From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
July 3, 2000

Operations
-----------

Plasma operations continued at Alcator C-Mod last week. Five run days were
scheduled and completed, following a boronization on the weekend of June 24. A
total of 79 plasma shots were produced with a reliability of 75%. This week's
experiments were in support of six MiniProposals, comprising research proposed
by the Particle Control Task Force, the Pedestal Physics Group, and the Core
Transport Group.

This week begins an extended scheduled maintenance period, with a
manned-access vent planned to begin in two weeks. Plasma operation will resume
in September. A schedule of planned activities can be found at
http://www.psfc.mit.edu/people/irby/short_term.pdf

Physics and Analysis
--------------------

The first run of the week, on Monday, investigated a different technique for
preparing the walls for plasma discharges following boronization. On Saturday,
6/24, we deposited about 1600 Ang of diborane onto the vessel walls using our
standard ECDC technique. Starting around 19:00 on Saturday we began glow
discharge cleaning in helium, which was continued through Monday morning. For
the first plasma shot on Monday we programmed a 1.9 second long, 1 MA fiducial
shot with a target density of 1e20 m^2, which had been run previously as the
first shot of the day on June 16; at that time, the shot required about
43 Torr-l of gas. The plan was to re-run this shot and see how much gas it
took after the extensive glow discharge campaign. Unfortunately, the first
shot disrupted after only 350 ms, so we loaded the wall somewhat with a shot
that could not be easily compared to the earlier fiducial. The following shot
ran full length and clearly required much more gas than the fiducial (62 T-l).
The next two shots only required 36 and 33 T-l of gas. The conclusion is that
it takes only 1 or 2 discharges to reload the walls, even after a very
extended period of wall conditioning. We also did two periods of between-shot
glow discharge in helium. Both of these periods had 30 minutes of glow at up
to 2.5 amps and 450 volts. Following the glow, we again ran the fiducial
discharge, and in both cases 45 T-l of gas was required. So, 30 minutes of
glow is worth about 10 T-l of gas. The second shot following the glow was
again at the 33 to 35 T-l value, again indicating recovery with 1 shot.

Tuesday's run continued earlier investigations of the effect of main chamber
recycling on H-mode thresholds. The level of recycling was varied by varying
the inner gap, for otherwise constant conditions. The main thrust of this run
was to establish the threshold L-H power and conditions for marginally limited
and well-limited plasmas. Unfortunately, these experiments were not
successful, although additional data for small inner gaps were obtained,
consistent with previous work in May of this year.

On Wednesday, we continued our studies of the "Enhanced D-alpha H-mode", with
exploration of the effect of plasma shaping on access to this promising
operating regime. In these experiments, with 'standard' 1 MA, 5.3 T plasma,
q95=4, the plasmas became mainly ELM-free at triangularity only slightly lower
than our 'standard' shape, at delta_av = 0.36. The pedestal widths did
generally get narrower, though they have yet to be examined in detail.
Attempts to go lower in triangularity led to impurity injections and
disruptions, perhaps due to the location of the outer strike point on the
divertor nose.

The pedestal physics group performed experiments on Thursday, with the
continuation of MP#271, on the scaling of pedestal parameters with current and
RF power. We started at 1MA and obtained some good data points at various
power levels, The pedestal gradient was observed to increase with input power,
in agreement with previous observations. At 1.2 MA and nl04=0.9e20 some
usable data was obtained, depite difficulties with maintaining the requested
RF power waveforms. The target density was increased to 1.1e20, and H-modes
were obtained at 2 power levels.

The Core Transport Group carried out two experiments on Friday, both
concerning the physics of internal transport barriers. The first experiment
completed the documentation of ion temperature profiles in the
spontaneously-generated ITB (so-called EN mode) following H->L transitions. We
successfully reproduced the conditions of the June 15 run on this topic, and
were able to obtain the profile data in a series of six shots.

The second experiment exploited the observation of ITB formation during
off-axis ICRF heating, where the absorption radius is displaced to the
high-field side. We have speculated that the existence of a "natural"
co-rotation in C-Mod may prevent the formation of ITB's in many circumstances,
and further, that if the rotation could be reduced or reversed, barriers might
form much more readily. In two recent experiments in this configuration the
normally co-current toroidal rotation was observed to slow down and even
reverse direction, and these discharges do show formation of density transport
barriers. The aim of the present experiment is to test our hypothesis by
measuring time-dependent profiles of T_i, V_phi, and V_theta. This would
allow us to calculate the shearing rate omega_{ExB}(r), which is the term
which is believed to stabilize turbulence. The physics of the barrier location
is also of interest since for these shots q0<1, in contrast to most
experiments with ITBs which have q0>>1, with very weak or reverse magnetic
shear. We had a number of good shots, suitable for profile analysis, although
there was a lot of radiated power, with the result that there are many L/H/L
transitions during each shot. The toroidal rotation consistently reverses
sign with strong density peaking. The maximum effect was seen during L-mode
phases. On several shots, strong mhd (m=1) was seen as the density peaked.
Typically the sawteeth ended before the onset of the MHD. At the end of the
run we ran limited shots to see if these effects would persist in an all
L-mode plasma. No peaking or flow reversal was observed in these cases.

ICRF System
------------

Preparations for the modification of the 4 strap antenna are well under way,
and are expected to be ready for the scheduled opening this month. Further
operation of this antenna this week as a 2 strap showed good efficiency and
reduced impurity generation; this result indicates that the planned
modification should be beneficial.

The original ICRF system (2 dipole antennas at D- and E-ports) continued to
deliver 2-3.5 MW in support of this week's experiments. For some experiments,
the J-port antenna was also used in combination with D and E to provide up to
5MW net ICRF power, with a low radiated power fraction.

DNB System
----------

During the run on Wednesday, June 28, the Motional Stark Effect Diagnostic
(MSE) saw first beam light. The 40kV beam was observed on edge channels of
the MSE system during a standard 1MA, 5.3T discharge. Small signals were also
observed on no-plasma shots with the beam firing into a gas target.