Plasma Physics Seminar ( Phys 769)

Prof. R.F. Ellis and the MCX Team
IREAP, University of Maryland

Latest Results on Stable Supersonic Rotation in the Maryland Centrifugal Experiment

Tokamaks undergo spontaneous transitions from poorly confined states to well-confined.  The Maryland Centrifugal Experiment(MCX) studies
enhanced confinement and stability produced by supersonic rotation about a linear confining magnetic field. MCX has a mirror geometry of
2.6m length, mirror ratio 2-20,maximum mirror field 1.9T, maximum midplane field 0.5T. Biasing of an inner electrode relative to the outer wall
produces a radial electric field which drives azimuthal rotation. MCX has achieved high density (n>1020 m-3) fully ionized plasmas rotating
supersonically with velocities of ~100 km/sec for times exceeding 8 ms under a wide range of conditions. Ion temperatures are 30 eV and
confinement times ~100 microseconds. Sonic mach numbers are 1-2 and Alfven mach numbers somewhat less than 1 for standard discharges.
We have recently increased the MCX pulse length by increasing the capacity of the driving capacitor bank by a factor of more than five and we
find that the plasma remains grossly stable for many milliseconds, much longer than MHD instability timescales. In particular, the voltage across
the plasma remains unusually constant during a discharge. Ion density, confinement time, and radial plasma voltage drop increase with
magnetic field strength; rotation velocity saturates at higher B. Best performance requires mirror ratios of 5 or greater. Most recently MCX
has demonstrated a transient( a few milliseconds )  enhanced mode of operation( HR mode) with sonic mach numbers greater than 3, larger
stored energy,  and confinement times of several hundred microseconds.  Data will be presented on the parameters of this mode and the
transition to the standard mode( SO mode ). New diagnostics include diamagnetic loops to measure plasma pressure, a new optical
spectrometer setup for improved Doppler rotation measurements, and a freewheeling capacitor bank, which allows us to directly measure
the rotational angular momentum confinement time. We will also discuss a possible upgrade path for MCX.