Beam Pinching Research Articles

Methods of calculating radial collapse velocity of short-<i>γ</i> diode field on Qiangguang-I accelerator

Electron beam pinching is a common physical phenomenon in the working process of high-current electron beam diodes. The radial collapse velocity (<i>V</i><sub>a</sub>) of the beam is an important index to determine the beam pinching and the working characteristics of the diode. The current research methods are based on optical diagnosis and theoretical estimation formulas for a specific diode. The radial collapse velocity of Qiangguang-I accelerator’s tight-pinched short <i>γ</i> diode can be obtained by the following three methods in this paper: 1) a theoretical formula, which is used to calculate the radial collapse velocity on the basis of the existing research results, and can very quickly determine the pinching situation because in this case this formula just needs a diode pinching current; 2) the method of calculating <i>V</i><sub>a</sub>, which is established based on particle-in-cell simulation. The simulation model includes the anode ion current, thus can simulate the pinching of electron beam more precisely; 3) a method of calculating <i>V</i><sub>a</sub>, which is given by measuring the pinch center offset and the γ-ray PIN waveform, because the Qiangguang-I γ diode is inconvenient for optical diagnosis. The radial collapse velocities obtained by the above three methods are 8.43, 8.70 and 7.89 cm<sup>2</sup>/ns respectively, and the relative difference among the three methods is < 10%. The third method obtains a slightly smaller value because the ion current assumed in the theory and simulation is H<sup>+</sup>. The ion current composition in the actual diode is complex, the diffusion speed is slower, then the radial collapse velocity is smaller. Compared with the typical <i>V</i><sub>a</sub> value (2–4 cm<sup>2</sup>/ns) of the Gamble II accelerator diode given by the Blaugrund team, the <i>V</i><sub>a</sub> value of the short γ diode of the Qiangguang-I accelerator is nearly doubled. The diode on Qiangguang-I, which works after a plasma opening switch (POS), has a very short rising time (less than 10 ns), and pinches quickly. In contrast, the rising time of the Gamble II accelerator diode is about 40 ns, which is different from the working status of the Qiangguang-I diode. This paper provides a new way to study the radial collapse velocity of high-current diodes.

Design and experimental research on a self-magnetic pinch diode under MV

A self-magnetic pinch diode (SMPD) integrating an anode foil-reinforced electron beam pinch focus and a small high-dose x-ray spot output was designed and optimized. An x-ray focal spot measuring system was developed in accordance with the principle of pinhole imaging. The designed SMPD and the corresponding measuring system were tested under ∼MV, with 1.75 × 2 mm2 oval x-ray spots (AWE defined) and forward directed dose 1.6 rad at 1 m. Results confirmed that the anode foil can significantly strengthen the electron beam pinch focus, and the focal spot measuring system can collect clear focal spot images. This finding indicated that the principle and method are feasible.

Different operation regimes of cylindrical triode-type electron accelerator studied by PIC code simulations

AbstractThe performance of the converging electron beam generated in cylindrical triodes is systematically studied by particle-in-cell code simulations. Depending on the cathode and grid potentials applied, different operation regimes are identified. For low voltages between cathode and grid, laminar flow and homogeneous beam energy density at the target (anode) is obtained. This applies both to the case of unipolar electron flow and to bipolar flow with counter-streaming ions. Hereby, the electron emission current is enhanced by about 50% for bipolar flow compared with unipolar flow. A further increase by about 20% is obtained when electron backscattering at the target is enhanced due to a change of target material from aluminum to tungsten. For cathode-grid voltages exceeding a critical value, laminar flow is replaced by non-laminar flow regimes. For unipolar electron beams, a virtual cathode forms between grid and target, which leads to an inhomogeneous power density at the target. For the specific geometry investigated and the cathode potential fixed at −120 kV, the cathode-grid voltage threshold for the formation of the virtual cathode is ~32 kV for Al targets and ~28 kV for W targets. For bipolar flow, the laminar flow regime already ends at cathode-grid voltages of ~23 kV (Al target) and ~20 kV (W target), respectively, and is replaced by magnetic insulation at the beam edge. For increasing cathode-grid voltage, the magnetically insulated region extends until beam pinching occurs.

