From: WOLFE@CMOD.PFC.MIT.EDU
Date: Mon, 23 Feb 1998 18:51:35 -0500
Reply-To: WOLFE@PSFC.MIT.EDU
Message-ID: <980223185135.25400767@CMOD.PFC.MIT.EDU>
Subject: ALcator C-Mod Weekly Highlights
Organization: MIT Plasma Fusion Center
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
Feb 23, 1998

Plasma operations continued last week on Alcator C-Mod. Four run days were
scheduled; three were completed successfully, and one had to be re-directed
due to instrumentation difficulties. Over 100 plasma shots were produced.

A fresh boronization was carried out over the weekend. Tuesday's run began
with a series of 800 kA fiducial shots to evaluate wall conditions and recover
reliable startups. Reproducible operation was established more quickly than in
past weeks. Several 1MA shots were then produced, with performance comparable
to similar discharges last week, indicating good wall conditions.

The remainder of Tuesday's run was devoted to H-mode confinement scaling
studies (MP#155). A power scan and part of an elongation scan were carried
out. Most of the afternoon was dedicated to a power scan in EDA H-mode. We
got from 2.0 to 4.1 MW of total power. Stored energy increased faster than
sqrt(ptot) and ranged from 0.13 to 0.23 MJ. At the lowest powers the
discharges became less EDA like (more like ELMfree) as determined from Halpha,
xray pedestal width, density rise, and impurity accumulation. Since EDA has
generally been found to have slightly lower confinement, this is in the
opposite direction to explain the somewhat surprising power scan results. In
the course of these experiments we achieved record stored energies, normalized
beta, and neutron rate: Wmhd = 0.233 MJ, H = 2.4, betaNormal = 1.6, Rdd =
1.5e14, prad/ptot ~ .3. Core conditions were quite similar to our best
previous results, though the neutral/divertor behavior is still different.

We were able to perform a modest kappa scan - from 1.5-1.7. No change in
confinement was observed. Because of the small range of the scan, we can rule
out only a very strong dependence on kappa.

Wednesday's run was in support of MP#189, "D(H) Minority and Mode Conversion
Heating". In recent experiments, the H/H+D ratio (as measured by passive charge
exchange) has been hovering around 2% and the total RF power absorbed to
injected RF power (Pabs/Pinj), measured by the discontinuity in the stored
plasma energy at an RF transition, has been typically 0.6-0.7 and sometimes
as low as 0.5. This is lower than in previous run campaigns where Pabs/Pinj
was 0.8-0.9. As part of MP189, the H/H+D was scanned from low to high and
high to low to determine the dependence of Pabs/Pinj on H concentration.
Concentrations between 5-10% have Pabs/Pinj of 0.8-0.9 and possibly higher
central heating power. Higher central heating power is supported by the
observation of sawtooth frequency locking with RF modulation and higher
central electron and ion temperatures. Sawtooth reheat analysis should allow
a direct comparison. At higher concentrations, the RF became more difficult
to operate and some evidence of mode conversion was observed. The operational
problems are thought to stem from stronger reflections from the cutoff layer.
This is supported by measurements from the inner wall Bdot probes opposite the
antennas. The Bdot signal decreases with increasing H concentration.
Evidence of D(H) mode conversion was observed for H/H+D~20-30%. Further
analysis is required for quantitative conclusions.

The schedule called for Thursday's run to be devoted to MP#190 "Off-axis Mode
Conversion Electron Heating & ITB Formation in D(He3) at ~8.0 T". The ultimate
objective of this mini-proposal is to produce an internal transport barrier
(ITB) triggered by MEASURED poloidal shear flows driven by mode converted ion
Bernstein waves (IBW). After only a few 8 tesla shots, it was determined that
the instrumentation which monitors the resistance of each segment of the TF
magnet was not functioning correctly. Since our procedures require that these
resistances be monitored during high-field operation, it was necessary to
terminate the 8T experiments for the day. The remainder of the run was then
devoted to further investigation of the effect of H concentration on Pabs/Pinj
at 5.4T. The results of H concentration scans from both experiments suggests
the H concentration is below the optimum under our normal operating
conditions. Increasing the hydrogen concentration slightly improves
Pabs/Pinj, but controlling the H fraction is difficult. Several options are
being explored. MP#190 has been rescheduled to next month.

