerator, the wave traveling time of the transition section should be designed as soon as possible. At last, a couple of contrastive experiments are performed on two kinds of IEBA. The experimental results agree with the theoretical analysis and simulated results.

Effects of diode current on high power microwave generation in a vircator

AbstractAn experiment of a virtual cathode oscillator (vircator) built on the low impedance intense electron beam accelerator Flash II is reported. A novel spectrum diagnosis method—a circulating dispersion line—is proposed. A thin oil layer coated graphite cathode is introduced in the experiment to decrease the delay time of the explosive emission process and obtain a homogeneous electron beam emission for improving the high-power microwave (HPM) generation efficiency. The effect of diode current on HPM generation in the vircator system is discussed. The HPM pulse width has a strong connection with the diode current waveform. For most shots, corresponding to the time that microwave emission starts, there is an inflection point in the diode current pulse. Compared with the case that no microwave is generated, the diode current increases more slowly following the inflection point. HPM generation terminates when the beam current reaches the self-pinching critical current of the diode.

Study on strip spiral Blumlein line for the pulsed forming line of intense electron-beam accelerators

AbstractThe pulse forming line (PFL) is the key part of the intense electron-beam accelerators (IEBA), which determines the quality and characteristic of the output beam current of the IEBA. Compared with the accelerator with traditional Blumlein line, an IEBA based on strip spiral Blumlein line (SSBL) can increase the duration of the output pulse in the same geometrical dimension. But the disadvantage of the SSBL is that the output voltage waveform at the matched load may be distorted, which influences the electron beam quality. In this paper, according to the electromagnetic theory, formulas for calculating the main electric parameters of SSBL (inductance, capacitance, transmission time, and characteristic impedance) are deduced. The effect of the geometric parameters of SSBL on the slowing coefficient is analyzed. The designed condition of SSBL for the output ideal voltage pulse in the matched load is obtained by theoretical analysis. Furthermore, the Karat code is used to simulate the output voltage waveform of SSBL on the matched load for different spiral angels. At last, a couple of contrastive experiments are performed on an electron-beam accelerator based on the SSBL with water dielectric. The experimental results agree with the theoretical and simulated results.

Coaxial Capacitive Dividers for High-Voltage Pulse Measurements in Intense Electron Beam Accelerator With Water Pulse-Forming Line

In this paper, a new coaxial capacitive divider is investigated. The electrical characteristics of the capacitive divider are theoretically analyzed, and the parameters of the capacitive divider are calculated and measured. Its rise time is about 8 ns, and the divider ratio is over 2000. The divider is employed to measure the pulsed high voltages of an intense electron beam accelerator with a water pulse-forming line (PFL) in our laboratory. The capacitive divider can directly measure the diode voltage within nanoseconds, and when combined with an integrator, it can measure the PFL charging voltage with a duration of several microseconds. Compared with conventional resistance dividers, the capacitive divider has more advantages, such as compactness, stability, a relatively high divider ratio, a fast response time, and not much of an effect on the accelerator.

Mode competition in Bragg resonator cyclotron resonance maser experiments driven by a microsecond, intense electron beam accelerator

Abstract We report an experimental study on mode competition in a high-g Bragg resonator cyclotron resonance maser (CRM) employing a microsecond, relativistic electron beam. The high quality annular electron beam of 20-80 A was produced through an apertured mask-anode. The high-Q Bragg resonator was designed to excite the TE31, mode CARM oscillation at 18-9GHz. Two configurations of Bragg resonators were used; with ripples half inward and with ripples fully outward with respect to a smooth tubular centre section. Bragg resonator microwave emission was compared to radiation emitted when the electron beam interacted with smooth tubes with the same diameter as the Bragg resonator centre sections. In the present Bragg resonator CRM experiments, three parasitic oscillations were the most serious competing modes, apparently suppressing the TE31 CARM oscillation: TE21, absolute instability, TE11 gyro-BWO, second and third harmonic TE51, mode. When a high current annular beam of 150-200 A was injected into the Br...

