Scenario for the establishment of an electron beam “squeezed state” in a magnetically insulated vircator

A 2.5-dimensional particle-in-cell code was used to simulate different scenarios for the establishment of an electron beam “squeezed state” in a magnetically insulated vircator. Vircators with and without anode foils were compared. It was found that the squeezed state is established in both cases but the dynamics of establishment differ. Pis’ma Zh. Tekh. Fiz. 23, 29–33 (November 26, 1997)     

Phase-locked ions and foil transparency in the radiation pressure acceleration regime

The transverse expansion of a thin foil accelerated in the RPDA regime can be exploited in order to increase the ion energy and the acceleration efficiency at the expense of decreasing the number of accelerated particles. In the relativistic regime, the ions become phase-locked with respect to the electromagnetic wave. The use of an optimal laser pulse shape makes it possible to keep the expanding foil opaque to the laser radiation. This provides a new approach in order to enhance the energy of laser accelerated ions significantly.     

Effect of positive ions on the microwave generation in a low-voltage vircator

The effect of vacuum conditions on the characteristics of microwave generation in a nonrelativistic electron beam with a virtual cathode (VC) formed in a retarding field (low-voltage vircator) has been experimentally studied. The ionization of residual gases in the working chamber of a low-voltage vircator leads to displacement of the VC out of the interaction space, which results in breakdown of the microwave generation. The experimental data agree with the results of numerical simulations.     

Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.     

On the possibility of plasma discharge initiation in the vicinity of an electrically insulated metal target bombarded by high-energy neutral particles

The possibility of plasma discharge initiation in the vicinity of an electrically insulated metal target occurring in plasma under the action of an incident flux of high-energy neutral particles is considered. Conditions for initiating discharge are determined and formulated as requirements on the incident flux parameters and plasma characteristics. The potential of a target, the electron concentration, and the electron current to the target surface in the presence of discharge are determined for an equilibrium state of the system.     

On the possibility of Bose-Einstein condensation in a solid

We treat the question of the existence of Bose-Einstein condensation (BEC) in Bose lattice systems. Simple examples are constructed that do display BEC while having a lattice symmetry. It is suggested, however, that these systems are not crystals in the ordinary sense. When a more conventional definition of a lattice is employed, we generalize the proof of Matsuda and Tsuneto that BEC does not exist in a Bose lattice, for a wider class of wave functions including two-body correlations. A solid with a finite vacancy concentration can display BEC, which is related to the BEC in the vacancy gas. We clarify the relation between BEC of vacancies and particles.     

Dynamics of the anode plasma in the magnetically insulated diode of the COBRA ion accelerator by modified laser deflectometry

A new type of optical system for measurements of laser beam deflection has been developed and implemented experimentally. The refractive index gradient in the anode plasma of a megavolt magnetically insulated diode in a high-power ion accelerator has been measured with submicron resolution. The refractive index distribution in the layer was measured during the pulse and the average electron density in the layer was determined. Pis’ma Zh. Tekh. Fiz. 23, 63–68 (May 26, 1997)     

On the possibility of preparing decoherence-free states in a cavity

The effect of decoherence in a quantum system can be viewed as a consequence of the interaction with the environment. As has been pointed out first by Dicke, in a system of N two-level atoms where each of the atoms is individually dipole coupled to the environment, there are collective subradiant states that have no dipole coupling to photon modes, and therefore they are expected to decay more slowly. We have recently proposed a scheme which is intended to create such states in a detuned cavity. We shall examine here the conditions under which our scheme can be used and compare them with the experimental possibilities. The analysis shows that our proposal can be implemented with present-day techniques achieved in atom—cavity interaction experiments.     

Cryogenic test of a superconducting half-wave resonator for the acceleration of heavy ions

We present the results of cryogenic tests on a superconducting half-wave resonator for the acceleration of heavy ions. The resonator was built out of OFHC copper and electroplated with 1.5 μm of Pb without chemical polish of the Pb surface. Measured properties include a low level Q of 4 × 10⁸ and a power dissipation of 6 W at 4 MV/m giving an energy gain of 1 MV per unit change to a particle of velocity 0.16 c.     

Influence of nonuniformity of the ion background on the oscillation frequency of a virtual cathode

The oscillation frequency of a virtual cathode in a diode gap is investigated as a function of the density of the anode ion layer. It is shown that when the densities of the ion layer are high, a stable electron bunch forms in the flux, which leads to an increase in the oscillation frequency. Pis’ma Zh. Tekh. Fiz. 24, 74–80 (November 12, 1998)     

Electron source and acceleration regime of a picosecond electron beam in a gas-filled diode with inhomogeneous field

It is experimentally demonstrated that, upon the application of a subnanosecond high-voltage pulse to the gap of a diode filled with air at atmospheric pressure, a bunch of runaway electrons is formed in a sharply inhomogeneous electric field near the cathode. The bunch duration does not exceed 50 ps, which is shorter than the electron flight time through the interelectrode gap in the continuous acceleration regime. This duration remained unchanged when the gap width was varied between 6 and 26 mm. The electron energy in the picosecond electron beam, as determined from the time-of-flight measurements in the drift channel behind the anode foil of the diode, agree with the results of numerical calculations of the electron acceleration dynamics in the vacuum diode approximation.     

