Propagation Of Relativistic Electron Beam Research Articles

Two-pulse generation of high-current relativistic electron beams and their transportation in the gaseous medium of a plasmochemical reactor

Experimental results on two-pulse generation and transportation of high-current relativistic electron beams (REBs) through the gaseous medium of a plasmochemical reactor (PR) are presented. The generation of two consecutive high-current REB pulses with a duration of 60 ns was achieved at the Tonus accelerator with modified schemes of high-voltage pulse formation. The first version of the formation scheme enabled pulse powers of 2 and 4.0–9.6 GW with a time interval between the pulses of 500 ns. The second version enabled one to generate pulses with powers of 1.8 and 16 GW and time interval between the pulses of 160 µs. The transportation parameters of an REB injected into a 1.4-m-long PR filled with an N2: O2 gas mixture are studied. The conductance of the plasma produced under the action of the electron beams is measured. It is shown that the schemes proposed provide more efficient (by 35–45%) transportation of the REBs in the reactor volume as compared to single-pulse high-current REBs of the same power and pulse duration.

Analytical theory of pulsed relativistic electron beams entering a vacuum

An electron beam with a high-current density, passing from a conductor into a vacuum, generates strong electric fields which significantly influence further propagation of the electrons. An analytical solution for a one-dimensional pulsed electron beam with an arbitrary energy distribution is found which allows one to calculate electric field distributions, electron density distributions and potential distributions in the vacuum region. The analytical solution is fully relativistic and is applicable up to the time at which the interpenetration of electron paths occurs. It may be useful in research with fs lasers relating to generation of relativistic electron beams, ion beams, and the pumping of x-ray lasers.

Application of resolution pattern based focal spot measurement technique during the development of a flash radiography system

A flash X-ray radiography system is developed using a relativistic electron beam (REB) generator and a field emission cathode. X-ray source size in any radiography system is an important parameter to determine image quality. In the field emission targets the source size is controlled by the anode–cathode geometry which eventually influences the radiation output and peak X-ray energy. The geometry was optimized by progressive monitoring of the X-ray source (focal spot) size. A method based on resolution of different spatial frequencies was used for the measurement of focal spot dimensions. Optimum focal spot size of 2 mm and maximum radiation output of 2 R at 150 mm distance were achieved. Radiograph of a radioactive source pencil was taken to corroborate the observations and it detected the active source.

Relativistic electron beam propagation in the Earth's magnetosphere

The global evolution of an artificially injected relativistic electron beam is simulated and examined. The study focuses on injections originating in the upper ionosphere, magnetically mirroring above the lower atmosphere where significant energy loss occurs, and so a long‐lived population arises in the inner magnetosphere from this particle source. This investigation is conducted by solving the bounce‐averaged relativistic kinetic equation for the electron distribution function for various L shells. It is found that the beam quickly spreads in MLT due to differential drift rates, eventually morphing into a fairly uniform shell around the Earth. Wave interactions are comparable to collisional losses in reducing the beam content. It is also found that the beam total particle loss rate is a complicated function of L and, for the chosen conditions, the total beam particle counts are 73%, 77%, and 52% of the initial count at t=24 hours after injection for L=2, 3, and 4, respectively. The loss rates at this time are ∼1%/hour (of the remaining beam strength) and very slowly decreasing with time. These loss rates and other features of the beam evolution are discussed in detail. There are now four distinct stages recognized in the evolution of an injected relativistic beam: (1) the immediate loss of particles injected at pitch angles mapping to the lower thermosphere; (2) the initial loss of particles injected right next to the loss cone by collisional scattering; (3) the continuation of this collisional loss along with the spread of the beant to all local times by differential drift rates; and (4) the transformation of the beam into a fairly uniform shell covering all energies and all local times, with the loss rate mainly governed by wave scattering. Other stages may exist beyond the 1‐day limit set on these simulations. While the study dwells on beam dynamics, it is also a general examination of the leading edge population next to the loss cone. This has implications for the physics of the naturally occurring radiation belt particles, as this region of phase space regulates the actual precipitation of these particles into the atmosphere. The applicability of this model for studying the natural radiation environment around the Earth is also pondered.