40级串联FLTD驱动杆箍缩二极管的箍缩特性模拟

40级串联FLTD驱动杆箍缩二极管的箍缩特性模拟

Fokker–Planck simulations for core heating in subignition cone-guiding fast ignition targets

On the basis of two-dimensional coupled relativistic Fokker–Planck and fluid simulations, the core heating by fast electron beam in subignition cone-guiding fast ignition targets is investigated. It was found that the magnetic field due to the thermoelectric current reduces the beam pinching due to the resistive magnetic field. Compared with the carbon-deuterium fuel used in the early phase of Fast Ignition Realization Experiment, phase-I, the core heating of deuterium-tritium (DT) fuel used in the later phase is less effective. One reason is the slower temperature relaxation between bulk electron and ion for the DT case, which is previously reported by Johzaki et al. [Phys. Plasmas 15, 062702 (2008)]. In addition, in the lower-Z DT case, the pinching effects by the resistive magnetic field are weaker due to smaller resistivity; the optical thickness for fast electron beam is smaller, which leads to further reduction in core heating efficiency. It was also found that the fast electron beam quality deteriorates during the transport in the only 10 μm thickness high-Z (e.g., Au and Cu) cone tip because of the collisional scattering and drag. As the result, the core heating efficiency considerably goes down compared to the case neglecting the transport in the cone tip.

Controlling charge and current neutralization of an ion beam pulse in a background plasma by application of a solenoidal magnetic field: Weak magnetic field limit

Propagation of an intense charged particle beam pulse through a background plasma is a common problem in astrophysics and plasma applications. The plasma can effectively neutralize the charge and current of the beam pulse, and thus provides a convenient medium for beam transport. The application of a small solenoidal magnetic field can drastically change the self-magnetic and self-electric fields of the beam pulse, thus allowing effective control of the beam transport through the background plasma. An analytic model is developed to describe the self-magnetic field of a finite-length ion beam pulse propagating in a cold background plasma in a solenoidal magnetic field. The analytic studies show that the solenoidal magnetic field starts to influence the self-electric and self-magnetic fields when ωce≳ωpeβb, where ωce=eB∕mec is the electron gyrofrequency, ωpe is the electron plasma frequency, and βb=Vb∕c is the ion beam velocity relative to the speed of light. This condition typically holds for relatively small magnetic fields (about 100G). Analytical formulas are derived for the effective radial force acting on the beam ions, which can be used to minimize beam pinching. The results of analytic theory have been verified by comparison with the simulation results obtained from two particle-in-cell codes, which show good agreement.

Preliminary Research Results for the Generation and Diagnostics of High Power Ion Beams on FLASH II Accelerator

The preliminary experimental results of the generation and diagnostics of high-power ion beams of FLASH II accelerator are reported. The high-power ion beams presently are being produced in a pinched diode. The method for enhancing the ratio of ion to electron current is to increase the electron residing time by pinching the electron flow. Furthermore, electron beam pinching can be combined with electron reflexing to achieve ion beams with even higher efficiency and intensity. The anode plasma is generated by anode foil bombarded with electron and anode foil surface flashover. In recent experiments on FLASH II accelerator, ion beams have been produced with a current of 160 kA and an energy of 500 keV corresponding to an ion beam peak power of about 80 GW. The ion number and current of high power ion beams were determined by monitoring delayed radioactivity from nuclear reactions induced in a 12C target by the proton beams. The prompt γ-rays and diode bremsstrahlung x-rays were measured with a PIN semi-conductor detector and a plastic scintillator detector. The current density distribution of ion beam were measured with a biased ion collector array. The ion beams were also recorded with a CR-39 detector.

Improved bremsstrahlung from diodes with pulse-heated tantalum anodes

Bremsstrahlung from high-power electron-beam diodes increases and becomes spatially more uniform when the tantalum anode is first pulse-heated to remove gas from the surface and interior, and then pulse-heated again to white hot just prior to and during the high-power pulse. Heating the tantalum eliminates protons, retards beam pinching, and increases the far-field X-ray dose relative to unheated tantalum. The radiation pattern becomes symmetric and hollow, producing a more uniform near-field dose distribution than for unheated tantalum. With a white-hot anode, the diode current is single-species Child-Langmuir until the voltage exceeds 1 MV, at which point it reaches critical current. These phenomena demonstrate reduced effects from ions in the diode. The increase in dose is a result of both reduced ion current and enhanced electron reflexing through the subrange tantalum foil. The heating technique is compatible with high-power generators such as Decade, whose x-radiation output would increase by as much as 30%.

