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University of Wisconsin Physics Department Department of Energy National Science Foundation

A picture of PCX plasma. The LaB6 plasma source is heated to 1400C, emitting electrons that ionize gas. Rings of permanent magnets, covered in white insulating cloth, create a multidipole magnetic field to confine the plasma. Electrode rings are inserted between the magnet rings and biased to spin the plasma.


 

Multipole Confinement

 

The PCX device is a one-meter diameter, one-meter tall stainless steel cylindrical vacuum vessel. Rings of alternating polarity permanent dipole magnets form a multi-dipole magnetic field to confine the plasma. Ions and electrons follow the magnetic field and are reflected by the mirror force at the higher field region of the cusps. The magnetic field is strong in a small region at the edge of the plasma, creating a large volume of uniform, quiescent, magnetic field free plasma.


The multidipole field is strong at the boundary but drops off quickly towards the center of the cylinder.

Each ring is made of 1.5 kG, 7/8" diameter, cylindrical boron ferrite ceramic magnets fastened with epoxy to water cooled aluminum rings. Cooling must be sufficient to keep the magnets below the approximate 250C maximum operation temperature. Each ring is electrically isolated from the plasma with a silica-based insulating cloth wrap. The magnet rings are spaced about 2" apart, allowing for insertion of probes between the rings.

Plasma Source


The plasma is ionized and heated with high energy primary electrons emitted by a heated lanthanum hexaboride cathode, which is biased with respect to a molybdenum mesh cage. Plasma density up to 1019 m-3 is expected, with 10-20 eV bulk electron temperature and 1 eV ion temperature. A coaxial circuit is used to minimize the magnetic field generated by the filament heating and bias discharge currents. An infrared thermometer measures the source temperature through a Pyrex window. The steady state plasma is continuously fueled and regulated with a gas flowmeter and software feedback loop.

Electrodes


To create rotation, electrode rings are installed in-between the alternating polarity magnet rings. The simple idea is that by applying a bias between electrodes, an electric field will cross the magnetic field and induce azimuthal ExB drift of the plasma. These edge-applied flows are expected to viscously couple to the unmagnetized region in the center of the experiment. By adjusting the potential drop between adjacent electrodes, the magnitude of the velocity can be controlled as a function of position.



Electrode rings inserted between the alternating polarity magnet rings are biased to create an electric field. Azimuthal plasma flow (of order 10 km/s) can be induced through ExB drift. 
 


We have installed the top and bottom set of electrodes first, because this gives us an early opportunity to create differential rotation by spinning the center of the plasma column. We will eventually install a center column of magnets and electrodes for better control of the inner boundary condition.

A Couette flow of plasma can be created between an inner an outer magnet/electrode stack. The top and bottom endcaps can act as a free-slip boundary condition.



Spinning the top and bottom endcaps of the plasma column would allow us to create counter-rotation between the top and bottom half. These types of flows are called Von Karman flows and have been shown to create dynamo in numerical simulations.


Machine Details


Vacuum is reached with cryogenic, turbomolecular, mechanical roughing, and dry scroll pumps. Various sized ports are available for diagnostics, plasma sources, electrical, water, and gas feedthroughs. A 20 kW deionized water cooling system has been installed for the internal permanent magnets, electrode rings, and plasma source assembly. To apply an axial magnetic field, an external Helmholtz coil will be installed.

Instrument control is achieved using National Instruments (NI) Compact Rio I/O modules and LabView software.  Data is recorded using MDSPlus, and can be accessed for real-time analysis throughout the duration of an experiment.