Friday's run was the second in support of MP#166, an investigation of Density
Limits. This phase of the experiment employed deuterium pellet
injection. Ohmic and RF L-mode discharges were studied at 0.6, 0.8 and 1.0MA
currents. Starting from a gas-fueled target density of about 1.5e20/m3, the
density was raised by injecting a series of D2 pellets. From 6 to 12 pellets
were injected on each shot. This turns out to be a rather difficult way of
studying the density limit. Convincing looking scenarios were obtained at 0.6
and 0.8 MA. We didn't quite have enough time to optimize at 1 MA. If the
pellets are injected too aggressively, we get extremely high densities >
10^21, highly peaked, P-mode/PEP mode type discharges. Additional pellets
will then cause disruptions. Impurity accumulation in the enhanced
confinement mode may be partly responsible. If we back off too much, the
plasma is too hot and pellet penetration is too shallow. We see good mass
accountability, but the density pumps out in as little as 5 msec. The RF
coupling is also sensitive; agressive pellet fueling can cause trips in the
RF. If these trips last too long, the plasma will cool and subsequent pellets
will penetrate through the axis. Typically, the plasma can take at most one of
these - additional pellets usually cause disruptions.

It seems always possible to exceed the empirical (Greenwald) limit transiently
with one or two big pellets. Exceeding it in quasi-steady state was not
possible in this run. On several shots, the pellet fueled density buildup
allowed us to gradually probe the limit. At 0.6 and 0.8 MA the plasma disrupted
about 10% below the empirical limit.

We had lots of misc. pellet phenomena - P-modes, PEP, snakes, MHD.
The early ohmic shots showed a 50% improvement in global confinement
during the P-mode.

Several weak H-modes were achieved during multiple deuterium pellet injection
with target densities above 4 x 10^20 m^-3 even with just 1 MW of ICRF
heating. Although they were weak, clear edge pedestals were observed on both
the edge soft x ray arrays and on the edge ECE electron temperature
measurements. In addition, large amplitude (several Gauss), low frequency (1
- 2 kHz), n=1 MHD oscillations driven unstable by the pellets were observed
during both H and L-mode phases of these discharges. During the L-mode
phases, the oscillations rotate in the electron diamagnetic direction, but
during the H-mode phases, the oscillations rotate in the ion direction.

MP 189 and MP190 were done in collaboration with Cynthia Phillips, Gerd
Schilling, and Randy Wilson of PPPL, who were visiting last week during these
experiments. In addition to participating in experimental operations and
analysis, Dr. Phillips and Dr. Wilson discussed the theory related to RF
driven flows with Paul Bonoli. Further theoretical and experimental work
is planned.

Gary Taylor of PPPL visited last week, and continued commisioning and
operation of the new 19-channel ECE diagnostic (GPC2). This was sucessful;
18 channels are now giving good signal to noise. He was able to measure
Te response during the D(H) modulation experiments and will be able to
carry out analysis of RF deposition with more detail than was previously
possible. Some further optimization of filters will be necessary for
edge measurements.

Chris Watts of Auburn Univ. visited 2/12-14, helping with the heterodyne
radiometer. This is also making good progress, and correlation as well
as edge profile measurements are now being made with the o-mode diagnostic.

With respect to the DNB, we completed tests and alignment of the electronics
in the thermocouple/SEM system that will be used to monitor the beam profile.
Fabrication and installation of the fast isolation amps for the system were
completed. Following integration of these two systems, the beam profile
monitor will be complete. Refurbishment of the control system for the Accel
supply, and preparation for conditioning of the mod/reg transformer oil
continued.

On Friday, Earl Marmar attended a meeting of the Fusion Physics Planning
Committee, which discusses U.S.-Japan bilateral fusion physics exchanges. The
meeting was held at the Princeton Plasma Physics Laboratory.

This week is also a plasma operations week. Four run days are scheduled.