LBNL-54918 HI FAN 1320 I n i t i a t i o n of long, free-standing z-discharges by CO2 laser gas heating C. Niemann*, A . Tauschwitz, D . Penache, S. Neff, R. Knobloch, R. Birkner, R, Presura*, D . H . H . Hoffmann Technische Universitaet Darmstadt, Schlossgartenstr. 9, 64-289 Darmstadt, Germany S.S. Y u , W . M . Sharp Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 9^720, USA sion [8,9]. As a first step experiments in Berkeley [10] as well as at GSI [11] have been started. In both cases the discharges are initiated i n metallic chambers. For this reason laser guiding of the channels is indispensable to prevent breakdown along the wall and to produce a discharge channel. Both experiments differ i n the laser- gas combination and the initiation mechanism but have very similar discharge parameters. While i n the Berke- ley experiment the discharge is guided by an UV-laser pulse creating a seed of electrons i n a gas fill of organic molecules, in the GSI experiment the gas along the chan- nel axis is heated by an infrared laser. This method was used first successfully by Olsen [2] at currents below 30 k A . Other possible laser-gas combinations are summa- rized i n [8]. High, current discharge channels can neutralize both cur- rent and space charge of very intense ion beams. Therefore they are considered as an interesting alternative for the final focus and beam transport i n a heavy ion beam fusion reac- tor. A t the G S I accelerator facility, 50 cm long, stable, free- standing discharge channels w i t h currents i n excess of 40 k A i n 2 to 25 mbar ammonia (NH3) gas are investigated for heavy ion beam transport studies. T h e discharges are initiated by a CO2 laser pulse along the channel axis before the discharge is triggered. Resonant absorption of the laser, tuned to the v% vibration of the ammonia molecule, causes strong gas heating. Subsequent expansion and rarefaction of the gas prepare the conditions for a stable discharge to fullfill the requirements for ion beam transport. This paper describes the laser-gas inter- action and the discharge initiation mechanism. W e report on the channel stability and evolution, measured by fast shutter and streak imaging techniques. The rarefaction of the laser heated gas is studied by means of a hydrocode simulation. This paper describes the laser gas heating and channel initiation i n the experiment at GSI. The channel stability and evolution are discussed to prepare the basis for the ion beam transport experiments and advanced channel diagnostics which will be reported in following papers. P A C S numbers: 52.80.Tn, 52.58.Hm, 41.85.Ja, 42.62.-b ll. EXPERIMENTAL I. I N T R O D U C T I O N SETUP The GSI laser channel experiment consists of a cylin- drical, stainless-steel discharge chamber of 50 cm length and 60 cm diameter with two high voltage electrodes on opposite sides, insulated from the chamber by plexiglas plates (figure 1). The chamber is filled with ammonia (NH3) at pressures between 2 and 25 mbar i n a steady gas flow. Before the discharge is triggered, a C O 2 laser pulse is fired into the chamber through a ZnSe entrance win- dow and a 15 mm diameter bore hole i n the anode. The laser beam leaves the chamber through a second ZnSe window in the cathode, where the energy is measured by a pyroelectric detector. W i t h a wavelength tuned to a molecular vibration of the N H 3 molecule, laser energy is efficiently coupled into the gas. This heating causes an expansion of the gas, creating a rarefaction channel along the desired path of breakdown on the chamber axis sur- rounded by a stabilizing gas wall. High current discharge channels have been investigated for a number of years as the mainline final focus scenario for light ion beam fusion [1-3]. Ballistic transport of light ion beams inside the chamber of an inertial confine- ment fusion reactor was impossible due to the high space charge of the intense ion beams. A sufficiently dense plasma can neutralize both beam space charge and cur- rent of such beams, making channel transport relatively insensitive to details of beam current, pulse shape and emittance. First experiments, based on wall stabilized [4,5] and wire initiated [6,7] discharges demonstrated ef- ficient ion beam transport over several meters. However, in a repetitively operated fusion reactor guiding struc- tures like insulators and wires inside the reactor chamber can not be used since they are destroyed in every micro explosion. Recently plasma channel transport was revived as an alternative for the final focus in heavy ion beam fu- *Electronic mail: C.Niemann@gsi.de ^present address: University of Nevada, Reno, N V 89557, USA

