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

Alcator C-Mod Weekly Highlights
April 19, 1999



Maintenance continued on Alcator C-Mod last week, with the primary effort
focused on the ICRF systems. One plasma run took place on Friday to
measure the influence of magnetic field strength on the opacity of the
deuterium Lyman series lines emitted from MARFEs existing at the inboard
midplane. In addition, several new diagnostics became operational for the
first time during this run.


Physics:

Recent simulations of midplane MARFEs obtained on Alcator C-Mod using
the CRETIN line-tranfer code have shown the large importance of magnetic
line broadening (the Zeeman effect) on the optical thickness of lines of the
deuterium Lyman series (Lyb to Lyh). This effect is of particular significance
in regimes where the Zeeman splitting and the Stark broadening are of the same
order of magnitude, as it then allows wavelength diffusion between separate
Zeeman components, thereby multiplying the effective line width and strongly
affecting the opacity of these lines. On Friday, a plasma run was devoted
to testing this process. There was full coverage of the inner wall by the
Chromex. The McPherson VUV spectrometer was looking at Ly_beta thru Ly_8 from
the relatively stable MARFE located at Z=-0.12 m. B_t ramped from 5.4 T to
3.24 T before the plasma disrupted at 0.9 s. Very preliminary analysis
shows no obvious change in the Ly_beta/D_alpha ratio, which would be the
signature of a changing Ly_beta opacity. Detailed analysis is continuing.

We obtained the first results from the recently installed Ly_alpha
detector last week. This diagnostic consists of a 20 channel silicon
detector, AXUV-20 EL, capable of measuring radiation in a very wide bandwidth.
Using a Ly_alpha filter (centered around 1216 Angstroms), we can now
image the Lyman alpha emission (from neutral deuterium) slightly below the
midplane in a tangential view. Covering a 4 cm radial range, near the
separatrix this instrument has a nominal 2 mm resolution, with a temporal
resolution of a few milliseconds (due to the high gain needed).
Results indicate a peaking of the radiation near the separatrix, increasing
with plasma and neutral densities, and decreasing in the presence of an H mode,
similarly to the H alpha emission. This instrument will give us a
measurement of the neutral density and ionization profiles in the main
chamber near the separatrix, important parameters for the edge dynamics.

The inner wall rangefinder also began operation. This instrument
measures differential wall movement between two closely spaced positions on
the inner wall. Initial results during low current disruptions, with barely
measurable halo currents, indicate 2 to 6 nm of relative motion during the
current collapse. Results from this new diagnostic will be correlated with
data from strain-gauges at the same location. We will use this data to
help benchmark codes being developed to model the inner wall upgrade.

A new high spatial resolution visible continuum array diagnostic is
operational. The chordal spatial resolution of the system, which uses a 2048x1
pixel CCD array detector, is about 0.7 mm at the edge of the plasma, in the
expected region of the H-mode pedestal at the outboard midplane. The camera
views the plasma tangentially, in the midplane, with coverage from 0.63 < R <
0.91 meter. The time histories of the central chord are in excellent agreement
with the single channel view which uses a narrower spectral filter, indicating
that the broader spectrum is dominated by free-free bremsstrahlung. Profiles in
ohmic and low power L-mode ICRF discharges were obtained with 2 msec time
resolution. Evaluation with H-mode discharges should begin during the upcoming
week's operations.


Engineering:

Work continued on testing and debugging the ICRF instrumentation and
control systems. During the run on Friday, FMIT#1 was brought back on
line with approximately 1.4 MW of power coupled into the plasma. The
remaining three transmitters will be brought back into operation over
the next few days of operation.

Work on the diagnostic neutral beam included design, layout, fabrication,
and testing of the last few printed circuit boards.

Travel and Visits:

Dr. Bill Noonan, our collaborator from the U. of Md was present for the
April 16 plasma run.

Soon-Mo Hwang, Dr S. J. Yoo, and Mr. J. Hong from the Korean Basic Sciences
Institute, which is home to the KSTAR project, visited MIT last week to
discuss diagnostics and MDS-Plus.

Starting this week, Professor Marco Brambilla
from the IPP in Garching will be visiting at the PSFC for a
one month period. Paul Bonoli will be working with Marco to implement
a new version of the TORIC ICRF code at MIT which treats
electron Landau damping of ion Bernstein waves more accurately.
They will also be coupling this new version of TORIc to an ICRF
current drive package.

Miklos Porkolab attended the 13th Topical Conference on Applications
of Radio Frequency Power to Plasmas and presented the Invited Talk:
ICRF Heating in Alcator C-Mod: Present Status and Future Plans
(The meeting was held in Annapolis, MD, April 12-14). Paul Bonoli also
attended this meeting and presented a paper entitled "RF Current Profile
Control Studies in the Alcator C-Mod Tokamak".

Amanda Hubbard was in Garching attending the first meeting of the new
ITER Pedestal expert group, April 12-16. She presented a talk on Enhanced
D-alpha H-modes, since this regime is of considerable interest for ITER-RC.
This meeting was held jointly with the Confinement and modelling expert groups;
Joe Snipes participated in the confinement database group. Joe has been asked
by the Threshold Database group to present the next paper from the group at
the upcoming H-mode Workshop in Oxford, England, 27 - 29 September.