From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights


Alcator C-Mod Weekly Highlights
Dec 22, 2003

Research operations continued at Alcator C-Mod last week. Four run days were
scheduled and completed. Experiments were carried out in support of RF
Physics, Burning Plasma Support, Transport and MHD research programs. Progress
on the Lower Hybrid project also continued.

No plasma operations are planned for the next two weeks. MIT will be closed
from Dec 25, 2003 to Jan 4, 2004. Operations are scheduled to resume the week
of January 5, 2004.

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

A fresh vessel boronization was carried out overnight on Monday. 100psi of
diborane were used (a "light" boronization, as had been done on 12/4), with a
slight modification to the duty cycle of the EC discharge, as we continue to
try to optimize the boronization procedure. Following the boron deposition,
which was completed around 11:30PM, ECDC in helium was carried out for about 7
hours, prior to the start of tokamak operation on Tuesday.

Four days of experimental operation were carried out last week. A total of 97
discharges were produced with a startup reliability of 87%. So far during the
current campaign (FY04), C-Mod has operated for 23.5 Research Days, equivalent
to approximately 6 weeks at our nominal 4 operating days/week. A total of
about 500 plasmas have been produced. We currently plan for 21 total weeks of
research operations in FY04.

The first half of Tuesday's run was devoted to RF antenna reconditioning,
following the boronization, and wall condition evaluation measurements. The
antenna reconditioning was accomplished in seven shots.

Following the reconditioning discharges, the remainder of the Tuesday run and
all of Wednesday's run was devoted to completion of MP#377, a comparison of
energy confinement in single and double null configurations. This MP
contributes directly to our Level 1 Milestone SC6-1b, "Compare energy
confinement, H-mode threshold and divertor particle dynamics in SN, DN and
inner-wall limited discharges in Alcator C-Mod". Dale Meade (PPPL) again
served as co-Session Leader for these experiments. Shots from current and
power scans from previous runs of this MP were reproduced, and a density scan
at 1MA was carried out. Both the target density and the H-mode density were
varied, the latter by puffing additional gas into the H-mode phase. The
highest line averaged density obtained was ~4.3e20/m^3, corresponding to ~0.6
n_greenwald. The energy confinement was not found to depend significantly on
density over this range. Higher densities were not explored due to limitations
on the ICRF system associated with high neutral pressures noted below. For
lower single null cases, two different values of upper triangularity were
used, with no apparent difference in confinement observed. Over the course of
the whole day, the confinement in DN discharges was about 10% higher than in
SN; this observation is consistent with results from the scans on 12/11. We
also again observed a slow degradation in confinement and plasma performance
as the run proceeded.

Thursday's run was devoted to MP#304 "H-Mode Regimes" from the Transport
Group. The main purpose of this day's experiment was to explore the boundary
between the EDA (Enhanced D-alpha) regime and the small ELM regime previously
observed as beta increased. Nearly steady-state EDA H-modes were produced
lasting about 0.9sec, with ICRF power up to 6.2MW and beta_N~1.3. However, the
operating window for such discharges was small, and the planned systematic
parameter scans were not accomplished. There were indications, from
break-in-slope analysis, that the RF absorption efficiency was low in these
discharges.

Friday's run was the first day devoted to MP#361, "Identity experiments with
JET on error field locked mode thresholds". This experiment is an ITPA high
priority joint experiment between C-Mod and JET. Tim Hender participated as
remote co-Session Leader for this run from the JET site in the UK. The
overall goals of the joint experiment are to confirm the toroidal field and
density scaling of the threshold for error field induced locked modes observed
on JET and DIII-D, and determine a size scaling for error field locked modes
through an identity experiment with JET. This would resolve the two key
uncertainties (toroidal field and size scaling) in determining the error field
threshold in a burning plasma experiment. The purpose of this first C-Mod
experiment was to establish a q95=3.2 discharge in the JET shape and determine
the maximum density at which the C-Mod non-axisymmetric coils (A-coils) can
induce a locked mode (and the correponding applied field). This result would
then determine the upper density range for the JET experiments, which are
planned for January, 2004. The maximum density for which locked modes were
actually obtained was nebar~3.6e20/m^3, using less than half the maximum
available applied 2/1 field from the A-coils. Higher densities, up to
6.5e20/m^3 were obtained, but high density in these ohmic 1.3MA discharges was
limited by a combination of available gas fueling and flux-swing. There was
insufficient time in the run to determine the locked mode threshold at these
higher densities, but previous scaling implies it should be within the range
of the A-coils.


