Radio Frequency Breakdown Research Articles

RF breakdown by toroidal helicons

Bounded whistlers are well-known for their efficient plasma production capabilities in thin cylindrical tubes. In this paper we shall present their radio frequency (RF) breakdown and discharge sustaining capabilities in toroidal systems. Pulsed RF power in the electronmagnetohydrodynamic (EMHD) frequency regime is fed to the neutral background medium. After the breakdown stage, discharge is sustained by toroidal bounded whistlers. In these pulsed experiments the behaviour of the time evolution of the discharge could be studied in four distinct phases of RF breakdown, steady state attainment, decay and afterglow. In the steady state average electron density of ≈1012 per cc and average electron temperature of ≈20 eV are obtained at 10−3 mbar of argon filling pressure. Experimental results on toroidal mode structure, background effects and time evolution of the electron distribution function will be presented and their implications in understanding the breakdown mechanism are discussed.

The radiofrequency breakdown in low pressure argon

The Monte Carlo simulation technique, previously used by the author (1996 Nuovo Cimento D 18 919) for computing, in the case of an ideal gas and plane-parallel electrodes, the threshold radiofrequency (rf) voltage for breakdown versus the gas pressure in the transition region from diffusion to multipacting controlled conditions, has been applied to the case of a real gas, argon. For this purpose the Monte Carlo algorithm has been modified to include the elastic and inelastic argon cross sections for electron collisions, the finite size of the electrodes and also a surface reflection coefficient for the primary electrons impinging on the electrodes. Furthermore, the so-called null-collision method has been introduced in order to handle the velocity dependency of the electron-atom collision frequency and, for a better description of the secondary emission characteristics of the electrodes, Vaughan's empirical formula has been preferred to the previously used Dionne's formula. The modified algorithm has been used for determining the threshold breakdown voltages for the experimental set-up conditions of Hoehn et al (1997 Phys. Plasmas 4 940). With the agreement with the experimental data being satisfactory, breakdown voltage values have also been determined for low-pressure conditions of practical interest in the case of argon rf capacitive reactors. In particular, indicative limits for the existence of the discharge at very low pressures are given and the behaviours of the pressure-dependent curves, providing the breakdown voltage versus the ratio of gap width over wavelength, are discussed.

Trapped electron acceleration by a laser-driven relativistic plasma wave

THE aim of new approaches for high-energy particle acceleration1 is to push the acceleration rate beyond the limit (∼100 MeV m−1) imposed by radio-frequency breakdown in conventional accelerators. Relativistic plasma waves, having phase velocities very close to the speed of light, have been proposed2–6 as a means of accelerating charged particles, and this has recently been demonstrated7,8. Here we show that the charged particles can be trapped by relativistic plasma waves—a necessary condition for obtaining the maximum amount of energy theoretically possible for such schemes. In our experiments, plasma waves are excited in a hydrogen plasma by beats induced by two collinear laser beams, the difference in whose frequencies matches the plasma frequency. Electrons with an energy of 2 MeV are injected into the excited plasma, and the energy spectrum of the exiting electrons is analysed. We detect electrons with velocities exceeding that of the plasma wave, demonstrating that some electrons are 'trapped' by the wave potential and therefore move synchronously with the plasma wave. We observe a maximum energy gain of 28 MeV, corresponding to an acceleration rate of about 2.8 GeV m−1.

Analysis of ICRE (ω ⩽ ωci) plasma production in large scale tokamaks

The authors study the possibility of target plasma radiofrequency (RF) production in the ion cyclotron range of frequencies (ICRF) (ω ⩽ ωci) in large scale tokamaks before the startup of an Ohmic discharge. A number of experimental and theoretical studies on dense plasma production in the ICRF in toroidal magnetic devices are reviewed. The criteria for optimal development of the RF discharge stages are analysed, i.e., for RF breakdown of the neutral fill gas in the vicinity of the antenna (non-wave stage, ne << na, where ne and na are the densities of electrons and atoms), and the wave stages of initial ionization (ne < na) and neutral gas burnout (ne approximately na) in the whole volume of the plasma torus. A number of requirements for the design of the antenna system are formulated for all stages of dense plasma ICRF production. A scenario for plasma ICRF production with slot type antennas in the ITER tokamak is proposed. Numerical simulations with a 0-D transport code show that in ITER a target plasma with an electron density of approximately=3*1012 cm-3 can be produced by coupling 3-6 MW of RF power to the plasma at a frequency of approximately=3 MHz. Such a level of RF power is sufficient not only for full ionization of the neutral gas but also for heating the produced plasma to electron temperatures of approximately 80 eV

