Optimization of Monte Carlo codes using null collision techniquesfor experimental simulation at low E/N

The use of the null collision method in Monte Carlo simulations of electron and ion motion in gases is briefly reviewed, with particular reference to conditions where only a handful of collision cross sections are required to describe the electron/ion-molecule interaction. An efficient algorithm is proposed for use in such conditions as an alternative to S.L. Lin and J.N. Bardsley's (1977) implementation of H.R. Skullerud's (1973) original null collision algorithm. The alternative implementation is described, and some examples of the advantages of the new algorithm are given for electron scattering from argon, helium, and the Reid ramp model gas     

Monte Carlo simulation of electron swarms in nitrogen in uniformE×B fields

The motion of electrons in nitrogen in uniform E× B fields is simulated using the Monte Carlo technique for 240⩽E/N⩽600 Td (1 Td=1×10^-17 V cm²) and 0⩽B/N⩽0.45×10^-17 T cm³ . The electron-molecule collision cross sections adopted are the same cross sections as those used previously for the numerical solution of the Boltzmann equation. The swarm parameters obtained from the Monte Carlo simulation are compared with the Boltzmann solution and with the experimental data available in the literature. In relation to E×B fields, it is concluded that the Monte Carlo approach provides an independent method of substantiating the validity of the equivalent electric-field approach     

Spatial and time characteristics of short gap, highdi/dt discharges

Under certain network conditions, vacuum circuit breakers may generate high-frequency currents. The quenching capability of vacuum circuit breakers for line-frequency currents and high-frequency currents plays an important role in the generation of unwanted voltage transients. This may occur when the gap distance at current zero is still too short to withstand the external voltage to the switch (TRV). The results of simulation calculations regarding these phenomena in the vicinity of current zero are described. Simulations are based on a detailed physical model, taking into consideration the basic conservation laws, the Maxwell equations, and the current continuity. The numerical solution takes into account the results of experimental streak photographs, revealing that the visible discharge covers only a small part of the contact diameter     

Simulations of V-I characteristics in a plasmafocus fun

The voltage and current characteristics in a plasma focus gun are simulated to yield the current efficiency and the rundown velocity of the current sheath. Since the discharge circuit is strongly influenced by the rundown dynamics of the current sheath, the simple snowplow model, which is modified to include the time-varying current and mass efficiencies, us used. The computer simulations are carried out for two separate experiments using deuterium and argon gas. A steady state of constant rundown velocity and saturated sheath current for both gases is correlated to the maximum holding voltage of the glass insulator. As much as 27% of the total discharge current for the deuterium gas and 33% for argon gas stay behind as leakage currents around the glass insulator when the radial compression phase begins     

Origin of the magnetic confinement force in the homogeneous Z pinch

A reexamination is made of the basic assumptions about magnetic force and magnetic pressure in a pinch. The confinement of a plasma by its own internal magnetic field is analyzed in a cylindrical geometry. It is shown that there are subtle but serious ambiguities in the usual theory, which can be cleared up by elementary arguments. The results which follow lead to an improved understanding of the mechanism responsible for the Z pinch     

A Z-pinch neonlike X-ray laser

A scheme using neonlike krypton ions is under intensive theoretical and experimental investigation to determine the feasibility of developing a pulsed power-driven laboratory X-ray laser. The scheme depends on discharging hundreds of kilojoules of electrical energy through coaxial cylindrical krypton gas puffs, generating a dense, hot, uniform, homogeneous, and highly ionized krypton plasma. The dynamics of energy absorption are such that self-generated magnetic fields compress and accelerate radially inward the outer plasma with speeds approaching 5×10⁷ cm/s. When the outer plasma impinges and stagnates on the inner plasma, shock waves are sent through the system as the plasma reverberates and bounces outward. Near the interface between the two interacting plasmas and along the axis, conditions appear to be conducive to the establishment of a population inversion with the subsequent emission of coherent soft X-rays with measurable gain. The results of numerical simulations support the notion that it is theoretically possible to achieve a population inversion and gain in three of the lasing lines provided that the appropriate plasma conditions are realized     