Physics
-------

Many high power ICRF H-mode plasmas have been observed to exhibit large
oscillating low frequency (< 20 kHz) m=1, n=1 modes near the H-L back
transitions. The m=1 mode frequency correlates well with the core plasma ion
rotation both in C-Mod and in JET. Generally, the modes and plasma rotate in
the ion direction during H-mode and in the electron direction in L-mode. At
the L-H transition, there have been a few shots over the years where the m=1
or even the m=3, n=2 mode rotation can be followed from just before to just
after the transition and the rotation is found to change sign from the
electron direction in L-mode to the ion direction just after the L-H
transition. The time resolution is of the order of 1 ms for Fourier
transforms of the magnetic fluctuations, so the rotation can be followed
through the transition and is found to typically change direction about 4 - 5
ms after the transition. Many of the recent shots with large m=1 modes also
show that the rotation changes sign from the ion to the electron direction at
the H-L transition. However, there were also a number of shots in which there
was a brief L-mode after a longer H-mode and the rotation remained in the ion
direction in the L-mode phase, even though it was rapidly spinning down.
Then, the rotation abruptly stops spinning down at the next L-H transition and
remains in the ion direction throughout the subsequent H-mode phase. The
effect may be profile dependent in that ELM-free H-modes have previously been
observed to have peaked rotation profiles, and these were the kinds of H-mode
that could continue to rotate briefly in the ion direction shortly after the
H-L transition, while ELMy H-modes slowed down and changed rotation direction
at the H-L transition.

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

The RF antennas reconditioned quickly following boronization on Monday
evening. No vacuum conditioning was done on D-port antenna but its recovery
was not remarkably worse than E-port. Operation in support of plasma
experiments successfully produced pulses up to the 6MW level, using all three
antennas. We have observed increased faulting by all antennas when the chamber
neutral pressure, as measured at the F midplane port, reaches 0.3 mTorr. This
limit appears to be soft in the sense that for some configurations the
antennas will operate to higher pressures. More analysis is required to
clarify this observation.

Lower Hybrid System
-------------------

The remaining circulators needed to isolate the klystrons from fault
conditions along the transmission line and in the launcher were
delivered on Thursday afternoon. These components were reworked by
the vendor after high power tests at MIT indicated some improvements
were required.

Bob Childs spent most of last week working with our coupler brazing
vendor. A data logger was setup and thermocouples installed to
monitor the oven and the samples. Very useful data from the oven
tests has already been obtained.

The forward launcher assembly arrived at MIT from PPPL on Friday.

DNB Systems
-----------

The DNB has continued to perform reliably this week at the same high
performance levels as last week. Beam currents are typically near 5A at 48kV,
with very few faulted shots.

Travel and Visitors
-------------------

Kwan Chul Lee from PPPL and UC-Davis visited Josh Stillerman on Wednesday to
get help configuring a PCI data acquisition computer. Data from one high
speed, 16 channel card will be taken as part of the main NSTX data archive.

Dale Meade (PPPL) was at C-Mod Tuesday and Wednesday to serve as Session Leader
for experiments on energy confinement in single and double null
configurations.

Chris Boswell visited MIT on Friday 19 December to discuss his work at JET
through the JET TAE collaboration with Miklos Porkolab and Joseph Snipes. His
ongoing calculations with the MISHKA code of Alfven Cascades in C-Mod were
particularly of interest.

Bob Childs was near Seattle, Washington last week working with our lower
hybrid coupler brazing vendor.



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