Plasma discharge in magnetically active plasma in the vicinity of an electrically insulated metal target bombarded by high-energy neutral particles

The possibility of plasma discharge initiation in the vicinity of an electrically insulated metal target occurring in a magnetically active plasma under the action of an incident flux of high-energy neutral particles is considered. Conditions necessary for initiating the discharge are determined and formulated as requirements on the incident flux parameters and plasma characteristics. The concentration of electrons and the electron current to the target surface in the presence of discharge are evaluated.     

Influence of feedback on the complex dynamics of an electron beam with a virtual cathode in a virtode

Results are presented of a numerical simulation of the nonlinear dynamics of a relativistic electron beam with a virtual cathode in a drift relativistic-electron-beam vircator system with and without external feedback. Pis’ma Zh. Tekh. Fiz. 24, 51–57 (March 12, 1998)     

Anomalous acceleration of interelectrode-plasma ions in the spark stage of vacuum discharge

A new method of obtaining high-energy ion beams using vacuum spark discharge is proposed. It is shown that ions are accelerated in the presence of a plasma cloud in the interelectrode gap, where the development of strong electron instability during the propagation of a cathode electron beam leads to potential buildup to a level exceeding the applied voltage. The appearance of anomalously accelerated ions of the interelectrode plasma is accompanied by a jump in the diode current.     

Experimental study of the transition to high-current regime of discharge with a hollow self-heated titanium cathode in nitrogen

It is established that rapid formation of a nitride layer on the surface of a self-heated hollow titanium cathode in arc discharge in nitrogen leads to an increase in the thermal stability and emissive properties of the cathode. Optimum regimes of cathode training that ensures rapid Ti → TiN transition over the entire cathode wall thickness in the active zone are determined, which allows the operation temperature to be increased above 2200 K at a discharge current of up to 50 A for the cathode with an inner diameter of 8 mm and a wall thickness of 1 mm. Results of measurements of the current-voltage characteristics of discharge, thermal regimes of operation, and chemical composition of the cathode are presented. The rates of erosion of the cathode operating in nitrogen and argon have been determined.     

The current of an annular electron beam with virtual cathode in a drift tube

An analysis based on the laws of conservation of energy and the z components of the field and particle momentum shows that a thin magnetized annular electron beam in a homogeneous drift tube behind a virtual cathode under stationary conditions occurs in a “squeezed” state corresponding to a slow left-hand branch of the current characteristic with a relativistic factor in the interval 1≤γ≤Γ^1/3. The beam current I ₁ behind the virtual cathode in a homogeneous drift tube can vary from zero up to a limiting value I _lim, while the injection currents (I ₂) and the current of electrons reflected from the virtual cathode (I ₃) for every stationary state are single-valued functions of I ₁ and fall within the intervals I _F/2≤I ₂≤I _lim and 0≤I ₃≤I _F/2, respectively, where I _F is the Fedosov current.     

On the possibility of creating a superfast-recovery silicon carbide diode

The possibility of a superfast (<1 ns) termination of the reverse current during the recovery of a 4H-SiC diode with a p ⁺ p ₀ n ⁺ structure is experimentally demonstrated for the first time. It is shown that the gate recovery process is much like that taking place in inverse-recovery silicon diodes.     

On the breakdown of acceleration waves in a vibrationally relaxing gas

Summary Explicit criteria for the breaking or non-breaking of acceleration waves in a plane, axially or spherically symmetric flow of a relaxing gas are obtained. The paper modifies a recent paper on the growth and decay of acceleration waves in transient gas flows with vibrational relaxation [Acta Mechanica33, 171–178 (1979)] by putting its analysis on a right footing, and by taking into account a general relaxing gas model. It is investigated as to how the effects of internal relaxation and wave front curvature influence the time of breaking. A remarkable difference between the behaviors of cylindrically and spherically converging waves is discovered.With 1 Figure     

A study of the electromagnetic properties of a resonator for multichannel acceleration of light ions

We present an experimental study of a modified Interdigital-H-resonator which can be used for multichannel acceleration of light ions. The distribution of the electromagnetic fields is investigated with a perturbation method. Furthermore, measurements of the resonance frequency, resonator parallel resistance and quality factor are discussed. The measurements have been done on three types of accelerating structures, which have a different capacitance per unit length. The results are compared with an elementary model, in which the resonator is described in terms of discrete circuit elements. It is found that the resonance frequency can be calculated with good accuracy. For the other parameters scaling laws are derived.     

Switching of output coupling in a grating coupler by diffraction transition to the distributed Bragg reflector regime

In a proposed switching grating coupler (SGC), the grating period Lambda is chosen so that the SGC converts from a first-order grating coupler to a third-order distributed Bragg reflector by means of a small change in the guided mode index. The principle for switching the radiated wave power and the effective aperture of the SGC were experimentally confirmed by use of the thermo-optic effect of a polymer waveguide. The extinction of the peak intensity of the wave radiated by the SGC and collected by an external lens was measured to be 5 dB with a temperature change of 10 degrees.