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The laser-gas interaction and the discharge initiation mechanism are presented. Resonant CO2 laser heating of low-pressure ammonia gas was applied to initiate 50 cm long, free-standing discharge channels.

Experiments at the Gesellschaft für Schwerionenforschung study the transport of heavy-ion beams in plasma channels in collaboration with the Advanced Reactor Innovative Engineering Study for Inertial Fusion Energy. One important aspect of channel transport is the suppression of instabilities. The susceptibility of discharge channels for magnetohydrodynamic instabilities is studied and their growth rate is measured with a fast framing camera. The images are analyzed with a computer code, determining the growth rates of the instability during later stages of the discharge. Magnetohydrodynamical (MHD) instabilities were observed only for high pressure and even then only at late stages of the discharge. The only observed MHD modes were kink instabilities and our measurements indicate that they pose no problem for transport channels with reactor parameters. [ABSTRACT FROM AUTHOR]

Ion-beam transport in space charge neutralizing discharge channels has been proposed for the final focus and chamber transport in a heavy-ion fusion reactor. A driver scenario with two-sided target illumination requires a system of two intersecting discharges to transport beams of the same charge from opposite sides towards the fusion target. In this article we report on experiments on the creation of free-standing, intersecting high-current discharge channels. The discharges are initiated in ammonia gas (NH[SUB3]) in a metallic chamber by two perpendicular CO[SUB2]-laser beams, which resonantly heat and subsequently rarefy the gas along the laser paths before the breakdown. These low density channels guide the discharges along the predefined paths and also around the 90° angles without any mechanical guiding structures. In this way stable X-, T-, and L-shaped discharges with currents in excess of 40 kA, at pressures of a few mbar were created with a total length of 110 cm. An 11.4 A MeV [SUP58]Ni[SUP+12] beam from the UNILAC (Universal Linear Accelerator) linear accelerator was used to probe the line-integrated ion-optical properties of the central channel in a T-shaped discharge. [ABSTRACT FROM AUTHOR]

High current discharge channels can neutralize both current and space charge of very intense ion beams. Therefore, they are considered an interesting solution for final focus and beam transport in a heavy ion beam fusion reactor. At the Gesellschaft fuer Schwerionenforschung accelerator facility, 50 cm long, free-standing discharge channels were created in a 60 cm diameter metallic chamber. Discharges with currents of 45 kA in 2 to 25 mbar ammonia (NH[sub 3]) gas are initiated by a CO[sub 2] laser pulse along the channel axis before the capacitor bank is triggered. Resonant absorption of the laser, tuned to the υ[sub 2] vibration of the ammonia molecule, causes strong gas heating. Subsequent expansion and rarefaction of the gas prepare the conditions for a stable discharge to fulfill the requirements for ion beam transport. The influence of an electric prepulse on the high current discharge was investigated. This article describes the laser-gas interaction and the discharge initiation mechanism. We found that channels are magnetohydrodynamic stable up to currents of 45 kA, measured by fast shutter and streak imaging techniques. The rarefaction of the laser heated gas is studied by means of a one-dimensional Lagrangian fluid code (CYCLOPS) and is identified as the dominant initiation mechanism of the discharge. [ABSTRACT FROM AUTHOR]

The diagnostic technique to determine the neutral gas density channel created by a laser in a neutral gas is discussed. The results show a density reduction near the axis, which compares favorably with one-dimensional code calculations.