Comparison of measured and calculated SF6 breakdown in rf electric fields

Measured breakdown characteristics of SF6 at 4–14 MHz are compared with the breakdown predicted with an electron-balance model. This balance includes terms for the formation of electrons from ionization of neutral molecules and electron losses by attachment to neutral molecules, electric-field-driven fluxes, and diffusion. With model parameters obtained from existing literature on electron properties in dc electric fields, the model predicts radio frequency (rf) breakdown voltages in agreement with the experimental values. The electron balance model is also expressed in a nondimensional form to aid in generalizing rf breakdown to other gases. The electron concentration profiles formed when including the electric-field-driven fluxes are compared with the sinusoidal distribution achieved for diffusion-controlled breakdown. The success of this model suggests that glow discharges can be modeled in a similar fashion.

Radio frequency breakdown and electron confinement time in a stellarator

Radio frequency ( f=350 kHz) inductive breakdown has been studied in a toroidal stellarator. The time delay between the application of the radio frequency pulse and the buildup of detectable plasma density is measured as a function of the discharge parameters. It is shown that the confinement time of energetic electrons (∼100 eV), produced during the breakdown, may be deduced from the breakdown characteristics. The experimentally determined confinement times are compared with predictions of neoclassical transport theory and are found to have functional dependence on the discharge parameters approximately consistent with plateau scaling, but to be shorter than predicted by the neoclassical diffusion model.

Validity of diffusion theory in radio frequency breakdown in molecular gases in longitudinal magnetic field

The breakdown of a gas excited by a radio frequency voltage of frequency 5.6 MHz has been studied in a cylindrical discharge tube 7.2 cm long and 2.9 cm in dia and fitted with two internal electrodes at a distance of 2.5 cm in hydrogen, oxygen and air within the pressure range of a few microns to 2 torr in the presence of a longitudinal magnetic field varying from zero to 800 G. Experimental results indicate that the breakdown is diffusion controlled and the values of (a/P) at differentE/P values calculations obtained by Brown as well as by Kihara’s theory have been compared with (a/P) values obtained in the literature. It is concluded that the diffusion theory is also valid when the frequency of the exciting voltage is scaled down to radio frequency provided the collision frequency is much higher than the exciting frequency. The change of diffusion length in the presence of longitudinal magnetic field has been obtained from measuredE/P values and comparison with theoretical values indicates that there is quantitative agreement for small (H/P) values whereH is the magnetic field. The calculated values of pressure at which the breakdown voltage shows a minimum in the presence of magnetic field is in very good agreement with experimental values. It is concluded that in the presence of magnetic field also the loss of electrons takes place predominantly by the process of diffusion.

Radio Frequency Breakdown in Penning Geometries with Nonlinear Fields

Radio frequency breakdown in air (10−3 to 10−6 Torr) is studied analytically and experimentally for two electrode configurations which lead to nonlinear Mathieu-like differential equations for the confined electron motion. Experimental data are identified with two energy-gain mechanisms which are described as the collisional and resonance modes. Breakdown voltages are predicted from stability plots which show several resonance minima where the observed rf breakdown voltages are very low. Resonance minima predicted by a linear analysis are shifted by nonlinear effects, from which it is shown that the collisional mode is apparently triggered by excitation of the 7.7-eV level of the nitrogen molecule with subsequent electron emission from the cathode surface. The resonance mode is found to be largely surface independent.

Radio-Frequency Breakdown in a dc Parabolic Potential Field

Electrons that are confined in a Penning discharge geometry by electric and magnetic fields are observed to gain energy from a superimposed rf field. By shaping the electrodes in the form of hyperboloids of revolution the prebreakdown motion can be accurately described by the Mathieu equation. Resonances are predicted when the frequency of the electron in the electrostatic well is a multiple of half the rf frequency. In the pressure range investigated (air at 10−2 to 10−5 Torr), values as low as 40-V dc and 3-V rf were observed at the first resonance under conditions where the Penning mechanism alone required over 400-V dc. Between resonances the Mathieu equation predicts runaway energy gain at considerably higher values of the rf voltage, but breakdown is observed at values much lower than the prediction. The explanation is found in energy gain through collisions with gas atoms. The collision resets the phase of the motion and allows the electron to continually gain energy from the rf field. Breakdown curves are calculated for the regions between resonances and are found to agree with experiment.