Plasma WindowOptions 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

• 10-6-10-9 Torr

• Chamber operation requires low to intermediate vacuum

• 10-3 - 10 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 P’s 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.

€

Q=πa4

8η lP P2 −P1( )

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 windowat 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 10-9 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 experienceFuel reforming using high pressure plasmas

Anode

Water outlet

Cathode

Water inlet

Air

Air/water/Naturalgas mixture

• 100 V, 12 A

• Air

• 2.5 bar to 1 bar pressure differential

• 3 mm diameter

• Single un-segmented narrow channel

Schematic diagram Discharge 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• 100-200 V, 10-30 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 50-200 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