Theoretical self-consistent relativistic electron beam models are developed which allow the propagation of relativistic electron fluxes in excess of the Alfven-La.wson critical-current limit for a fully neutralized beam. Development of a simple, fully relativistic, self-consistent equilibrium is described which can carry arbitrarily large currents at or near complete electrostatic neutralization. A discussion of a model for magnetic neutralization is presented wherein it is shown that large numbers of electrons from a background plasma are counterstreaming slowly within the beam so that the net current density in the system, and therefore, the magnetic field, is nearly zero. A solution of an initial-value problem for a beam-plasma system is given which indicates that magnetic neutralization can be expected to occur for plasma densities that a.re large compared with beam densities. It is found that the application of a strong axial magnetic field to a uniform beam allows propagation regardless of the magnitude of the beam current. Some comparisons are made with recent experimental data.

Calibrated density measurements have been obtained of the coronal plasmas around exploding 7.5-40 m W wires carrying 15-120 kA per wire for 30-70 ns. X-ray radiographs of the exploding wire plasmas using 2.5-10 keV photons from a Mo wire X-pinch backlighter enabled measurements of areal densities of W ranging from 2ϫ10 17 /cm 2 , equivalent to 0.03 m of solid density W, to about 10 19 /cm 2 . The rapidly expanding ͑few mm/s͒ coronal plasmas surrounding the slowly expanding ͑Ͻ1 mm/s͒ residual wire cores have areal densities up to about 2ϫ10 18 /cm 2 . Single 7.5 m wires tested with 100 kA had as much as 90% of the initial wire material in the coronal plasma. Coronal plasma W number densities were estimated to be up to a few times 10 18 /cm 3 , while core W densities as low as a few times 10 20 /cm 3 were observed. With linear arrays of four ͑eight͒ 7.5 m wires carrying 30 kA ͑15 kA͒/wire, up to 35% ͑25%͒ of the initial W wire material was in the plasma around and between the wires at 46-48 ns after the current started. Preheating the wires to drive off adsorbed gases and hydrocarbons increased the W mass in the coronal plasma and made it more uniform then when wires were not preheated.

The dynamics of the dense plasma near the cross point of an X pinch has been investigated using 1 ns x-ray backlighting images at different moments relative to the start of 100 ns [full width at half maximum (FWHM)] 200 kA current pulses. If the two metal wires are fine enough (e.g., 10 μm W or 17.5 μm Mo) to form a pinch at the cross point, accompanied by an x-ray burst, with the available current pulse, then the images show three stages of development: a radial explosion/expansion phase; an implosion during which a dense Z pinch of 200–300 μm length forms at the cross point together with plasma jets which move axially away from that point; and a breaking up of the Z pinch, coincident in time with one or two x-ray bursts, after which a 300 μm gap opens up. For W, the backlighter minimum sensitivity is 1017/cm2 areal density, and the dense Z pinch is estimated to have a volume density close to 1021/cm3. Shock waves appear to be expanding at about 50 μm/ns from the end points of the collapsing Z pinch, where the plasma was the most dense.

Using an X-pinch configuration, we have determined that micropinches produced by exploding-wire z pinches can have densities approaching solid density and temperatures of 0.5-1.8 keV, depending upon the wire material used. These plasma parameters, determined from x-ray spectra recorded using an x-ray streak camera, vary drastically on time scales ranging from <10 to 100 ps. Computer simulations require radiation loss to reproduce the observed plasma implosion, suggesting that a radiative-collapse hypothesis for micropinch plasma formation may be correct.

There has been considerable interest recently in the production of high-power microwaves by the pulsed intense relativistic electron beams which have become available in the last few years. 1 * 2 Experimental observations of high-power microwaves have been made in a variety of beam configurations, including beams injected into a few hundred milliTorr of neutral gas, 3 and magnetically focused annular beams propagating in vacuum (<10" 3 Torr) in the presence of special metal boundaries 4 ' 5 and magnetic field perturbations. 6 Theoretical models have been developed, and calculations have been carried out to try to explain the observations, 7 " 10 but they have been hampered by an incomplete knowledge of the properties of the electron beam. In the present study, we control the transverse energy of beam electrons by varying the static spatial magnetic compression to which the beam, propagating in a straight, metallic drift-tube wave guide, is subjected. We find that if this transverse energy exceeds a certain minimum value, microwave power in excess of that possible by a single-particle mechanism is obtained. Moreover, we find this to be a characteristic of microwave production using the perturbed magnetic-field configuration. A theoret-7 R. to be published, for even stronger cluster properties in the low-fugacity region.ical model based on an interaction between the e observed wave-guide mode and the electron beam gives unstable waves which agree well with observations.The experimental configuration used for the present study is shown schematically in Fig. 1. The electron beam is produced by applying a highvoltage pulse from a 7-12, 50-nsec pulse-forming line to a foilless diode. 11 The diode voltage and current are 350-650 kV and 10-25 kA, respectively. The beam propagates in a 4.7-cm-i.d., thin-walled, stainless-steel drift tube immersed in a quasistatic (10-msec risetime) magnetic I field applied coaxially to the drift tube by a 22-cmdiam, 1-m-long solenoid. By varying the distance d in Fig. 1 between the cathode and the end of the solenoid from -2 cm (i.e., cathode 2 cm inside the coil) to 18 cm, the magnetic field near the cathode relative to that in the middle of the coil is varied from -0.55 to -0.1. Thus, the distance d controls the magnitude of the radial component of the magnetic field near the cathode. The interaction of the beam electrons with the radial field produces the desired transverse energy. Lucite witness plates obtained in the middle of the solenoid for d --2 and 1 cm are shown on the right-The role of the transverse energy of a magnetically focused intense relativistic electron beam in the emission of microwaves is investigated experimentally and theoretic ally " 752