Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Mon Apr 10 16:51:42 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
April 10, 1995

Alcator C-MOD continued plasma operations this week. We are presently
operating with the direction of the toroidal field and plasma current
reversed, i.e. with the ion-grad-B drift away from the divertor structure at
the bottom of the chamber. Four run days were scheduled and completed this
week.

Following configuration of the bus work for reverse field operation, and
development of a reverse-field startup scenario, physics experiments began
Tuesday with an exploration of H-mode thresholds. Low-q discharges at a
reduced field of 3.5Tesla were produced in an effort to obtain the threshold
for ohmic H-mode in lower SN plasmas with the field reversed. It was expected
that the threshold power would be higher by about a factor of 2 than in the
normal configuration, for which we typically observe transitions at
P/(nBS)~.022. No H-mode transitions were obtained, even for currents up to
.95MA (q_psi~2.5); the lower bound for the threshold in these experiments was
therefore found to be P/(nBS)>.045.

Studies of the scrape-off layer and divertor with reverse field showed
qualitative and quantitative differences from our standard results. A density
scan was carried out at 0.8MA current, and a second scan at 1.0MA was begun.
Conditions in the inner and outer divertor legs appear to have reversed. The
outer divertor is now colder (Te around 5 eV) while the inner divertor is hot
(Te around 25 eV). At sufficiently high density, a time-evolving partial
detachment (as density increased) could be observed on the inner divertor which
is remarkably similar to that observed previously on the outer divertor: as Te
drops below 5 eV the electron pressure at that probe falls - indicating
pressure loss and detachment. The puzzle now becomes: "how does the asymmetry
in inner/outer divertor leg connect to the B X gradB direction or the Ip
direction?" Trace amounts of CH4 were injected through the NINJA capillaries
on the inner midplane and outer divertor shelf. Clear C-II and C-III plumes
were seen directed towards the inner divertor. We now can say that the flow is
always directed towards the divertor, regardless of the B X gradB direction.

The dimensionless scaling experiments begun in March were continued with the
field reversed to suppress H-modes at the higher powers. These experiments are
part of a coordinated effort with DIII-D to test non-dimensional similarity in
plasmas with very different dimensional parameters. The high-power,
low-density portion of the dataset (nebar ~1.5e20) was completed, with Prf up
to 3.2MW, Ptot up to 4.7MW. The reversed field operation was successful in
keeping the plasma out of H-mode up to the highest powers tested. The higher
density dataset (nebar~3e20) still needs to be completed.

Robert Granetz presented a seminar on disruptions and halo currents at
Columbia University on Friday. Martin Greenwald presented a seminar on
transport results in C-MOD at the Courant Institute at NYU. Miklos
Porkolab, Paul Bonoli, Garry McCracken, and Jay Kesner attended the Sherwood
Conference.

Last week Stefano Bernabei and Nevelle Greenough from PPPL visited M.I.T. for
discussions related to Lower Hybrid Current Drive experiments on C-MOD.