Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Mon Jun 5 10:40:12 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
June 5, 1995

Plasma operation continued on Alcator C-MOD last week. Four runs were
scheduled and substantially completed, although one run was terminated
prematurely due to a vacuum incident. The experiments included first operation
at 8 Tesla toroidal field, and ICRF heating in D-He3 plasmas; other
experiments carried out included continuation of SOL and divertor detachment
studies in the "flat plate" configuration and ICRF H-mode studies in D-H at
5.3 Tesla.

The first operation of C-MOD at 8.0 Tesla was achieved on Tuesday. The TF was
raised shot-by-shot from our previous maximum of 6.5 T in small increments, to
evaluate performance at each level. A nonlinearity in the TF regulator
behavior at about the 7.5T level was noted and fixed. The field was then
raised to 8.0 Tesla, and the plasma current increased to 1 MA. Initial heating
experiments with He3 minority were begun. Operation at the increased field was
found to be reliable, with all systems performing nominally. Cooldown times
between shots were about 25 minutes, slightly longer than anticipated.

The D-He3 heating experiments were continued on Wednesday, at a field of 7.9
Tesla. A scan of He3 concentration was carried out at a density of 1.6e20/m3
and a current of 1MA. The observed heating was fairly insensitive to He3
concentration in the intermediate range of He3 concentration (n_He3/n_e <
10%, as estimated by spectroscopic diagnostics).

Wednesday's run was terminated somewhat early when a gusset protection tile
broke loose from the top of the machine and fell onto the lower TCI
interferometer window, resulting in a crack which breached vacuum. Although
the gate valve over the window closed automatically, isolating the leaking
window, machine cleanliness was degraded enough that operation could not be
immediately continued. Overnight discharge cleaning (ECDC) was carried out
and Thursday's experiment was run as scheduled, except that density feedback
employed the visible bremsstrahlung signal rather than the interferometer. The
interferometer window was successfully replaced Thursday night, and the TCI
was back in operation on Friday. The design of the Mo gusset tiles, which
protect these stainless steel structures from disruption damage, is being
reviewed, and we anticipate modifications will be implemented during the next
maintenance period.

The run on Wednesday continued investigation of the "flat plate" or "open"
divertor configuration, in which the outer strike point is positioned above
the nose on the upper surface of the divertor structure. These experiments
were carried out at 1MA and 5.3 Tesla. Again pressure detachment of the
outer divertor was not observed even at the highest densities, ne_0>3.5e20/m3.
At these central densities the electron temperature at the plate went as low
as 3eV, with a local density up to 2-3e21/m3.

Friday's run was a continuation of the ICRF H-mode experiments, using
H-minority heating at 5.3 Tesla and 1MA plasma currents, with ICRF power of
2.5 - 3MW. The experiments were designed to study the relationship between
triangularity and H-mode quality and ELM behavior. The target equilibria were
SN diverted, typically in the slot geometry, i.e. with the outer separatrix
terminating on the floor tiles rather than the inclined plate. This
configuration enables us to maximize the lower triangularity. It was found
that midplane neutral pressure was the dominant factor correlating with H-mode
behavior in these discharges, and that the midplane pressure was strongly
correlated with how much of the scrapeoff flux maps from the outer to inner
target plate. The higher upper triangularity cases in this sequence became
close to double null, with more of the SOL flux connecting to the upper
surface; these had higher midplane neutral pressure and lower quality H-modes
or remained in L-mode.

Drs. Real Decoste and Francois Martin from Tokamak de Varennes visited MIT on
Thursday and Friday. Discussions were held with the C-MOD divertor and
plasma control groups on topics of mutual interest. Dr. Decoste presented a
seminar on Divertor Studies in TdeV.

Upcoming events: Next week will be the final operating week of the current
campaign. Alcator C-MOD will then enter a scheduled maintenance and upgrade
period lasting through the summer. Major new systems to be installed during
this time include a divertor Thomson scattering diagnostic (in collaboration
with PPPL and PSI), and a prototype divertor cryo-pump. Plasma operation will
resume in the fall.