X-band dielectric Cerenkov maser amplifier experiment

An X-band dielectric Cerenkov maser amplifier experiment is reported. The amplifier system consisted of a solid, thermionically generated electron beam propagating through a cylindrical waveguide partially filled with an annular, dielectric liner. The input signal was provided by a tunable (9-10.3 GHz) magnetron with power up to 10 kW. Electron beam voltages and currents of up to 250 kV and 100 A could be generated for 1 μs pulse durations. The system was configured to operate in the TM₀₁ mode of the dielectric-lined waveguide. In this experiment the gain of the system with respect to the length of the dielectric liner was studied at a fixed input frequency of 10.3 GHz. At electron beam parameters of 160 kV and 60 A, a power gain of 24 dB over 56 cm of interaction length was measured for an input power of 4.5 kW, corresponding to a maximum RF amplified power of 1.15 MW and 12% efficiency     

Performance characteristics of a high-power X-bandtwo-cavity gyroklystron

The operating characteristics of a two-cavity X-band gyroklystron experiment are reported. Beam voltages and currents up to 440 kV and 200 A, respectively, are generated in 1 μs pulses by a thermionic magnetron injection gun. Velocity ratios (ν_⊥ /ν_z) near one in the output cavity are used to achieve peak powers of 24 MW near 9.87 GHz. The maximum saturated efficiency of more than 33% occurs at a beam voltage of 425 kV and a current of 150 A. A large signal gain in excess of 34 dB is realized by operating the input cavity just below the start oscillation threshold. Details of tube stability and the dependence of amplification on magnetic field profile, input signal parameters, and various beam quantities are presented     

Experimental study of a Ka-band gyrotron backward-waveoscillator

A gyrotron backward-wave oscillator (gyro-BWO) has been operated and magnetically tuned over the frequency range 27-32 GHz. Tuning by varying the electron beam voltage was effective over a smaller frequency range (Δf~1 GHz). Output power was as large as 7 kW, corresponding to a device efficiency of 19%. This large efficiency value was unexpected, and related analysis indicates it may be associated with the nonuniform magnetic field profile in the interaction region     

Development and test of an L-band magnetron

An L-band relativistic magnetron has been designed, built, and tested at approximately 1.1 GHz. The authors have obtained a peak RMS power of 2.4 GW in one waveguide, and a total of 3.6 GW from two waveguides. The overall efficiency reached 9%, compared with the input power. In contrast with previous S- and X-band magnetron results, the L-band magnetron impedance does not collapse at the end of the pulse, and microwave generation tends to last as long as high voltage is applied. This implies that large total energies can be produced by applying long high-voltage pulses     

Electrostatic ion-cyclotron wave experiments in the WVU Qmachine

The influence of transverse, localized, DC electric fields (TLEs) on the current-driven electrostatic ion-cyclotron (CDEIC) instability is being investigated in a Q machine. A small (diameter ~10 ion gyroradii) segmented disk electrode is being used to excite the mode in a narrow electron-current channel along which exists a radial electric field between regions that magnetically map to the different circular segments (separated by a radial gap of ~3 ion gyroradii). Experiments aimed at demonstrating a TLE dependence in the threshold current for mode excitation are described. A comparison of observed and predicted mode frequencies over a range of magnetic field strengths is presented for the benchmark case of no applied transverse electric field. When the electric field is present, ion-cyclotron fluctuations are observed for cases in which the current is below the CDEIC instability threshold     

A frequency-locked, high power, X-band dielectric Cerenkovmaser

A high-power frequency-locked dielectric Cerenkov maser oscillator is presented. The device consists of an annular electron beam traveling down a dielectric-lined waveguide (wall radius 1.74 cm, liner thickness 2-3 mm, linear ∈=10). A 100 kW input signal is injected into the drift tube between the diode and liner, through a rectangular TE₁₀ to cylindrical TM₀₁ mode converter. When operated with a beam current of 500 A and a total output power of 13 MW, the device displays no RF quenching through the full width of the 100 ns beam pulse. Two higher current cases with power as high as 280 MW but severe RF quenching are presented. The results are compared with linear theory, and quenching mechanisms are discussed     

Study of gain in C-band deflection cavities for afrequency-doubling magnicon amplifier