Particle beam fusion research at IAE in Beijing

The research of particle beam fusion and its related problems at The Institute of Atomic Energy in Beijing is evolving along the following five directions: 1. Pulsed power technology. An 80 GW intense electron beam accelerator has been built. Switch research is ongoing. 2. Diode research. The experimental research and theoretical simulation of electron pinch in the diode have been carried out. The pinch process has been investigated by measuring the area collapsing velocity of the pinching electron ring on the anode surface. Expanding velocities of the cathode and anode plasmas have been observed. The diode with large area cathode of 38 × 5 cm2 can produce a 46 kA electron beam current with a beam cross-section of 36 × 4 cm2 and good uniformity for pumping of a KrF laser. 3. Energy deposition of the electron beam on the targets has been studied by means of measurements of the intensity of soft X-rays, the energy spectrum of blow-off ions, the visible light spectra and the rear surface velocity of the target. These experiments show that the results are in agreement with the classical interaction mechanism for high Z targets, but it is several times higher than the classical result for low Z targets. 4. Electron beam propagation in neutral gases with various pressures space-and current-neutralization processes have been investigated. 5. The production of a KrF laser pumped by an electron beam. A laser beam with an energy of 13 J and pulse duration of 70 ns has been obtained.

Measurement and Interpretation of Electron Angle at MABE Beam Stop

This analysis shows that radiation measurements combined with a sophisticated simulation provides a simple but powerful tool for estimating beam temperature in intense pulsed annular electron-beam accelerators. Specifically, the mean angle of incidence of a 60 kA, 7 MeV annular electron-beam at the beam stop of the MABE accelerator and the transverse beam temperature are determined. The angle is extracted by comparing dose profiles measured downstream of the stop with that expected from a simulation of the electron/photon transport in the stop. By calculating and removing the effect on the trajectories due to the change in electric field near the stop, the beam temperature is determined. Such measurements help give insight to beam generation and propagation within the accelerator.

Radio-Frequency Accelerators for Multi-Kiloampere Electron Beams

Most present work on intense pulsed electron beam accelerators is aimed toward linear induction accelerators. In this paper the authors consider the application of resonant accelerators to the generation of pulsed multi-kiloampere electron beams. The main relative advantages are that low cost and high voltage gradient can be achieved by eliminating ferromagnetic isolation cores; that cavity designs are simpler (with optimum utilization of vacuum insulators); and that there are no high voltage shorting switches. The last feature implies that RF accelerators may operate at higher repetition rate and duty cycle. The authors discuss RF cavities filled with water. The high attainable dielectric constant raises the stored energy density by a factor greater than 60 for enhanced pulsed operation. The lower impedance of the cavity also permits CW operation in the 10-50 A level. The discussion treats, in order, the functions a water-filled cavity can perform and the configuration of a high current RF accelerator; the power losses in the cavity walls and dielectric medium; the effects of water heating and cavity distortion on the resonant frequency; the baseline parameters for a 750 kV cavity; and, in the last section, the problem of beam loading in CW operation.

A Beam Current Monitor for Intense Electron Beams

The use of interceptive probes for measuring the current density profile of electron beams is a common technique in accelerators producing low current beams. Severe requirements are imposed on interceptive devices when current densities of the order of kiloamperes per square centimeter are encountered. In this case, space charge effects, large secondary electron production, and probe heating are important considerations. This paper describes a simple and fast (pulse rise ca. 0.4 ns) interceptive ''electric'' probe which can be used in repetitively pulsed intense electron beam accelerators with high current densities. The device is sensitive only to the beam charge present within the probe volume. To accomplish this, the probe uses a ''built in'' electrostatic shield. The device consists of two parts: a sensor tip made of graphite and a stainless steel support/positioning tube. The entire structure is designed to approximate a ''capped off'' coaxial transmission line or, in other words, an open-ended but shielded line. The outer conductor constitutes the electrostatic shield. The theory behind the probe is described as well as its performance testing at the Lawrence Livermore Laboratory Experimental Test Accelerator. By mechanically scanning this probe through the beam as a function of radius it was found thatmore » the beam current density could be determined.« less