Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Mar 5 16:56:34 1996
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
March 4, 1996

Plasma operations continued on Alcator C-MOD last week. This was the final
week of the winter campaign, and four run days were scheduled and completed,
with three extended runs lasting until 9PM. The last two days of operation
featured reversed toroidal field and current. This week was quite successful,
with good data obtained in support of six Mini-proposals.

Rho-star scaling experiments in H-mode were carried out on Monday. These
experiments, which are part of a PhD thesis research program, involve
operating at different toroidal fields with density and temperature adjusted
to give constant nustar and beta, leaving the normalized gyrosize as the
plasma parameter being varied. Earlier runs had addressed L-mode plasmas;
H-mode experiments are technically more difficult, because of the difficulty
of controlling the density. In this run, we were successful in producing an
H-mode at 2.75Tesla, using second harmonic ICRF heating, that forms a good
match in nustar and beta with H-mode shots at 5.5T. Beta-normal for these
shots was 1.4.

A 12 1/2 hour run on Tuesday was devoted to dissipative divertor studies in
high power H-mode plasmas. Neon, argon, and nitrogen were injected as
radiating impurities. Nitrogen was most successful in increasing the fraction
of radiation outside the separatrix, and in producing divertor detachment. The
main effect with Ne and Ar was to raise the core radiation. Nitrogen puffing
produced detached-divertor H-modes with good core confinement (H~1.8).
Shots from this sequence included some with the highest stored energy and
neutron rates obtained on C-MOD to date. The UV spectrum was scanned with the
McPherson spectrograph viewing the x-point region; the scan confirmed that
nitrogen was a strong radiator around the x-point, with states up to N-VII
(hydrogen-like) contributing.

A piggyback experiment on Monday's run involved the use of "killer" pellets
(gold and silver) to dissipate the plasma energy rapidly and reduce the
electromagnetic and plasma energy ending up on divertor hardware due to VDE
disruptions. These experiments are in support of ITER, which plans to use such
a technique to ameliorate disruption damage. VDE's were induced by turning off
the vertical position feedback, and gold or silver-doped polyethylene pellets
were injected using the LPI. Two attempts to inject gold pellets were
unsuccessful. Four silver-doped pellets were injected, and in one case the
pellet was found to speed up the quench and reduce halo currents.

On Wednesday, the buss work was changed to allow reverse-field and current
operation for the last two operating days of the campaign. Wednesday also
served as the maintenance day for this week. A fresh layer of boronization was
applied over Wednesday night.

Following checkout of all the coil systems and power supplies in the reverse
configuration, and a sequence of shots to optimize the startup, the first
reverse-field run was devoted to completion of the dimensionless identity
comparison with DIII-D (MP85). The plan for these experiments, a collaboration
between DIII-D and C-MOD, was to produce L-mode shots with the same values of
nustar, rhostar, and beta on the two machines (with different sizes and
fields) to test the concept of dimensionless similarity. Initial experiments
on both machines were carried out last year. The present run was intended to
remedy some discrepancies in radiated power fraction and obtain additional
profile data for the detailed comparison. The results are now being analyzed.

The first part of Friday's run involved a study of the dependence of
cross-field SOL drifts on the field direction. Previous measurements, prior to
boronization, showed there were strong in-out divertor asymmetries which
reversed with toroidal field; these were ascribed to the effects of
cross-field drift flows. Boronization raised the SOL power and changed the
atomic species mix and the radiation. The run in reverse field was carried out
for comparison with data obtained in the normal (ion grad-B drift toward the
x-point) direction. Two density scans were performed, at currents of 0.8MA and
1MA, with data being taken in the ohmic phase and at ICRF powers of 1.5 and
3MW. In addition to the edge studies, the same shots were used for an
investigation of density dependence of L-mode confinement (MP140A). H-mode
transitions were obtained for the first time on C-MOD in the reverse-field
configuration.

The second part of the run was in support of a proposed dimensionless identity
experiment to be carried out in collaboration with ASDEX-U, analogous to the
DIII-D comparison described above. The main technical difficulty in this
comparison is matching the shape, since ASDEX-U operates with a considerably
lower triangularity than is normal for C-MOD. WE succeeded in reducing the
upper triangularity to 0.25, but did not get to the nominal target shape of
delta_u=0.1; delta_l is restricted by the divertor structure to be around 0.5.
Profile scans using the HIREX were carried out with nebars of 2.5 and 2e20.
Several (unwanted) H-mode transitions were observed.

The 1995-96 winter operating campaign is now concluded. Since October 1995 we
have had 54 run days in support of 30 Mini-proposals; a total of 1090 plasmas
were produced. C-MOD is now beginning an extended maintenance and upgrade
interval. Plasma operations will resume this summer.