Relativistic electron-beam generation in a gas-loaded foil-less diode

Relativistic electron beams were generated in a gas-loaded foil-less magnetized diode. The helium gas was employed to fill the diode, and the experimental results have shown that the diode operated normally without the closure problem when the gas pressure was below a certain value (typically, 100 mTorr in the presence of an externally applied guide magnetic field). It has been observed that the current of the electron beam could increase with increasing the gas pressure, and that the presence of helium gas could reduce the operating voltage of the diode producing an electron beam with lower kinetic energy, which implies a decrease in the diode impedance. The externally applied magnetic field along the axial direction also exerted an influence on the electron-beam generation in the gas-loaded diode.

Beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a medium-pressure rarefied gas

A theoretical model is given, along with a numerical analysis of the evolution of beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a rarefied gas at medium pressure. It is shown that the self-stabilization of beam-plasma discharge as a result of longitudinal inhomogeneity of the density of the discharge plasma makes it possible for the beam to traverse the beam chamber with relatively low total energy losses, including ionization losses and energy losses in the generation of oscillations. During the dissociative recombination of electrons and ions of the discharge-driven plasma, heat is released and spent in raising the temperature of the gas. The investigated collective-discharge mechanism underlying heating of the gas for a relativistic beam can be more efficient than the classical heating mechanism due to ionization losses of the beam in pair collisions of its electrons with gas particles.

Propagation of an intense relativistic electron beam in an annular channel

The propagation of an annular intense relativistic electron beam inside a cylindrical drift tube is affected by many factors such as external magnetic field, drift tube geometry, the beam self electric and magnetic fields, etc. It is shown, experimentally and theoretically, that the deleterious influence of the self-fields on the electron flow can be reduced when part of the beam return current is allowed to flow on a grounded conductor that is inserted coaxially within the beam. The improvement manifested itself by reducing the axial magnetic field needed to confine the electron beam or alternately by lowering the electron transverse velocities.

Relativistic electron beam propagation in the Earth's atmosphere: Modeling results

Linear accelerators (linacs), capable of producing 5 MeV energy electron beams at 80 mA currents, are now down to a size that allow them to be flown on sounding rockets or balloons. This opens up new opportunities for atmospheric/ionospheric modification experiments where the mesosphere and thermosphere regions of the atmosphere can be perturbed down to 40 km altitude. In this paper beam propagation and atmospheric perturbation effects are studied by Monte Carlo simulations and by analytical means. It is shown that the earth' magnetic field severely limits the radial expansion of the beam otherwise induced by electron‐neutral collisions. It is also shown that the so‐called “envelope‐equations” from high‐energy laboratory physics adequately describe beam propagation in the upper atmosphere. The plasma density and electric conductivity modifications to the atmosphere are calculated from the Monte Carlo simulations. Inside the beam the conductivity in the 40–50 km altitude region is enhanced more than one order of magnitude by a 10 µs‐duration pulse. Some ideas for future scientific investigations are discussed, including the generation of electrical discharges by beams injected over thunderstorm regions.

Generation of a large diameter intense relativistic electron beam for the triaxial relativistic klystron amplifier

We built and tested a new electron beam generator that is capable of generating a 66 cm diameter annular intense relativistic electron beam of energy of 450 keV and current of 60 kA. The new generator incorporates a coaxial-to-triaxial mode converter which allows us to generate and (in the future) to propagate the electron beam between two coaxially grounded conductors. We designed the beam transport system to minimize the axial magnetic energy needed to confine the electron beam. An even lower impedance device can be built by scaling up the size, generating a higher electron beam current. This beam geometry (sheet beam) is useful for efficient generation of high power microwaves.

