William Peter

Advances in cold cathode physics and technology

We review recent progress in the physics and technology of cold cathode electron emitters. The characteristics of emission from field emitter arrays, photocathodes, and ferroelectrics are presented, together with a summary of the understanding of the physics involved. The paper concludes with a description of L-band micropulse gun, based on secondary emission in an RF cavity. Emphasis is placed on cathode development for electron guns to drive microwave tubes and RF accelerators.

Theory of plasma injection into a magnetic field

Analytic and self‐consistent solutions for the propagation of a low‐beta, large‐gyroradius plasma across a transverse magnetic field are derived. It is shown that if ω2pi/Ω2i≫(M/m)1/2, the beam can propagate into the field by means of an E×B plasma drift. The cross‐field velocity of the plasma in this case is found to be very nearly equal to the beam injection velocity u0. The theory is discussed with reference to recent proof‐of‐principle experiments on cross‐field propagation.

Propagation of a wide ion beam into a magnetic barrier

A fully relativistic and self‐consistent Vlasov equilibrium model is presented and solved for the general problem of the motion of a neutralized ion beam in a transverse magnetic field. The radius of the beam is taken to be much larger than any characteristic length of the system so that the model is, in effect, one‐dimensional. The beam has density n0 and velocity v0 and enters a vacuous region with an externally applied transverse magnetic field B0. It is found that the distance that the ions penetrate into the barrier is determined primarily by the longitudinal electric field produced by the electrons, so that their penetration length is much less than the ion gyroradius. In the case v20/c2≪m/M−B20/16πn0Mc2 the model equations can be solved analytically. In this case the injected plasma is quasi‐neutral and the ions closely follow the electrons. The penetration length of the plasma is then the geometric mean of the electron and ion gyroradii due to the magnetic field Bc= (B20+16πn0Mv02)1/2. A discussio...

The weakly nonlinear theory of density waves in a stellar disk

Quasilinear theory is applied to the adiabatic wave-star interaction of a differentially-rotating stellar disk of a galaxy. Under the influence of growing spiral waves the velocity dispersion of stars increases and the surface mass density becomes more peaked. The resulting distortion in phase space leads to a decrease in the growth rate of the waves, and as the result the Jeans instability should end after a few rotations of the system. The theory is confirmed by N-body computer simulations.

Vacuum breakdown and surface coating of rf cavities

Electrode surface coating may play an important part in overcoming power limitations in rf cavities for voltages far above the electron multipacting limit. In such cases, the prinicpal use of the coating is not to reduce secondary emission but to isolate electrode whiskers from the cavity chamber and to serve as a trap for slow electrons. Restrictions on the layer thickness are derived theoretically, and calculated for conventional accelerator cavities.

Synchrotron radiation spectrum for galactic-sized plasma filaments

The radiation spectrum for synchrotron-emitting electrons in galactic-sized Birkeland current filaments is analyzed. It is shown that the number of filaments required to thermalize the emission spectrum to blackbody is not reduced when a non-Maxwellian electron distribution is assumed. If the cosmic background radiation (CBR) spectrum (T=2.76 K) is due to absorption and re-emission of radiation from galactic-sized current filaments, higher-order synchrotron modes are not as highly self absorbed as lower-order modes, resulting in a distortion of the blackbody curve at higher frequencies. This is especially true for a non-Maxwellian distribution of electrons for which the emission coefficient at high frequencies is shown to be significantly less than that for a Maxwellian distribution. The deviation of the CBR spectrum in the high-frequency regime may thus be derivable from actual astrophysical parameters, such as filamentary magnetic fields and electron energies in the model. >

A Generalization of the Child-Langmuir Relation for One-Dimensional Time-Dependent Diodes

The steady-state Child-Langmuir relation between current and applied voltage has been a basic principle upon which all modern diode physics has been based. With advances in pulsed power technology and diode design, new devices which operate in vastly different parameter regimes have recently become of interest. Many of these devices cannot be said to satisfy the strict requirements necessary for Child-Langmuir flow. For instance, in a recent pulsed electron device for use in high-current accelerators, the applied voltage is sinusoidal in time. In another case, development of sources for heavy ion fusion necessitates understanding of transient current oscillations when the voltage is applied abruptly. We derive the time-dependent relationship between the emitted current and time-dependent applied voltage in a nonrelativistic planar diode. The relationship is valid for arbitrary voltage shapes V(t) applied to the diode for times less than the beam-front transit time across the gap. Using this relationship, transient and time-dependent effects in the start-up phase of any nonrelativistic diode can be analyzed.

Current‐voltage relation in a time‐dependent diode

A simple time‐dependent relation between the current and voltage pulse in a one‐dimensional diode has been obtained. The relation is applicable to diodes in which the voltage or current pulse changes appreciably during the beam transit time across the diode gap. A simple application of the results to eliminating current transients in ion diodes is presented.

Hydrodynamic collimation of precessing jets

Numerical experiments of a precessing adiabatic jet similar to SS 433 are presented which display hydrodynamic collimation on a large scale as suggested by an earlier work (Eichler 1983). The differences between this work and that of Kochanel & Hawley (1990), who argued against hydrodynamic collimation, are explained

A comparison of the dielectric and plasma wakefield accelerators

A comparison of two advanced accelerator concepts, the plasma wakefield accelerator (PWA) and the dielectric wakefield accelerator (DWA), is presented. Emphasis focuses on the peak accelerating gradients and transformer ratios that can be achieved in these two devices. The effect of finite plasma size on the PWA is also analyzed. For the same cavity geometry and drive beam current, it is found that the dielectric wakefield accelerator can generate a peak field that is comparable to the field that can be generated in the plasma wakefield accelerator. Provided that ideal beam pulse shaping can be achieved, the transformer ratio for the PWA is ten times larger than that for the DWA. A change in this ideal pulse shape results in closer agreement for the transformer ratios. Given these encouraging preliminary results for the DWA it is concluded that the simplicity of employing a passive structure as an accelerating medium warrants further experimental testing of the dielectric wakefield accelerator.