Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Wed Feb 28 12:08:14 1996
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Feb 26, 1996

Plasma operations continued on Alcator C-MOD last week. This was a difficult
week, with technical problems resulting in the effective cancellation of three
of the four scheduled run days. An extended run (12 1/2 hours) was carried out
successfully on Friday, allowing most of the content of two of the scheduled
experiments to be accomplished. The first 8 hours of this run was devoted to
research in support of two graduate student theses.

Operation on Friday of the previous week had been curtailed because of the
failure of a breaker feeding 13.8kV AC power to the poloidal field supplies.
The burned out motor on this breaker was replaced on Monday, and operation
scheduled to resume on Tuesday. However, the pf breaker still was not
functioning correctly. An unrelated problem with the pulsed gas feed system
was successfully debugged and fixed. On Thursday, the PF breaker was removed
from its cabinet and replaced by a breaker from one of the toroidal field
supplies. This enabled us to operate at reduced field while awaiting repair of
the old PF breaker. Since the run scheduled for Friday, already rescheduled from
earlier in the week, called for fields of 3.4T and below, this was an
acceptable option. Our standard technique for running low-field plasmas is to
initiate the discharge at high field, e.g. 4.8T, and ramp the TF down to the
desired operating point. Since this option was not available with only two TF
cabinets operating, we reverted to a low-field startup scenario, which
required some setup and development, which was carried out between 7:30 and
9PM on Thursday.

Friday's run began with experiments in support of MP#141, RF edge effects. The
purpose of these experiments was to study the thresholds and saturation
levels of edge ion heating and RF decay waves during Parametric Decay
Instabilities in ICRH plasmas. This research is involved in the thesis
projects of two Ph.D. students. Principal diagnostics included the PCX neutral
particle analyzer and RF probes, including a newly installed scanning probe.
Experiments were carried out at 3.4T and 2.8T, with the RF power being varied in
steps from 0.1 to 0.8MW. It was felt that the studies would be best carried out
in L-mode, but H-modes occurred on many of these shots. Nevertheless, useful
data was obtained.

The breaker which had been removed from the power room was inspected and
repaired onsite by an outside contractor during Friday morning. This breaker
was replaced in the power room, and confirmed to be operating nominally, and
the run was continued, with full-field capability restored.

At 5PM a new experimental sequence was begun, to study access to the
ERS mode and to pursue our studies of PEP modes, which involve related
phenomena. These experiments were carried out at a toroidal field of 5.3T. ERS
experiments were carried out during the current rise portion of the discharge,
with ICRF power being applied as early as 50msec after initiation. While the
onset of sawteeth was somewhat delayed and electron temperatures above 5keV
were produced during the current rise phase, no clear signatures of ERS mode
were observed. Li pellets were injected into the current rise plasmas, both to
try to diagnose the current density profile and to induce early PEP mode
effects. However, penetration of even the largest Li pellets was limited
because of the high electron temperatures, and none reached the axis. Further
experiments along these lines will try to use multiple, closely spaced pellets
to improve penetration.

PEP mode studies were also carried out in the later portion of the same
discharges. Li pellets were injected into an ohmic target at 0.8sec, about
25msec before application of ICRF heating. Laser blow-off impurity injection
was employed to determine medium z impurity particle confinement, which turned
out to be essentially infinite in the PEP mode phase. Varying amounts of ICRF
power were employed following the pellet injection, and the duration of the
high central confinement PEP mode was found to be extended at lower injected
power levels.

Bob Pinsker from GA was visiting Alcator this week, and participated in the RF
edge studies and the ERS/PEP mode experiments.

Next week will be the last plasma operations week of the current Alcator C-MOD
campaign.