Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Mon Mar 20 17:26:52 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
March 20, 1995

Phase IIB plasma operations on Alcator C-MOD are continuing. Four runs were
scheduled this week. The principal experiments were all related to edge and
divertor studies, including SOL characterization, a comparison of impurity
screening in limiter and divertor plasmas, and a detailed investigation of
plasma characteristics in the region around the X-point. In addition,
significant progress was made in our collaboration with LLNL in remote tokamak
operation.

The planned run on Tuesday, March 14, was compromised by a leak which occurred
during pre-operation discharge cleaning. While the leak was closed and
additional ECDC was performed, the vacuum conditions were not good enough to
proceed with the planned run, an investigation of the role of strike-point
location on divertor detachment. However, a group of MIT and LLNL researchers
were quite successful in making preparations for our upcoming demonstration of
remote tokamak operation, including actually programming and running three
shots from the remote site.

Steve Horne and Tom Fredian of the C-MOD Group travelled to the Livermore
Remote Experimental Site (RES) to work with Tom Casper, Bill Meyer, and Jeff
Moller of LLNL on preparations for operating C-MOD from the RES. A video
conference link between a workstation in the C-MOD control room and the RES
along with an IRC (Internet Relay Chat) channel were used for communications.
A four-way split screen video transmission from the Alcator end provided the
remote team with two views of C-MOD control room personnel, an in-cell view of
the tokamak, and a real-time plasma TV display. C-MOD standard data display
and control tools (SCOPE, PCS, IDL animation routines, etc.) were tested.
Data displays were run both by connecting (TELNET) to computers in the C-MOD
cluster, and directly on the UNIX machines in the RES using RPC's (Remote
Procedure Calls). After establishing satisfactory communications protocols
and observing the operation, the remote team was temporarily given control of
the run. Horne, the remote Physics Operator, re-programmed the shot sequence
using the PCS interface, gave the ready signal to the Engineering Operator at
MIT, and produced three successful shots. We believe this to be the first
instance of trans-continental operation of a tokamak in history. A
mini-proposal (MP095) calling for a full C-MOD experimental run, with Session
Leader, Physics Operator, and additional scientific personnel located at the
LLNL Remote Site, has been approved and will be carried out in the near
future.

Overnight discharge cleaning restored the vacuum condition of the machine
sufficiently to resume edge physics experiments on Wednesday. The SOL
characterization study (MP084) in ohmic lower single null discharges was
completed with data at low currents (400 and 600 kA). Results are currently
being analyzed.

A comparison of impurity screening in limiter and divertor discharges was
carried out on Thursday. Argon was injected using the usual gas puffing
technique and scandium was injected by laser blow-off. Spectroscopic
observations from the HIREX instrument were used to monitor the impurities in
the core. Use of the non-recycling Sc data permits derivation of the core
impurity transport coefficients, which will be used in the MIST code, which is
being used to obtain the total concentrations of the injected impurities.

The purpose of Friday's experiment was to investigate the plasma
characteristics in the region around the X-point in a number of Alcator C-Mod
plasma regimes (sheath-limited, high-recycling, and detached). The
study included measurement of the visible, VUV, and x-ray spectra along chords
viewing the X-point region, the standard bolometer and Reticon array data and
spectra from the cx analyzer. Using these diagnostics we explored the makeup
of the large volumetric emission near the x-point measured by the divertor
bolometer array. In particular the relative contributions of impurities and
neutrals are of interest. We will also address the opacity of the plasma to
the emission in the Lyman series of hydrogen. Ion temperature measurements in
the region just above the x-point were obtained by both charge exchange and
HIREX instruments.

The radiation (defined by the peak in emissivity from the divertor bolometer
array) is normally between the inner nose and the x-point when the x-point is
in its standard position, and jumps to the x-point at detachment. When the
x-point was shifted up and out the emissivity peak moved to the x-point.
Under these conditions we did not observe any obvious movement of the
radiation peak at detachment. When the x-point was shifted back down in 1cm
steps the emissivity returned to a more standard pattern. These bolometry
observations are consistent with those of the Reticon arrays. VUV spectra show
very little Mo emission from the x-point region. Also emission from H-like
and He-like carbon was absent. Lower charge states of C, O and Ar (ground
state transitions) were observed and will be used to quantify the
contributions of these impurities to the x-point radiation emissivity.

Our new optical disk jukebox for long-term on-line data storage has arrived
from the vendor and been placed in service. The new device, with a capacity of
approximately 170 Gbytes, supplements our older 90 Gbyte system, which was
approaching its capacity. C-MOD is presently producing up to 1 Gbyte of
compressed data per run day.

Absolute calibration of the YAG Thomson scattering diagnostic was carried out
on Saturday, using Raman scattering from hydrogen. The tokamak was back-filled
with up to 600 Torr of hydrogen for the calibration.

Rich Hawryluk and Ken Young visited M.I.T. this week for consultations on the
PPPL/C-MOD colaborations in FY96 and 97. Norton Bretz of PPPL is visiting today
for more detailed discussions on upcoming collaborations.

Ben Welch (U. Md) was at C-MOD this week working with his Optical Multichannel
Analyzer diagnostic and participating in the week's runs.

Martin Greenwald participated in informal meetings of the ITER Confinement and
Confinement Database Expert Groups at the San Diego Joint Work Site. He
presented results from the C-MOD experiments on H-mode thresholds and
described progress in our non-dimensional scaling comparisons with DIII-D.