Search Results (7 total)

In low-pressure capacitive radio frequency discharges, two mechanisms of electron heating are dominant: (i) Ohmic heating due to collisions of electrons with neutrals of the background gas and (ii) stochastic heating due to momentum transfer from the oscillating boundary sheath. In this work we show by means of a nonlinear global model that the self-excitation of the plasma series resonance which arises in asymmetric capacitive discharges due to nonlinear interaction of plasma bulk and sheath significantly affects both Ohmic heating and stochastic heating. We observe that the series resonance effect increases the dissipation by factors of 2–5. We conclude that the nonlinear plasma dynamics should be taken into account in order to describe quantitatively correct electron heating in asymmetric capacitive radio frequency discharges.

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30–100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95–105 nm.

We present a method to generate a flat (large horizontal to vertical emittance ratio) electron beam suitable for linear colliders. The concept is based on a round-beam rf photoinjector with finite solenoid field at the cathode together with a special beam optics adapter. Computer simulations of this new type of beam source show that the beam quality required for a linear collider may be obtainable without the need for an electron damping ring.

We present the first observation of self-amplified spontaneous emission (SASE) in a free-electron laser (FEL) in the vacuum ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.

We consider the effect of free-streaming axion emission on numerical models for the cooling of the newly born neutron star associated with SN 1987A. We find that for an axion mass of greater than ∼10^-3 eV, axion emission shortens the duration of the expected neutrino burst so significantly that it would be inconsistent with the neutrino observations made by the Kamiokande II and Irvine-Michigan-Brookhaven detectors. However, we have not investigated the possibility that axion trapping (which should occur for masses ≥0.02 eV) sufficiently reduces axion emission so that axion masses greater than ∼2 eV would be consistent with the neutrino observations.

We numerically evaluate the axion emission rate from nucleon-nucleon, axion bremsstrahlung for arbitrary nucleon degeneracy(ies). Our numerical rates agree with analytical results previously derived in the degenerate and nondegenerate limits. While the conditions in the newly born, hot neutron star associated with SN1987A are semidegenerate, the nondegenerate, analytical rate is found to be a very good approximation (accurate to better than a factor of 2), while the degenerate, analytical rate overestimates axion emission by a factor of ∼20–100.

The stability problem for strongly inhomogeneous plasma equilibria is treated by the method of relaxation under constraints. By use of statistical mechanics, a thermodynamic potential is derived for two-dimensional plasma configurations. As a consequence of a suitable constraint the plasma carries a nonvanishing electrical current. The state variables include the average electromagnetic fields. With the aid of the second law of thermodynamics the thermodynamic potential provides a necessary and sufficient stability criterion.