The final beam transport in the reactor chamber for heavy ion fusion in preformed plasma channels offers many attractive advantages compared to other transport modes. In the past few years, experiments at the Gesellschaft für Schwerionenforschung (GSI) accelerator facility have addressed the creation and investigation of discharge plasmas, designed for the transport of intense ion beams. Stable, self-standing channels of 50 cm length with currents up to 55 kA were initiated in low-pressure ammonia gas by a CO2-laser pulse along the channel axis before the discharge is triggered. The channels were characterized by several plasma diagnostics including interferometry and spectroscopy. We also present first experiments on laser-guided intersecting discharges.

High-current discharge channels are ideally suited for the focusing and transport of intense charged particle beams. The azimuthal magnetic field provides a strong focusing force, which acts symmetrically towards the discharge axis. A sufficiently dense and hot plasma can also neutralize the beam current and space charge of very intense ion beams, relevant to a number of future applications. In this paper we present experiments on high-current discharge channels designed for the transport of heavy ion beams. A spectroscopic method is introduced, which allows us to determine both the plasma temperature and density in hydrogen–nitrogen plasmas, from comparisons of the measurements with computer calculations. The temperature is derived from a comparison of experimentally obtained relative nitrogen-line intensities with a collisional radiative rate modelling of the nitrogen plasma. The electron density is determined by a detailed line shape analysis of the Stark-broadened hydrogen Balmer lines.

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 (NH3) in a metallic chamber by two perpendicular CO2-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 Ni+1258 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.

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 (NH3) gas are initiated by a CO2 laser pulse along the channel axis before the capacitor bank is triggered. Resonant absorption of the laser, tuned to the v2 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.

The aim of the presented experiments is to study the transport of a heavy ion beam in a high-current plasma channel. The discharge is initiated in NH3 gas at pressures between 2 and 20 mbar by a line-tuned CO2 laser. A stable discharge over the entire electrode gap (0.5 m) was achieved for currents up to 60 kA. Concerning the ion beam transport, the magnetic field distribution inside the plasma channel has to be known. The ion-optical properties of the plasma channel have been investigated using different species of heavy ions (C, Ni, Au, U) with 11.4 MeV/u during six runs at the Gesellschaft für Schwerionenforschungs-UNILAC linear accelerator. The high magnetic field allowed the accomplishment of one complete betatron oscillation along the discharge channel. The results obtained up to now are very promising and suggest that, by scaling the discharge gap to longer distances, the beam transport over several meters is possible with negligible losses.

For final beam transport in an IFE reactor three alternatives are mainly discussed. These are neutralized ballistic transport, serf-pinched transport, and plasma channel transport. Discharge plasma channels were investigated in the recent years at GSI Darmstadt and at LBNL Berkeley in a number of experiments. Different initiation mechanisms for gas discharges of up to 60 kA were studied and compared. In the Berkeley experiments laser ionization of organic vapors in a buffer gas was used to initiate and direct the discharge while at GSI laser gas heating and ion beam induced gas ionization were tested as initiation mechanisms. Measurements of temperature, electron density, gas density, and magnetic field distribution in the channels are compared with results of beam transport experiments at the GSI UNILAC accelerator and with MHD simulations of the ID-fluidcode CYCLOPS, which was developed in Berkeley. Good agreement between plasma diagnostics results, measured ion optical properties and MHD simulations was found. Parameters that are required for a reactor application are a discharge current of 50 kA, a channel diameter below 1 cm, a pointing stability better than 500 µm, and MHD stability for more than 10 µs. These parameters have been demonstrated in the recent experiments. The results imply that transport channels work with sufficient stability, reproducibility and ion optical properties in a wide pressure range and for various discharge gases.