We consider a simple model in which a small fraction of runaway electrons produce the often-observed x-ray emission in pinches. We show the turbulent collision frequency scales as v−3, from fairly general arguments. Simple scalings result, linking the number of runaways with their energy. Fitting to a recent experiment gives a pinch field exceeding the applied field by ∼400. Future experiments can check the scaling without knowing the pinch electric field.

Physics has been the forerunner of much of modern science, but mankind supposedly lacks the courage of its convictions. In the early 1960s, steady state was in retreat and a group at Princeton University began working on implications of an earlier hot stage. String theory is mind numbing in its mathematical complexity, and predictions are very difficult. As a flashlight for showing where to step, the string theory seems useless, as it has made no prediction of a past event not anticipated by conventional, inflated Big-Bang theory.

Relativistic electron beams propagating in long plasma columns must be well focused to cause efficient plasma heating. Expansion of the beam area due to scattering lowers efficiency. We calculate the beam spreading expected from errors in the ambient magnetic field. We then include scattering from both electrostatic and magnetic waves generated by the beam itself. All these effects can be important in contemplated experimental regimes. However, it may prove possible to ’’tune’’ beam-plasma heating processes to avoid significant beam spreading.

This paper reviews the current state of understanding of the universal phenomena that high power microwave pulses are shorter than the applied electrical pulse. Higher power reduces pulse duration, limiting present-day sources to a few hundred joules. It is asked whether this limitation is fundamental, or there are means to avoid it entirely. The answer seems to be that no single mechanism is responsible. Rather, there are layers of effects which may need to be addressed separately. This paper categorizes experimental observations in terms of candidate pulse shortening mechanisms such as gap closure, primary and secondary electron bombardment of walls, and RF breakdown. Pulse shortening mechanism theory (microwave field interaction with the beam, resistive filamentation, enhanced closure, etc.) is summarized and compared to observations. Suggestions are made for additional experiments and diagnostics to help separate out causes. Finally, the means of reducing or eliminating pulse shortening are reviewed.

The authors examine qualitatively the requirement that field-particle interactions in turbulence be diffusive. The basic assumption necessary for turbulently dissipative (resonantly broadened) models is that the field-particle correlation time viewed along the particle orbit be much smaller than the field-field correlation time in the laboratory. They assume the orbit variables x(t) and v(t) are normally distribution, but the electric fields need not be. They then use simple arguments from probability theory to derive the particle propagator for a weakly turbulent plasma in a strong magnetic field. They use this propagator to find the effects of cross field diffusion on single wave pulses and plasma echoes.

Electromagnetic emission produced by a propagating electron beam in a cylindrical drift chamber can be amplified by axially reflecting screens. Radiation appears at the first and second plasma harmonics with linewidths ∼0.1 νp. Amplification scales with νp2 and lags electron-beam voltage by several hundred nanoseconds, implying that electrostatic waves moving at the electron thermal speed must traverse the resonator before amplification begins. Rotating the reflectors beyond 30° lessens amplification, suggesting a broad reflection property.