Relativistic electron beam generation in a plasma-filled diode with foilless injection into a dense plasma

In the experiments an relativistic electron beam (REB)-plasma interaction, the foilless injection of REB from a magnetized diode is of special interest due to the low spread angle of the beam and high repetition rate of the shots. In the experiments presented, the problem of diode shortening in the presence of a dense plasma from the interaction chamber has been solved using a special drift pipe as an anode of the foilless diode. The electron beam (U/sub d//spl sim/0.7 MeV, t/sub b//spl sim/100-200 ns) produced by an axially symmetric magnetically insulated diode has been injected into a magnetized hydrogen plasma column with density ranging from 1/spl middot/10/sup 15/-3/spl middot/10/sup 16/ cm/sup -3/. It has been found that the anode pipe substantially reduces the plasma flow into the diode gap, but does not stop it completely, thus the REB generation in a plasma-filled diode has taken place. In some regimes of the beam generation it becomes possible to increase significantly the injected current and total energy deposition of the beam in comparison with the case of a vacuum magnetized diode of the same geometry. The experiments have shown effective dense plasma heating under the foilless injection.

Angular velocity spread of relativistic electron beam generated by high current diode

Theoretical analysis and numerical simulations of the Relativistic Electron Beam (REB) generation in a high current diode immersed in an external magnetic field has been done. The calculations have confirmed that the generated beam is homogeneous and monoenergetic in a broad central region. In the case of cylindrical diode the mixing of electron trajectories has been observed only in a narrow periphery beam region. The angle between particle trajectories and external longitudinal magnetic field varies chaotically from 0° to −25°. This phenomenon suppresses the excitation of two stream instability excited by the REB in the plasma column.

Experimental and numerical studies of sheet electron beam propagation through a planar wiggler magnet

Detailed experimental studies on sheet relativistic electron beam propagation through a long planar wiggler are reported and compared with numerical simulations. The planar wiggler has 56 periods with a period of 9.6 mm. Typically, the wiggler field peak amplitude is 5 kG. The experimental efforts are focused on controlling the deviation of the beam toward the side edge of the planar wiggler along the wide transverse direction. It is found that a suitably tapered magnetic field configuration at the wiggler entrance can considerably reduce the rate of deviation. The effects of the following techniques on beam transport efficiency are discussed: side focusing, beam transverse velocity tuning at the wiggler entrance, and beam spread limiting. High beam transport efficiency (almost 100%) of a 15-A beam is obtained in some cases. >

Microwave growth from the beam breakup instability in long-pulse electron beam experiments

The beam breakup (BBU) instability has been investigated in high-current, long-pulse electron beams propagating through microwave cavities. Experiments are performed using a relativistic electron-beam generator with diode parameters: 0.7–0.8 MV, 1–15 kA, and 0.5–1.5 μs. The magnitude of the solenoidal magnetic field places these experiments in an intermediate regime between strong focusing and weak focusing. The electron-beam transport system consists of ten identical pillbox cavities each containing a small microwave loop antenna designed to detect the TM110 beam breakup mode. The TM110 microwave mode is primed in the first cavity by a magnetron tuned to the resonance frequency of 2.5 GHz. The BBU instability growth is measured through the amplification of the 2.5 GHz microwaves between the second and tenth cavities. Strong growth (25–38 dB) of the TM110 microwave signal is observed when the initial cavity is primed exactly on resonance, with a rapid decrease of the growth rate off-resonance. The magnitude of microwave growth is consistent with the predictions of BBU theory.

Simulation studies of a klystronlike amplifier operating in the 10-100 GW regime

A coaxial drift tube allows propagation of an ultra high-power relativistic electron beam (500 keV, >or=100 kA, 100 ns). The modulation of a large-diameter (12.6 cm) intense, relativistic electronic beam (500 keV, 16 kA) by an external microwave source via particle simulation is studied. The annular beam, enclosed within a coaxial drift tube, is found to be fully modulated by a low-power external RF source at a frequency of 1.3 GHz. It is shown that for such an intense beam, a highly nonlinear interaction takes place at the modulating gap, producing highly coherent bunches of electrons. This finding is similar to earlier research in which such modulations were studied for an intense beam propagating in a hollow drift tube. It is further shown that, unlike the hollow drift tube case, the coaxial configuration is easily scaled to high power. Here, a very large diameter (26 cm) intense beam (460 keV, 100 kA) is fully modulated at 1.3 GHz to obtain 31 GW of RF beam power. >

Relativistic electron-beam generation in a plasma-filled diode and its interaction with plasma

A relativistic electron beam of 150–300 keV, 1–2 kA, and 40–80 ns generated in a plasma-filled diode is injected into a magnetized hydrogen plasma of density 1011–1014 cm−3. Beam-plasma interaction was observed to be strong when the plasma density in the diode and interaction region was ≂1012 cm−3. The effective collision frequency was estimated to be around 10−2 times the ion plasma frequency. The plasma heating levels of ≂1016 eV cm−3 μs−1 were inferred. The observed net current multiplication was explained by two-stream instability.

