53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011;
Salt Lake City, Utah
Session JI2: 3D Equilibrium, Stability and Control
2:00 PM–5:00 PM,
Tuesday, November 15, 2011
Room: Ballroom BD
Chair: Mike Mauel, Columbia University
Abstract ID: BAPS.2011.DPP.JI2.1
Abstract: JI2.00001 : Stabilization of the Resistive Wall Mode and Error Field Reduction by a Rotating Conducting Wall*
2:00 PM–2:30 PM
Preview Abstract
Abstract
Author:
Carlos Paz-Soldan
(University of Wisconsin-Madison)
The hypothesis that the Resistive Wall Mode (RWM) can be
stabilized by high-speed differentially-rotating conducting
walls
is tested in a linear device. This geometry allows the use of
cylindrical solid metal walls, whereas a torus would require a
flowing liquid metal. Experiments over the past year have for
the
first time explored RWM stability with a rotating copper wall
capable of achieving speeds ($r \Omega_{w}$) of up to 280 km/h,
equivalent to a magnetic Reynolds number ($R_m$) of 5. The main
results are: 1) Wall rotation increases the stability window of
the RWM, allowing $\approx$ 25\% more plasma current ($I_p$) at
$R_m$ = 5 while maintaining MHD stability. 2) Error field
reduction below a critical value allows the observation of
initial mode rotation, followed by braking, wall-locking, and
subsequent faster growth. 3) Locking is found to depend on the
direction of wall rotation ($\hat{\Omega}_{w}$) with respect to
the intrinsic plasma rotation, with locking to both the static
wall (vacuum vessel) and rotating wall observed. Additionally,
indirect effects on RWM stability are observed via the effect of
wall rotation on device error fields. Wall rotation shields
locking error fields, which reduces the braking torque and
inhibits mode-locking. The linear superposition of error fields
from guide field ($B_z$) solenoid misalignments and
current-carrying leads is also shown to break symmetry in
$\hat{\Omega}_{w}$, with one direction causing stronger error
fields and earlier locking irrespective of plasma flow. Vacuum
field measurements further show that rotation decreases the
error
field penetration time and advects the field to a different
orientation, as predicted by theory. Experiments are conducted
on the Rotating Wall Machine, a 1.2 m long and 16 cm diameter
screw-pinch with $B_z \approx 500$ G, where hollow-cathode
injectors are biased to source up to 7 kA of $I_p$, exciting
current-driven RWMs. MHD activity is measured through 120 edge
$B_{r}$, $B_{\theta}$, $B_z$ probes as well as internal Bdot,
Langmuir and Mach probes. RWM eigenfunctions are found to be
skewed towards the anode end, likely due to anode-directed axial
flows measured to be $\approx$ 6 km/s. Eigenfunctions also
illustrate increased helicity at higher $I_p$ and helicity is
reversed with $B_z$, while wall counter-rotation is found to
reduce mode helicity.
*Supported by Department of Energy grant \#DE-FG02-00ER54603, National Science Foundation grant \#0903900, and the Natural Sciences and Engineering Research Council of Canada.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.DPP.JI2.1