Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue May 16 10:35:15 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
May 15, 1995

Plasma Operations continued last week on Alcator C-MOD. Four run days were
scheduled and completed, with over 100 plasma shots obtained. The toroidal
field was increased to 6.5 Tesla for the first time, in support of experiments
employing direct ICRF electron heating in the mode conversion regime. Other
runs were concerned with divertor detachment physics.

Divertor detachment characteristics at 1MA currents were studied during
Tuesday's run. A density scan was carried out with the strikepoint on the
vertical plate. Detachment was first observed at a line-average density around
2.4e20/m3; as the density was increased over the next several shots the
detachment appeared to extend further up the divertor plate. At nebar>3e20/m3
the divertor appeared to be detached along the entire plate. Confirmation of
this result awaits analysis of the Fast Scanning Probe data from these shots.
In summary, the detachment threshold in terms of density appears higher at 1MA
than for 800kA, and the detachment phenomenology differs in that the detached
region, as determined from the pressure drop, moves up the divertor face
over a range in density, rather than detaching over the entire plate at once.
An attempt was also made to determine the density necessary to produce
detachment in the flat plate configuration at 1MA, but no detachment
was observed at the highest densities obtained.

Divertor detachment was also the subject of the run on Friday, which
investigated the phenomenon in the presence of strong ICRF heating, and
employed impurity puffing to induce detachment at high RF powers. It was found
that the amount of puffed neon required to detach the divertor (with P_RF ~
1MW ) had deleterious effects on the core plasma. However, using methane as
the injected impurity was more successful. At a density nebar= 2.4e20/m3 we
were able to detach during the pre-RF, Ohmic phase, then attach with the
increased input power supplied by the RF, and finally detach again during the
RF phase by puffing methane. At this density we were able to find the methane
(C) thresholds for detachment at RF powers of 1 and 2 MW. With RF power of
1MW, the methane detaches the re-attached divertor at a C level which is about
a factor of 2 above the intrinsic level. The impurity increase due to the puff
is not apparent on Z_eff, which is about 1.55 throughout the RF phase of the
discharges. The required C level for detachment at 2 MW is about 3 times the
intrinsic C level; Z_eff increased from 1.4 to 1.6.

The runs on Wednesday and Thursday were devoted to direct electron heating in
the mode conversion regime, using a H-He3 target plasma at a toroidal field of
6.5 Tesla. This marked our first operation at 6.5 T, and all systems performed
reliably. Scans of He3 concentration and magnetic field were carried out to
study and optimize the mode conversion heating. Very strong localized electron
heating was observed, with local absorbed power densities up to 25 MW/m3.
Central electron temperatures above 5keV were obtained. The RF power was
modulated in order to carry out perturbative transport studies to allow
measurements of electron transport coefficients to be made, using GPC as the
main diagnostic tool. These data are being analyzed. Dick Majeski from PPPL
was at MIT and participated in these experiments.