A study of the propagation of a quasistationary relativistic electron beam in a dense gas. II. Experimental studies

Results obtained from experimental studies of the interaction between a quasistationary electron beam (300 keV, 20 A, 500 μsec, 0.3 cm) and a dense gas are presented. These investigations were carried out in air at atmospheric pressure and, for the purpose of modeling the propagation of a magnetostricted REB, at reduced pressure (3–300 torr). The rarefaction dynamics of a channel heated by the beam are studied and its maximum propagation distances in a dense gas are determined. The results obtained are compared with the theoretical model examined earlier.

A study of the propagation of a quasistationary relativistic electron beam in a dense gas. Part I. A physical model of the interaction

A study of the propagation of a quasistationary relativistic electron beam in a dense gas. Part I. A physical model of the interaction

Current multiplication during relativistic electron-beam propagation in a subTorr-pressure gas

The current multiplication observed when a relativistic electron beam is propagated in an initially neutral gas at subTorr pressures (0.1–0.3 Torr) is generally attributed to the two-stream instability. A general expression for this current gain is derived for a scattered beam propagating in a neutral gas, which takes into account a quasihydrodynamic expression for the beam energy loss. Moreover, the beam-to-plasma density ratio is obtained as a function of time for a given beam pulse by numerically solving the electron energy and continuity equations. It is found that at a given beam-to-plasma density ratio, the current gain increases with increasing the beam energy, with the increase being most rapid for the small scattering angles. A finite average scattering angle is seen to reduce the current multiplication. The experimentally observed peak current gains are in good agreement with the predicted peak gains obtained by taking into account the time-evolved beam-to-plasma density ratio.

Propagation of wiggler focused relativistic sheet electron beams

A recent design concept for millimeter-wave free-electron lasers [J. Appl. Phys. 60, 521 (1986)] would require the stable propagation of a sheet electron beam through a narrow waveguide channel. Experimental results reported in this article support the feasibility of such a configuration by demonstrating the stable propagation of relativistic sheet electron beams through a narrow waveguide gap (3.2 mm) using focusing by a short-period electromagnet wiggler. 90% of the electron current in a 100-keV sheet electron beam was transmitted through a 5-cm-long channel with peak wiggler fields of 800 G. Almost 80% of a 400-keV beam was similarly confined with a 1600-G wiggler field. The data were consistent with single electron trajectory models, indicating that space-charge effects were minimal. No evidence of beam breakup or filamentation instabilities was observed.

IBEX magnetic field extraction and propagation experiments of intense high-energy electron beams

We have investigated the extraction and propagation of intense relativistic electron beams from an immersed foilless diode into a magnetic field-free region. It was established experimentally that under matching conditions, the extracted beam propagates into and through the axially varying magnetic field region with minor perturbation of the equilibrium. The independent variables of the experiment were beam current, beam radius, magnetic field strength, and gas pressure in the field-free region. The accelerator used was IBEX. [J. J. Ramirez, J. P. Corley, and M. G. Mazarakis, in Proceedings of the Fifth International Conference on High-Power Particle Beams, San Francisco (Physics International, San Leandro, CA, 1983), p. 256.] The propagation of the extracted beam was studied up to 60 cm downstream from the extraction foil. A strong, fast growing filamentation instability was observed after extraction. Time-resolved measurements of the beam current, net current, and current distribution were performed by using segmented Faraday cups. Time-integrated measurements of beam rotation and emittance were also recorded. The observed beam parameters and equilibrium radii before and after extraction are compared with theory and numerical calculations.