Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Mon Feb 6 18:37:39 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Feb. 6, 1995

Plasma operations continued on Alcator C-MOD last week with a full four days
of experiments scheduled and completed. The main experiments were concerned
with effects of divertor geometry and ICRF heating. This was quite a
productive week, with a total of over 100 successful plasma shots obtained
over the four run shifts.

Studies of the effect of divertor geometry on detachment were continued.
A systematic investigation of the 'radial' extent of detachment, i.e. the
flux surface location in the SOL (referenced to the midplane) where pressure
is no longer constant along a flux surface, was carried out. The dependence of
this quantity on strikepoint location was studied, for slot, inclined plate,
and open divertor geometries; the strikepoint was positioned at different
locations along the inclined plate to obtain more detailed data for this
configuration. Data were taken with ramping and constant densities at two
values of current (0.5 and 0.8 MA).

For the inclined plate cases, detachment threshold at the separatrix does not
seem to vary much as the strike point location is varied. Some differences
were observed at different strike point locations depending on the rate of
density rise; less density rise is needed to detach up to the vicinity of the
divertor nose when the strike point location is moved closer to the divertor
nose. There is a fairly narrow density range from the onset of detachment
until the detachment reaches the nose of the outer divertor. At 0.8 MA this
window is approximately between n~1.7 and 2.5e20 m^-3.

The comparison of flat-plate and inclined-plate divertor continues. At 0.5 MA
the detachment threshold for the inclined and flat-plate geometries were
n~.8 and 1.5e20 respectively. This result is similar to the 0.8 MA results
where the inclined plate and slot had a threshold of n~1.7e20 and the
flat-plate at n~3e20.

ICRF heating studies this week included scans of toroidal field (resonance
location for our fixed frequency 80MHz sources) and minority concentration.
The toroidal field was scanned between 3.5 and 5.5T at nebar~1.8e20/m3 and
Ip=0.8MA with approximately 2.5MW of RF power. The ohmic stored energy (EFIT)
was found to be independent of the toroidal field, at about 47kJ for the
present condition. With on-axis heating, the stored energy increases to 80kJ
during the L-mode phase and 90kJ during the brief H-mode phase (see below). The
stored energy increment decreases as the resonance layer is shifted away from
the center. With the resonance layer shifted half way out the stored energy
(RF heated) was 60kJ, and no stored energy increase was observed when the
resonance layer was shifted by r/a >= 3/4. H-mode transitions were observed on
all shots with resonance layer located at r/a <= 1/4. We still have some
difficulty maintaining constant power through L-H transitions, but we were
able to keep reasonable power across the transition and observe the stored
energy increase (from L to H-mode) on some shots.

A hydrogen minority concentration scan was performed at two densities,
nel_04=0.7 and 1.1e20 m^-2. Heating (stored energy increase) degraded
as the H concentration was increased from 2% to 15%. At nel_04=1.1e20, the
stored energy increased from an ohmic level of 46kJ to 80kJ with 2.5MW of RF
power in L-mode with no H (concentration < 2%), but with 15% H (22% H mix in the
plenum) the stored energy only increased to 52kJ. There is a definite
indication that heating is better at nel_04=0.7e20, especially at higher H
concentrations. The stored energy with 2.6MW of RF power at this lower density
with 15% H was 69kJ.

In support of the ICRF toroidal field scan, a new startup scenario was
developed. Initiation was produced during the TF ramp-up, at a field of about
3.4 tesla. This technique allows the flattop TF to be changed shot-to-shot
without affecting the breakdown. It also reduces the TF magnet heating at
higher fields by decreasing the time the coil is at full current.

The YAG Thomson Scattering diagnostic operated successfully this week,
following a re-alignment carried out on Monday. Data are obtained for electron
density and temperature every 20msec during the pulse; three spectrometers
(spatial locations) are presently operational, with additional spectrometers
in preparation. Electron temperatures obtained are consistent with the
independently calibrated ECE system.

Jim Bialek(PPPL,TPX) and Hutch Neilson(ORNL,TPX) visited MIT for the initial
meeting of the CMOD Halo Current EM task group, which was formed to help resolve
issues of halo and eddy current paths in the vacuum vessel and PFC's of
ALCATOR-CMOD . Participants from MIT included Bob Pillsbury, Len Myatt, Jim
McCarrick, Jim Irby, Dave Gwinn, Herb Becker, Jeff Freidberg and Joe Sorci. The
fundamental mission of the group is to provide forces for stress analysis, based
on actual 3D measurements of halo currents observed in the experiments. It is
expected that there will also be insights gained as to the flow of these
currents in the walls and other hardware, contributing to the development of a
useful design and analysis tool applicable to other projects.

On Monday-Wednesday, Jan. 30-Feb 1, Prof. Miklos Porkolab attended the
US-Japan Workshop on Steady State Tokamaks, in Livermore, and presented a talk
on Access to Advanced Tokamak Regimes in TPX through Profile Control. He also
discussed possible collaborations between C-Mod and the LLNL Tokamak Physics
Group. On Friday he attended the TPX Physics Workshop, in San Diego, and made
a presentation on the latest profile control scenario work.