Theoretical modeling and experimental characterization of a rod-pinch diode

The rod-pinch diode consists of an annular cathode and a small-diameter anode rod that extends through the hole in the cathode. With high-atomic-number material at the tip of the anode rod, the diode provides a small-area, high-yield x-ray source for pulsed radiography. The diode is operated in positive polarity at peak voltages of 1 to 2 MV with peak total electrical currents of 30–70 kA. Anode rod diameters as small as 0.5 mm are used. When electrode plasma motion is properly included, analysis shows that the diode impedance is determined by space-charge-limited current scaling at low voltage and self-magnetically limited critical current scaling at high voltage. As the current approaches the critical current, the electron beam pinches. When anode plasma forms and ions are produced, a strong pinch occurs at the tip of the rod with current densities exceeding 106 A/cm2. Under these conditions, pinch propagation speeds as high as 0.8 cm/ns are observed along a rod extending well beyond the cathode. Even faster pinch propagation is observed when the rod is replaced with a hollow tube whose wall thickness is much less than an electron range, although the propagation mechanism may be different. The diode displays well-behaved electrical characteristics for aspect ratios of cathode to anode radii that are less than 16. New physics understanding and important properties of the rod-pinch diode are described, and a theoretical diode current model is developed and shown to agree with the experimental results. Results from numerical simulations are consistent with this understanding and support the important role that ions play. In particular, it is shown that, as the ratio of the cathode radius to the anode radius increases, both the Langmuir–Blodgett space-charge-limited current and the magnetically limited critical current increase above previously predicted values.

Effects of self-generated electric and magnetic fields in laser-generated fast electron propagation in solid materials: Electric inhibition and beam pinching

We present some experimental results which demonstrate the presence of electric inhibition in the propagation of relativistic electrons generated by intense laser pulses, depending on target conductivity. The use of transparent targets and shadowgraphic techniques has made it possible to evidence electron jets moving at the speed of light, an indication of the presence of self-generated strong magnetic fields.

Pseudospark produced pulsed electron beam for material processing

An intense pulsed electron beam produced by a pseudospark discharge is used for material processing. The electron beam propagates in a self-focused manner in the background gas. Hardly 12 ns after the beginning of the discharge the fraction of space charge neutralization is about 96%. To sustain the neutralization effect high energy electrons (E <500 keV) are accelerated in radial direction at the beam head, due to strong electric field gradients. At current maximum the beam pinches due to its own magnetic field. At peak current of 400 A and charging voltage up to 16 kV power density reaches 10/sup 9/ W/cm/sup 2/ on the target surface. Some results of copper thin films are presented. Due to the high expansion velocity of 10/sup 4/ m/s of the ablated target material a copper-matrix has been masked.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Numerical Investigations of Electron Beam Energy Deposition into the Gas Medium of an Excimer Laser

Electron beam propagation in a dense gas medium is numerically investigated. All the main phenomena that determine electron beam behavior in a gas (scattering and energy losses of the electrons on the gas atom molecules, ionization and excitation, electron thermalization, beam pinching, and influence of a magnetic guide field) are taken into account. The initial beam energy and the gas chamber dimensions are varied in a wide range: typical gas mixtures for the excimer lasers are considered. Graphs are given that allow the choice of the optimal electron beam energy that provides the maximum efficiency of the beam energy deposition into the gas, depending on the gas chamber dimensions.

Electron flow in a relativistic electron beam pinched diode collective acceleration system

Investigations of the electron component of the composite collectively accelerated ion and electron beam produced in a relativistic electron beam pinch diode are reported. The experiments consisted of examining the Kα x-ray signal induced by the propagating electron beam and comparing this with the Kα signal produced in the anode. The measurements dealt with propagation velocity, beam front current density as a function of distance from the anode, and the temporal structure of the beam front as a function of distance. Time dependent structure consisting of several peaks was observed. The velocity of the beam front was seen to increase with the addition of CH2 to the anode.