An experimental study of the gain between two half-wavelength, 5.7-GHz TM₁₁₀ mode pillbox cavities, separated by a quarter-wavelength drift space, and powered by a 170-A, 500-keV electron beam immersed in an 8.1-kG magnetic field is reported. These cavities constitute the first section of a planned multicavity deflection system, whose purpose is to spin up an electron beam to high transverse momentum (α≡υ⊥/υ_z⩾1) for injection into the output cavity of a frequency-doubling magnicon amplifier. A gain of ~15 dB was observed in the preferred circular polarization, at a frequency shift of approximately -0.18%, in the opposite circular polarization, at a frequency shift of approximately +0.06%. These results are in good agreement with theory     

High-peak power Ka-band gyrotron oscillatorexperiments with slotted and unslotted cavities

A K_a-band gyrotron oscillator powered by a compact pulseline accelerator has been operated using oscillator cavities with and without axial slots. The oscillator was operated at high voltage (~900 keV) and high current (~500 A) in the approximate frequency range of 20-50 GHz. The use of axial slots has been shown to suppress low-starting-current whispering-gallery modes, in particular, modes of the TE_m2 type, allowing stable operation in a linearly polarized TE₁₃ mode. A peak power of 35 MW has been observed at 6% efficiency     

A deconvolution technique for B-dot signals from aplasma-driven electromagnetic launcher

A novel technique is being developed to quantify the current distribution in the plasma armature of an electromagnetic launcher (EML). The technique relies on data from B-dot probes inserted above the barrel of an EML. The current distribution is found by taking the fast Fourier transform of the integral of the B-dot signal and deconvolving it with a geometry-dependent weight function. The result allows calculation of the total plasma length and total current magnitude. The author describes the signal-analysis technique, discusses results obtained from theoretical B-dot signals, and suggests possible sources of error which may be encountered when deconvolving experimental B-dot signals     

Determination of current distribution in EM gun armature by leastsquares fitting of BSi (small-induction) coilvoltage

The electromagnetic (EM) plasma rail-gun current distribution was determined by fitting (using the method of least squares) the voltage induced on small induction (B_Si) coils to a derived function model. The voltage function model was derived using the Biot-Savart equation. The model was derived assuming that the current flowed in sheets perpendicular to the rails. The sheets of currents varied in time and in the rail direction but were assumed to be constant (at a given time) perpendicular to the bore direction. The plasma current distribution in the bore direction, the initial length, and expansion characteristics of the plasma were determined from B _Si coil voltage measurements taken at short time increments. Fitted parameters correlate well with measurements taken by other sensors     

Effect of finite V⊥ on Cerenkov FEL in adielectric loaded waveguide

Allowance for finite V_⊥ of an annular electron beam propagating through a dielectric-loaded waveguide immersed in an axial magnetic field opens up the possibility of excitation of TE modes. The interaction is observed using Cerenkov and cyclotron resonances. On approximating the field distribution at the electron orbits to some suitable form, an analytical solution to the Vlasov equation is obtained, leading to a comprehensive dispersion relation for azimuthally symmetric TE_0n modes. In the special case where all the guiding centres lie on the axis, a fluid treatment for the arbitrary azimuthal-mode number is applicable. In these cases the growth rate increases with V_⊥     

The influence of an electronegative gas admixture on the intenseE-beam transport and induced plasma currents in argon

The propagation of an electron beam, with electron energy ~1.2 MeV, beam current ~8 kA, and an induced plasma current is investigated using a metal drift tube ~0.5 m long filled with argon at atmospheric pressure. The behavior of electron concentration and electron temperature was calculated for several concentrations of the SF₆ admixture. The comparison of the results of the experiments and numerical calculations shows that the dropping of plasma current when electronegative gas is added is due to the electron attachment to the SF ₆ molecules. The results of the experiment and the calculations provide the evidence of at least three mechanisms responsible for the increase of the REB transport efficiency when electronegative gas is present     

High-power X-band amplification from an overmodedthree-cavity gyroklystron with a tunable penultimate cavity

Experimental results for a 10 GHz TE₀₁ mode three-cavity gyroklystron with a tunable penultimate cavity are presented. The electron beam was produced by a pulse line modulator and a magnetron injection gun which operates to 433 kV and 225 A with 1 μs flat-top. Three-cavity circuits have produced a peak power of 27 MW with efficiency of 32% and pulse energy of 39 J. A maximum gain of 50 dB was achieved at a peak power of 20 MW, and a maximum efficiency of 37% was achieved at a peak power of 16 MW