From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: sci.physics.plasma


Alcator C-Mod Weekly Highlights
May 3, 1999

There were two days of plasma operations on Alcator C-Mod last week. Two runs
originally scheduled were lost due to a fault in the alternator drive motor on
Wednesday and problems with the ICRF systems on Friday. This week is a
maintenance week, with no runs scheduled.

Thursday's run completed the ohmic portion of MP#222, on impurity compression
and enrichment. This proposal is an attempt to extend the compression and
enrichment database to gases other than argon and investigate the effect of
the divertor bypass on impurity behavior. Argon and krypton were puffed into
Ohmic plasmas with nl04 ranging from 0.67 x 10^20 to 1.48 x 10^20 with
Ip=0.8MA. The detachment threshold for these plasmas was determined to be
~1.25 x 10^20.

The scaling of argon compression with density was evaluated; these compression
values are reduced by a factor of ~2 from the previous run campaign. The
krypton data are yet to be analyzed. The deuterium compression also scales
with density in a way similar to that observed in the previous run campaign,
with the value of the deuterium compression reduced by ~25%. Taken together
with the argon compression data, these results seem to indicate that the
divertor is not as "closed" as in the previous run campaign.

The divertor bypass does indeed have an effect on the impurity
behavior. The difference on the argon compression with the bypass open and
bypass closed is a factor of about two. (The compression is higher with the
bypass closed.) The deuterium compression is also about two times lower with the
bypass open.

Divertor detachment reduces the impurity compression greater than a factor of
two. After detachment occurs, the deuterium compression remains constant at
the pre-detachment level. Both of these observations are similar to those
seen in the 1998 run campaign.

The effect of the bypass on impurities is found to be a function of density.
At very low density, the bypass does not appear to have any effect on the
impurity levels in the core and the divertor, and thus the impurity
compression remains constant. At medium density, nl04=0.84 x 10^20, opening
the bypass during the shot decreases the impurity compression by ~2.5x. This
is due both to an increase in the core concentration and a reduction in the
divertor impurity concentration. At high density, nl04=1.1 x 10^20, opening
the bypass causes the divertor plasma to detach. The combination of open
bypass and detachment results in the argon compression being reduced by a
factor of ~6.

Physics and Analysis:
---------------------

Carbon "plume" imaging continued during piggy-back experiments
last week. In these experiments, trace amounts of ethylene gas
are injected via the F-port vertical scanning probe. Calibrations
taken at the end of the runs indicate that as few as 2 x 10^16
molecules were injected per probe scan. Exposure times for the
PPPL and LANL Xybion intensified cameras were optimized for
viewing CIII light. Optimization of the camera exposures for CII
light is still required.

An interesting result was observed during the dynamic flapper scans on
successive shots. During these discharges, the flapper was closed at the
beginning of the shot, and opened at 0.9 seconds, while the probe scan times
were at 0.65, 0.85, and 1.05 seconds. On at least one shot a dramatic
difference was seen in the CIII emission before and after the flapper opening,
with a strong asymmetry in the "plume" with the flapper open (indicating a
strong flow towards the divertor). Correlations with discharge parameters
still need to be made, and further analysis is required.

The density rise following the H-mode transition in C-Mod has been
analyzed. It is found that the density rise for a number of shots is well-fit
by an exponential. This implies that the ion source profile stays fairly
constant across the H-mode transition and that there is a step change in
transport. Furthermore, the density rise was compared across 2 shots that were
essentially identical for core characteristics. However, the bypass was
changed (open/closed) for these 2 shots leading to very different divertor gas
pressures. Although the divertor pressure drop in both cases was more than
enough to explain the core density rise, we cannot account for the neutrals
from the divertor that do not go to the core. This implies an important role
for the wall.

Evidence for a resistive interchange mode during the current rise phase has
emerged from detailed analysis of the ECE signals on a single shot, at a time
when the pressure profile is slightly hollow. During the current ramp of Shot
990302013, highly localized fluctuations were observed on only one of the
inner channels of GPC, which is very unusual. The pressure gradient of the
inner q=5 surface, which lies near this ECE channel, was predicted to be
marginally positive by the reconstructed pressure profile. Such a hollow
pressure profile with q_min > 1 in a low beta plasma has been predicted to be
unstable in terms of resistive interchange mode. As it is highly localized, a
resistive interchange mode is quite challenging to observe within current
C-Mod diagnostic capabilities.

