12 The spectral line shapes are temperature independent up to 20 K, showing the predominance of rigid-lattice spin-spin relaxation mechanisms. Thus field-dependent phonon processes [see R. Orbach, Proc. Roy 0 Soc. London, Ser. A 264, 485 (1961)] are not importantThe use of relativistic electron or ion rings trapped in magnetic mirror fields to produce absolute-minimum-2? configurations for confining thermonuclear plasmas was originally proposed by Christofilos, 1 ' 2 and McNally. 3 As presently envisioned, rings of several hundred MeV ions will be needed for a practical fusion reactor. As recently proposed 4 such ion rings may be produced by adiabatic compression of low-energy ion rings produced either by slow pulse injection of H 2 + ions 5 or by injection of intense ion beams which have recently become available. 6 In an effort to investigate the stability of such proposed ion rings, model experiments with easier-toproduce electron rings were performed by the Astron group at Lawrence Livermore Laboratory and more recently by the relativistic-electroncoil-experiment (RECE) group at Cornell.At Lawrence Livermore Laboratory attempts to produce field-reversing electron rings by multiple-pulse injection of 6-MeV, 650-A linac pulses proved unsuccessful. 7 However, single-pulseinjection experiments using an additional toroidal field, JB e , approximately equal to the basic mirror field, B z0 , and a tank filling of 100 mTorr here. In addition, exchange coupling in similar structures has been found to be negligible [E. Bucher, H, J. . hydrogen gas led to the generation of electron rings exhibiting axial field changes, bB/B ZQ^ 40%, and overall lifetimes of several milliseconds. On the other hand, field-reversing rings (6B/B Z0 ^ 190%) were first produced at Cornell University by injection of intense 400-keV, 20-kA, 80-nsec electron beams in an Astr on-type magnetic-mirror geometry, 8 * 9 and with an annular beam injected through a magnetic cusp field. 10 The rings appeared to decay stably, with the exception of the well-known precessional mode 11 which was found to be stabilized by the image currents in the conducting walls or by adding small toroidal or quadrupole field components to the basic mirror field. 12 ' 13 Also, distortions of the applied magnetic field of up to 1% did not appear to trigger instabilities. 14 The overall ring lifetimes, however, were limited to less than 30 /xsec due to collisional diffusion 15 of the fast electrons in the background gas of several hundred millitorr of hydrogen which was needed to obtain sufficient ring trapping.The present paper reports on new experiments in which higher-energy electrons (^2.1 MeV) are injected into a new facility, RECE-Christa. In Field-reversing electron rings, exhibiting initial axial field changes 6B/B e0^ 170%, and having overall lifetimes of greater than 1300 jusec, have been generated by injection of 2.1-MeV, 30-kA electron-beam pulses into the relativistic-electron-coil-experiment (RECE)-Christa mirror field, B Z Q(Z) . These lifetimes, which are co...

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Ions or ionized neutrals released from solid surfaces by electron beam impact can be accelerated and trapped in the beam potential causing beam disruption. Experiments have been performed on the DARHT-I accelerator (1.7 kA, 19.8 MeV, 60 ns) to study this phenomenon. The beam, focused to a range of diameters, was transmitted through thin targets made of various materials. The time evolution of the beam radial profile was measured downstream of the target. For low current density, the downstream-beam radial profile was time invariant as expected for a pure electron beam. At higher current density, the downstream beam was clearly disrupted during the pulse followed by a large-amplitude transverse centroid instability. Two-dimensional calculations using the Lsp particle-in-cell code show that if the space-charge-limiting ion current is allowed to flow after the target surface temperature increases by about 400 K, the main features of the experimental observations are replicated. Three-dimensional Lsp calculations show growth of the ion hose instability at a frequency close to that observed in the experiments.

We have developed two-dimensional calorimetry with infrared imaging of beam targets to optimize and measure the energy-density distribution of intense ion beams. The technique, which measures a complete energy-density distribution on each machine firing, has been used to rapidly develop and characterize two very different beams—a 400 keV beam used to study materials processing and an 80 keV beam used for magnetic fusion diagnostics. Results of measurements, using this technique, varying the diode applied magnetic field strength and geometry, anode material type and configuration, and anode-cathode gap spacing are presented and correlated with other observations. An assessment of calorimeter errors due to target ablation is made by comparison with Faraday cup measurements and computer modeling of beam-target interactions.

Twelve experienced and seven naive male subjects with a similar Vo2max, performed an incremental work test on an electronically braked bicycle until exhaustion. High venous lactate concentrations were recorded in both groups (naive-11.74 +/- 2.43, experienced-13.96 +/- 2.36 mmol/liter). No significant relationship could be demonstrated between Vo2max, venous lactate concentration, Borg ratings of perceived exertion, and the serum cortisol response. The postexercise increase in serum cortisol was significant in both experienced (59%) and naive (138%) groups, and was significantly greater in the naive as compared to the experienced subjects. The present results indicate that the psychoendocrine response to a novel situation was a major determinant of the serum cortisol response to maximum work.