Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue May 9 18:48:36 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
May 8, 1995

Plasma operations continued on Alcator C-MOD last week. Four run days were
scheduled and completed, with over 100 successful plasma shots produced.
Principal experiments were concerned with divertor detachment and ICRF H-mode
physics.

The effect of RF power on divertor detachment (MP#073) was investigated on
Tuesday. The primary purpose of this run is to control the power flowing into
the SOL (using RF) independent of the plasma current. Using this control we
searched for the detachment threshold and characteristics as a function of SOL
power. Radiated power in the divertor and the source rates for different
impurities at the first-wall surfaces (CHromex spectrograph) were monitored.
The McPherson spectrograph was used to monitor the UV emission from the
x-point region. With the core density set at approximately nebar=2.5e20/m3, RF
power was varied from 0 to 1.5MW; these powers into the main plasma raised the
power flowing into the SOL from 0.5 to 1.25MW. At the highest levels of power
the outer divertor plasma appeared to reattach. At higher density, full
reattachment was not obtained, even for RF powers up to 2.4 MW and P_SOL up
to 2 MW.

The effect of strike-point location on detachment was studied during
Wednesday's run. Using an equilibrium with the outer strike-point located
above the nose of the divertor, we raised the density (shot to shot) and
obtained the divertor parameters for comparison with the more standard
operation - vertical plate divertor. We swept the strike point across probes
located on the upper surface of the divertor structure to obtain SOL plasma
profiles. In particular, we found that the threshold for detachment (pressure
not constant along a flux surface) was significantly higher for the flat-plate
divertor than for operation with the vertical plate. The threshold was of
order nebar=3.5e20/m3 as compared to around 2-2.5e20/m3for the vertical plate
case.

H-mode operation with ICRF heating was investigated on Thursday and Friday.
ICRF power up to 2.7MW was injected into a variety of plasmas, with the main
goals being to map out the operating space and determine optimum conditions in
terms of H-mode confinement and ELM behavior. The outer gap was varied
systematically between 1.2 and 2.2 cm. Some variation in x-point height was
also explored. Line average density of the target plasma was varied over a
modest range, from 1.3 to 2.2e20/m3. Plasma current was varied from 600kA to
1.2MA. Good H-modes were obtained throughout this parameter range. Typically
the L-H transition was followed by an ELM-free period of up to 70msec, during
which the density would rise significantly, followed by an Elmy phase. Under
some conditions we observed oscillation between ELMy and ELM-free behavior as
the density evolved. In no case were we able to keep the density from
increasing during the ELM-free period. H-factors in these discharges were
modest, around 1.3-1.4 times the ITER89-P L-mode value.

AET has delivered a prototype transfer line for the divertor cryopump. This
line, with its associated feedthroughs, will allow two cryopump modules to be
supplied with liquid nitrogen and helium.

M.I.T. hosted a TPX physics workshop this week, attended by physicists from
around the country, many of whom took advantage of the opportunity to visit
the control room and hold informal discussions with C-MOD staff. Amanda
Hubbard and Paul Bonoli presented results of their work at the workshop.

Bob Pinsker from the DIII-D Group continued his visit, working with the C-MOD
ICRF Group. In addition, Gary Jackson and John Robinson visited from GA for
discussions with Earl Marmar and Darren Garnier regarding design of the
Lithium pellet injector. Dr. Jackson also had discussions with Ian
Hutchinson, Jim Irby, Catherine Fiore, and Joe Snipes regarding prospects for
boronization on C-MOD.

Glen Wurden is visiting from Los Alamos to begin installation of a
new diagnostic on C-MOD. The new equipment arriving from LANL includes a fast
Kodak camera system & Pentium controller PC , with GPIB optical links, video
links, assorted lenses, heavy-duty tripod, and a 15' imaging fiber bundle.
The system will be used for fast radial and tangential imaging in visible
light, with filters for different spectral bands, in order to look at MHD
structures in the plasma edge, view antenna arcing, view pellet injection,
etc. The system was setup and checked out Thursday evening, and first plasma
light was seen with it on Friday May 5, viewing in a midplane port at Bay C,
from about 4 meters out. This was simply to confirm operation in the magnetic
field, and check out computer links. Tests were successful, and Friday evening
the system was moved in to a distance of 2.4 meters from the inner wall,
viewing through a 200 mm lens at f22 aperture.