ICRF Systems
------------

The run on Tuesday was devoted to optimization of ICRF coupling by adjusting
the target plasma conditions, including gaps and matching of the plasma shape
to the antenna geometry. At issue is both maximizing the antenna loading and
avoidance of fault indications. Progress was made in identifying target
plasmas with satisfactory behavior. However, in the course of the run, both
transmitters #1 and #2 gave indications of faulting, indicating possible
problems with the final amplifier tubes (FPA). During subsequent testing into
dummy loads, these faults persisted. During this testing transmitter #2
experienced a tube arc which resulted in damage to the FPA tube, rendering it
inoperable. Examination of the transmitter #1 is continuing, and the status of
its FPA tube is not yet known.

The damage to the tube in transmitter #2 appears to indicate a failure of the
protection circuitry which is intended to prevent damage in the event of a
tube arc. An inspection and review of the protection circuitry has begun,
involving engineers from PPPL as well as MIT. As a precaution, all ICRF
operation has been suspended until satisfactory completion of this review and
implementation of any recommended remedial actions has been accomplished.


Diagnostic Neutral Beam
-----------------------

All power and directly associated control systems for the DNB have now been
operated and tested to full performance with the exception of the
modulator/regulator for the acceleration voltage supply. The mod/reg
construction is near completion. At present, some of the engineering
support for the DNB is being diverted to the ICRF; however, we expect to
test the mod/reg system prior to the completion of the present CMOD
operational campaign and to be prepared to run the beam in the test cell
during the scheduled outage beginning in the summer.


Visits and Travel:
-------------------

Rejean Boivin visited LHD in Nagoya, where he presented a paper "High
Resolution Bolometry in Alcator C-Mod," by Boivin, Goetz, Marmar, Rice, and
Terry. He also attended the ITER Diagnostic Expert Group meeting, where he
presented an informal talk on Novel Diagnostics in Alcator C-Mod on behalf of
the Alcator Group.

Rejean Boivin, Martin Greenwald, Ian Hutchinson, Dmitri Mossessian, and Thomas
Pedersen presented papers at the Transport Task Force Meeting in Portland.

Miklos Porkolab attended the SEAB Review of the Fusion Program at PPPL
on Thursday and Friday, April 28,29. He also had a meeting with Joel Hosea
to discuss the Lower Hybrid plans for C-Mod.

Dr. Ricky Maqueda from LANL was on-site last week. The LANL IR camera is
operating routinely with a view of the outer divertor structures and the outer
strike point. The most striking result is that no toroidally symmetric
features of surface heating are observed. In particular, there is no toroidal
band of surface heating at the nominal position of the outer strike
point. Surface heating is typically observed on the edges of the divertor
tiles, especially on the "nose" tiles. The heated edges are on the ion side
(as defined by the plasma current direction). Surface temperatures are
typically less than 50 deg. C, and the hottest spots are around 200 deg. C.
Heating is also observed on specific divertor tiles. This is a result of poor
thermal contact and/or because they are raised slightly. Dr. Maqueda is
working on correlating some of the observed hot spots with the strike point
position and on trying to observe any effects of divertor detachment.

Charles Skinner of PPPL made preliminary measurements of the He II linewidth
and shift using a channel from the tangential fiber array which is usually fed
to the Kaiser spectrometer for analysis of the helium beam. The He II
measurements may potentially allow the determination of ion temperatures and
toroidal drift measurements near the separatrix. Analysis is underway.

Dr. S. Zweben of PPPL visited C-Mod on Apr. 28-29 to work with Brian LaBombard
and Jim Terry on the imaging of turbulent edge fluctuations. Clear
filamentation of the edge H-alpha light emission was seen using the
LANL camera at exposure times of 10 microsec. This turbulent structure
had a few-cm scale in the poloidal direction and a meter-length scale
in the toroidal or parallel direction (as seen previously). This structure
seemed to disappear during H-modes, but this could just be due to the
higher frequency associated with H-mode turbulence. The plunging
probe was used to puff Helium and Deuterium near the separatrix, and
clear visible light images were recorded with a 1 microsecond exposure
time. However, these images have not yet shown any filamentation inside
the 1-cm scale of the puff cloud, despite the large small-scale fluctuations
seen by the nearby Langumir probes.