As seen in Fig. 1(a), the diabatic signal does indeed increase as the value of n increases. In addition, both ionization features are evident at slew rates differing by a factor of 2 from that used to obtain the data in Fig. 1.When both lasers are polarized parallel to the direction of the electric field, then theoretically no | m x | = 2 can be produced and, in this case, we observe a dramatic decrease in the signal obtained at the diabatic threshold. In view of this test, and since the probability for diabatic passage in a given field slew rate is larger for \m l \ = 2 states than for | raj = lor |raj = 0 states, we conclude that the diabatic high-field peak is due primarily, if not wholly, to |raj = 2.The signal at electric fields between the adiabatic threshold and the diabatic threshold is attributed to a combination of adiabatic and diabatic passage through level crossings. As n increases, the ratio of this intermediate signal to the diabatic signal drops rapidly.These measurements demonstrate, for the first time, the passage of highly excited atoms from low electric fields to ionizing electric fields along predominantly diabatic paths. Clearly, it is important to account for the possibility of diabatic thresholds whenever one observes atomic states of high n using field ionization.A compact, toroidal configuration of magnetized plasma is produced by a combination of Z-and 0~pinch discharges. A paramagnetic toroidal field is produced by currents circulating in the plasma on closed flux surfaces.We report here our initial results on the formation of a plasma confinement configuration with the generic name of spheromak. 1 ' 2 This compact toroidal configuration has both toroidal (B^) and poloidal (B r ,B 2 ) magnetic field components with the toroidal field maintained by circulating plasma currents rather than by an external coil through the toroidal hole, as in a tokamak. Configurations of this type were first studied theoretically in an astrophysical context. 3 * 4 Related laboratory experiments involving plasma guns, 5 * 6 electron beams, 7 and pinches 8 have been performed. Our experiment, called paramagnetic spheromak (PS-1), makes use of Z-and 6 -pinch techniques to produce a prolate spheroid configuration. The results show, for the first time, that it is possible to establish the desired closed poloidal flux surfaces with a stabilizing paramagnetic toroidal field (i.e., with the peak magnitude of B(p near the magnetic axis). Figure 1 illustrates the formation phase. We start with a cylindrical deuterium gas column of radius 11.4 cm and a pressure of 15 mTorr. The column contains an axial bias magnetic field {-B z ) produced by I 9 currents 9 in an external, single-turn mirror coil with a mirror ratio of 1.1. The bias field is produced by a 20-kV, 18-\xY capacitor bank capable of producing fields with magnitude up to 8 kG. Typically fields of 4 kG are used. The field rises in 3 to 5 jusee (depending on the external inductance) and is clamped. Following this a Z-directed current shell is produced by disc...

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A new and relatively simple method has been developed to focus and guide electron beams without the use of a magnetic field. The scheme relies on the electrostatic charging of a highly resistive wire in the presence of a beam. The beam is then strongly guided and focused by the oppositely charged wire. In addition, the highly anharmonic nature of the wire potential leads to rapid phase-mix damping of transverse beam displacements and radial pulsations.

We have made detailed measurements of the timedependent beam quality of an ATA pulse transported on a KrF laser-photoionized benzene channel. We have found that the tail portion (T > 20-30 ns) of the pulse becomes severely degraded both at high currents (I > 3 kA) and at long transport distances. Non-axisymmetric channel ion motion is suspected as the predominant source of the problem. We discuss various experiments which varied laser properties (timing, fluence, profiles), background ion pressure, and electron beam current in order to study the physics behind this degradation.

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Spectral emissions of the Experimental Test Accelerator beam in 500-Torr synthetic air have been measured in the wavelength range 250–700 nm for beam currents of 4.5 and 8 kA. Intense emissions are identified as radiation from nitrogen. Relative intensities agree well with Franck–Condon factors. Wet air results in a dramatic decrease (by a factor of 2–3) in intensity over the scanned range. This effect is presently attributed to increased hose motion. Near 700 nm, emissions that are not observed in dry air, are identified as emanating from water vapor, nitric oxide, and oxygen. The pressure dependence of emitted intensities at 337.1 and 391.4 nm were measured from 80 μ to 500 Torr of nitrogen as well as dry synthetic air. Results obtained for an 8-kA beam are not explained by a time-dependent Boltzmann air chemistry code which predicted very well previous measurements for a 1–kA beam.

UCID--20264 DB85 004410The Advanced Test Accelerator (ATA) is a linear induction electron accelerator whose design parameters are 50 MeV, 10 kA and 70 ns pulse duration. The key physics issues affecting performance of the accelerator involve beam dynamics during transport through the accelerator structure.In this report, we present experimental results describing the initial operating phases of ATA. These results illustrate the complexity of the beam transport phenomena but also indicate the means to stabilize beam dynamics. Improvements in beam transport which result from deploying various stabilization techniques are also presented.