Newsgroups: sci.physics.plasma
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Dec. 12, 1994

Operations continued last week on Alcator C-MOD. Four run days were scheduled,
with three principal experimental topics. The tokamak operated reliably, and
all the planned experiments were substantially completed.

A comparison of impurity screening efficiency between diverted and limited
plasmas (MP#056) was carried out. A known amount of argon, a "recycling"
impurity, was puffed in from a midplane valve, and the resulting argon content
of the plasma was measured using data from the HIREX spectrometer. All shots
had 0.8MA plasma current and 5.3 tesla toroidal field; three different
densities, in the range 0.8 : 2.4e20/m3 were studied.Results show that limited
plasma allow roughly twice as many impurity particles (6-8%) to reach the
plasma core as in diverted L-mode plasmas (3-4%); in all cases the fraction is
relatively low. Preliminary data on diverted H-mode plasmas shows about double
the L-mode fraction.

One run was devoted to ohmic transport in nearly circular plasmas with
dimensions close to those of Alcator C (part of MP#046). The purpose was to
make as close a comparison as possible between transport scaling on Alcator C,
which showed neo-Alcator scaling, and Alcator C-MOD, which shows an L-mode
like scaling in ohmic plasmas. Discharge geometry for this run was: R=0.63m,
a=0.18m, kappa=1.1, Bt=5.5T (at the magnetic axis). For comparison, Alcator C
had R=0.64m, a=0.17m, kappa=1, and typically Bt >/= 6T. Scans of density and
plasma current were carried out to cover as much of the operating space as
possible. Currents were in the range .25<Ip<.55 MA, corresponding to 6>q>2.8;
line average density was in the range 0.7 < ne < 2.4 e20/m3 . Results are
presently being analyzed.

Two run days were allocated to ICRF tuning and conditioning, using the D-port
antenna. Acceptable tuning was established quickly, and on the first day power
was brought up to approximately 1.8 MW and substantial heating was observed.
Also on the first day, a moderately high triangularity (delta >0.4) limiter
target plasma was established, and a start was made on developing a diverted
plasma with a better match to the antenna. On the second day, more work on
coupling to and heating diverted plasmas was carried out. Up to 1.8MW was
coupled into the plasma (2MW from the transmitter). The plasma was moved up
1cm to better match the antenna shape, while maintaining the strike points in
the divertor. The RF pulse length was also increased; up to 0.5 second pulses
at powers up to 1.75MW were run successfully. Good heating was observed. A
number of elm-free H-modes were obtained at RF powers over 1.5MW. In general,
it can be said that we have re-established our previous rf performance and
gone beyond it in rf pulse length and H-mode transitions.