Simulation of the Wisconsin-Argonne Plasma Wakefield Experiment

The plasma wakefield accelerator (PWFA) is an advanced accelerator concept that uses the large electric fields that can be generated in a plasma to accelerate charged particles. We present the results of a self-consistent two-dimensional simulation of the first experiment designed to test this concept. Linear theory predicts for this experiment an accelerating gradient of approximately 95 MV/m. However, the simulations indicate that a much larger accelerating field is achieved in the plasma. This enhancement is due to strong beam pinching, which is not treated self-consistently by a linear theory. Wave steepening due to a nonlinear modulation of the background plasma is also observed. This steepening results in a phase shift that degrades the acceleration.

Dynamics of a virtual cathode oscillator driven by a pinched diode

A new method is demonstrated for extracting radiation from a virtual cathode oscillator in transverse electric (TE) waveguide modes. The dominant radiation mechanism occurs in the region of the virtual cathode, and is not due to reflexing of electrons. Microwave radiation occurs simultaneously or just after beam pinching in the diode. Electrostatic signals show simultaneous occurrence of the virtual cathode and microwave radiation. At the same time, the electron population divides into a beam population and a reflexing electron population. Inhibition of pinching by an axial guide field suppresses microwave radiation.

Picosecond beam propagation for gas-loaded free electron lasers

A gas-loaded FEL requires electron beam propagation through pressures of ≅ 1 atm. Interaction with the resulting plasma can lead to beam pinching, beam expansion, or various plasma instabilities. Here we discuss calculations and an experiment which demonstrate that the 4 ps electron bunches of a linear accelerator are short enough to avoid major disruption from these effects while traversing 1 m of H 2 at 1 atm.

Electron-Beam Pumped Excimer Laser Device for Uniform, Large Volume Excitation

Coaxial diode electron-beam device for excimer laser pumping has been designed and constructed. In designing this device, we have especially considered spatial uniformity of energy deposition, pumping efficiency and electron-beam pinch. Experimental observation on diode impedance, beam pinch, and energy deposition uniformity are compared with standard formula or a code calculation on electron transport.

Steady-state treatment of relativistic electron beam erosion

The head of a relativistic electron beam propagating into un-ionized or weakly ionized gas is not self-pinched and expands freely, causing the beam to take on a ’’trumpet’’ shape. The region where the beam pinches, referred to as the ’’pinch point’’, moves steadily back into the beam because of the reduced ionization rate at the expanding beam head and energy loss to the induced electric field. This beam head ’’erosion’’ is modeled by assuming that the axial beam profile is stationary in a reference frame moving with the pinch point. This assumption allows the beam equations to be written in time-independent form, and radial averaging then yields a set of one-dimensional ordinary differential equations for the beam radius and energy, the mean pinch force, and the background conductivity. Solution of these equations with appropriate boundary conditions gives both the erosion rate and the beam axial structure. The results of extensive numerical calculations are presented, along with analytic estimates of the erosion rate, degree of current neutralization, and axial scale lengths.

Super-ion-beam accelerator for the ignition of thermonuclear reactions

A pulsed, multi-stage, high-voltage accelerator is proposed which should be capable of producing intense ion beams of many million amperes and many million volts. Super ion beams produced by this type of accelerator can exceed the limiting Aifvén current for light ions, typically 107 A, at which beam pinching occurs. The beam pinching of these super-beams permits them to be precisely focused onto a thermonuclear target. With such an accelerator it seems to be possible to reach a beam voltage of 108 V with a beam current of 107 A. The resulting beam power of 1015 W should be more than sufficient to ignite a DT thermonuclear microexplosion. By the formation of a stable ion beam superpinch within a thermonuclear target, such a large beam power in conjunction with the strong self-magnetic field of the beam may even lead to the ignition of the DD and perhaps HB11 thermonuclear reactions.

The Localized Pinch Model as a High Energy Ion Collective Acceleration Mechanism

We present analytic and numerical calculations of the localized pinch model for ion collective acceleration by intense relativistic electron beams. These studies demonstrate the model's postulated synchronous acceleration of the ion clump and beam pinch. However, the configuration is found to be phase unstable, terminating the acceleration process before significant ion energy is acheived. Therefore, although the localized pinch model may be an appropriate explanation for some previous experimental observations of collectively accelerated MeV ions, it does not appear to be a suitable mechanism for high energy ion acceleration.