From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly HIghlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
Dec 15, 2003

Research operations continued at Alcator C-Mod last week. Four run days were
scheduled and completed. Experiments were carried out in support of RF
Physics, Burning Plasma Support and Advanced Tokamak research programs.
Progress also continued on the Lower Hybrid Project.

Plasma operations are planned to continue this week.

Operations
----------

Four days of experimental operation were carried out. A total of 118 plasma
discharges were produced with a startup reliability of 93%. In general, all
tokamak systems performed well this week, resulting in an average of nearly 30
successful plasmas per day, the highest production of the present
campaign. Computer and data system reliability were considerably improved,
although a server crash on Thursday did result in about a one hour delay.

Runs on Tuesday and Wednesday were devoted to MP#376, "Study of the Efficiency
of ICRF Minority Heating and Investigation of the Cause of Low H-mode Plasma
Performance". Scans of plasma shape, antenna-plasma gap, density, current,
and power were carried out in an effort to identify parameters which impact
ICRF efficiency and plasma performance in H-mode discharges. Comparisons were
also made between the J-port and D- and E-port antennas in H-mode. All the
experiments were carried out using H minority heating in either D or He
majority plasmas at 5.2-5.4 tesla (f=78-80.5 MHz). The best plasma
performance during these experiments, carried out on the second and third day
following boronization, was among the best of the past several years, with a
number of cases achieving stored energy above 200kJ. The plasma response was
fairly insensitive to outer gap and upper and lower triangularity. In
addition the plasma confinement improved with plasma current. The increase in
stored energy was nearly linear with RF power in H-mode. Degradation in
performance during a given run day was observed, as well as some degradation
from day to day, as indicated by comparison of fiducial discharges with rf
power up to 5MW.

Thursday's run was the first day devoted to MP#377, "Energy confinement in
double-null discharges". This MP forms part of the experimental program in
support of our Level 1 Milestone SC6-1b, "Compare energy confinement, H-mode
threshold and divertor particle dynamics in SN, DN and inner-wall limited
discharges in Alcator C-Mod". Dale Meade (PPPL) was at MIT to serve as
co-Session Leader for this run. The experiments concentrated on comparison of
double and near-double null discharges at moderate density (n/n_G~0.4) and
currents of 1 and 1.2MA (q95~4.1,3.4) with lower single null plasmas at the
same parameters; ICRF power was ~4.5MW on all shots. A small (~10%)
improvement in confinement was observed in the DN cases, especially at the
higher current. In all cases on this day we obtained multiple H-modes, with
apparent increases in radiated power during the H-phase. There were
indications that the effectiveness of the boronization was substantially
reduced relative to earlier in the week.

Friday's run was in support of MP#374, "ITB dependence on triangularity". The
purpose of this experiment is to explore the influence of the plasma
triangularity on the ITB quality, and specifically to see if obtaining an ITB
is easier, and if higher densities and better confinement can be achieved by
varying the shape. The ITB's were formed by use of off-axis (high-field side)
ICRF heating with Bt=4.5T, using up to 3MW of RF power. As in previous
experiments of this sort, the ITB evolves from an EDA H-mode and is
characterized by a strong peaking of the density and pressure profiles. Good
ITB discharges were obtained over a range of upper and lower triangularity in
lower SN discharges, and also in well-balanced DN discharges. We were able to
obtain ITBs with up to 3MW of ICRF power in double null operation. Access to
ITB's appeared to be easier with larger values of lower triangularity
(delta_L), but further analysis of the data is required. In contrast to the
higher power experiments of the previous day, and to the previous run devoted
to this ITB MP, radiated power was not excessive, and the EDA H-mode targets were
readily achieved.

ICRF System
-----------

The ICRF Systems were employed for plasma heating in all four runs this week.
Early in the week, FMIT#3 required additional tuning due to high FPA screen
current. Optimizing the tune allowed operation to resume up to nominal full
power into plasma (1.5 MW). On Thursday, the FMIT#2 deionized water exchange
valve stuck open resulting in a low level alarm in the C-Mod main DI water.
We will install additional monitoring to ensure an overflow is detected, and
this valve will be replaced.

In comparative experiments it was noted that, although the plasma response in
L-mode was similar for the D&E-port and J-port antennas, an increase in the
rate of impurity accumulation with the J antenna was identified in H-mode.
These experiments were done at higher plasma current which also increased the
impurity confinement time. With the increased impurity confinement time,
small differences in the impurity production rate were more obvious. Further
investigation of the phase dependence of the impurity production rate need to
be investigated to provide some data on whether this difference is attributable
to basic antenna design or antenna phasing (J-port is [0,pi,pi,0] compared to
D and E-port which are [0,pi]).

Clean RF operation required lowering the F-side neutral pressure to <0.3mTorr,
which imposes a restriction on the maximum density of the target discharge.
Operation has been achieved at higher neutral pressures but the antennas are
more likely to fault, particularly J-port.

During an upward-biassed (SSEP>0) near-double null discharge on 12/11, a 65
msec sawtooth period was observed . "Monster" sawteeth (period >50 msec) have
recently been observed on several other upper single null discharges. In this
case a 5.6 keV central electron temperature was obtained. A phase scan with
the J-port antenna may give information on what is causing the long period
sawteeth.

DNB Systems
------------

The DNB is now back in full operation. Beam performance this week has been the
best since this beam system was installed. The beam has been very reliable and
is delivering 5A, 50 ms, 48 kV beam shots with less than 10% change in beam
current during the shot. The spurious faults that had reduced the reliability
of the high voltage beam systems have been eliminated. Changes included
improved high voltage power supply grounds, repair of one non-bonded fault
indication wire, and improvement of power supply filtering in the inverter
modules.

Lower Hybrid System
-------------------

MIT Engineers carried out a final inspection of the Forward Waveguide assembly
at PPPL on Wednesday. The inspection went well, low-power RF testing has now
been completed, and final fitup to the port extension is in progress.

Travel and Visitors
-------------------

David Mikkelsen (PPPL) was at MIT this week learning from Nils Basse about the
C-Mod reflectometer systems and their data analysis procedures. Discussions
with Nils, Steve Wolfe and Steve Scott produced several good leads for
developing experiments in C-Mod to probe turbulent transport.

Dale Meade was at MIT 12/10-11 and served as co-Session Leader during
Thursday's experiment. Gerd Schilling was at MIT 12/9-12 to
participate in all ICRF-heated experiments.

Bill Beck, Bob Childs, and Rui Vieira visited PPPL last week for an
inspection of the lower hybrid forward waveguide.







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