Plasma Window Options and Opportunities for Inertial Fusion Applications Leslie Bromberg Ady Herskovitch* MIT Plasma Science and Fusion Center ARIES meeting UCSD January 10-11, 2002 *Brookhaven National Laboratory, NY

HIBD-Chamber Vacuum Interface Heavy Ion Beam Driver requires high vacuum for operation Torr Chamber operation requires low to intermediate vacuum Torr Because of the large openings required for beam propagation, large gas throughput across the HIB final focus and the chamber exits Large vacuum pumping speeds required Not clear whether it is possible to maintain that large pressure differential with the available space for pumps.

Throughput calculations In the viscous regime (usually p > 100 mTorr), the throughput through a channel can be calculated from Dushman Here, Q is the throughput, is the gas viscosity, a is the diameter, and Ps are the pressure Increased viscosity and decreased number density results in decreased flow through the opening. If the channel is filled with a thermal plasma, both the viscosity increases and the number density decreases, decreasing the particle throughput.

Plasma Window Under certain circumstances, plasmas can function as vacuum windows. plasmas can be confined in vacuum (by electric and magnetic fields) with minimal wall contact provide increased impedance to balance large pressure differential This plasma window establishes a barrier to gas flow creating a hot plasma discharge that results in higher effective viscosity lower number density Plasma windows can separate high pressure and atmosphere high and low vacuum

Schematic of plasma window operation

Viscosity dependence on temperature g/cm s = 0.1 Pa s independent on density! For intermediate temperatures, ~ (MT ) 1/2

Plasma window diagram

Plasma window at MIT

Plasma window

Plasma window pumping at low pressure side

Plasma window parameters Limited experience with arc diameter range from 2 mm to 11 mm in diameter. Electrical power consumption scales roughly as the arc diameter 10 kW/cm of arc diameter. 7.5 kW/cm of arc diameter if venturi is used in the high pressure chamber

Plasma windows experience Best high-pressure results obtained to-date using argon as both the high- pressure, the low pressure and the arc gas. High pressure to atmospheric pressure 5 bar chamber separated from 1 bar chamber 2.85 bar absolute was isolated from 0.6 mbar The use of atmospheric arc plasmas to establish a vacuum-atmosphere interface been demonstrated 2.36-mm diameter 40- mm long arc. When coupled to a three-stage differential pumping system the background pressure of 5 x bar was reached Results recently duplicated with a 5- mm diameter 30- mm long arc. rf emission from the arc is negligible

Particle/photon transport through plasma window Transport of particles through plasma windows has been demonstrated 175 keV electron beam was transported from the vacuum into the atmosphere 2 MeV proton beam was successfully transmitted through a plasma window with negligible energy losses X-ray transmission experiments through a plasma window were performed at the National Synchrotron Light Source (NSLS) at BNL National Spallation Neutron Source and some of its experiments ad planning to use the plasma window concept 2- inch diameter plasma window is being considered for a 1-inch proton beam

Plasmatron experience Fuel reforming using high pressure plasmas Anode Water outlet Cathode Water inlet Air Air/water/Natural gas mixture 100 V, 12 A Air 2.5 bar to 1 bar pressure differential 3 mm diameter Single un- segmented narrow channel Schematic diagramDischarge in air

Plasma window scaling Power consumption seems to be proportional to plasma window diameter The higher the mass of the gas, the higher the viscosity Xe would provide reduced throughput for comparable plasma conditions The power is reduced for higher mass of gas reduced thermal conductivity Power consumption decreased by high-Z operation Power consumed reduced by decreased pressure lowered radiation losses decreased conduction losses Pumping effectiveness is due to thermal effects At low pressure, plasma has small effect on window conductance 1-10 mTorr operation results in nothermal discharges, not effective for vacuum window operation. Turning on neutral beams ion sources decrease the pressure in the chamber by about a factor of 2 (nonthermal effect due to particle extraction at high velocities).

Plasma options for plasma windows Technology has been demonstrated by use of high power arc discharges V, A Arc discharges have disadvantages Need of electrodes at both ends Electrode wear/erosion is substantial; limitation on lifetime Inductive discharges offer an alternative approach: No electrode wear Large, more uniform plasmas (temperature is flatter) Requires loop/loops around axis of plasma window However, less efficient coupling.

Plasma windows for applications to HID Demonstrated technology for intermediate pressure (in the viscous regime) minimum pressure at low pressure side is < 100 mTorr Not clear how low it can reach with different gases and different pressure at high pressure side of window Power consumption, per window, is probably on the order of W for 1 Torr operation with Xe Induction plasma may be more attractive for HID applications Electrical currents in plasma window can be effectively shut down on a microsecond time scale to allow beam to propagate, if necessary Simple preliminary experiments at MIT will explore conditions relevant to IFE