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
Oct 25, 1999

Plasma operation resumed last week at Alcator C-Mod. Magnet testing was
carried out on Monday, followed by a series of plasma shots. Plasma runs were
carried out Tuesday-Friday. This week's runs were devoted to ICRF
conditioning, evaluation of the performance of the J-port antenna, and
exploration of the sensitivity of heating to minority (H) concentration. A
total of 64 plasma shots were produced, with a start-up reliability of nearly
90%.

Following cooldown, a series of TF magnet tests were carried out to evaluate
the single-turn resistance measurements in the vicinity of H-port. Prior to
the recent warm-up these measurements had included some anomalously high
readings at low current (<1kA) , while measurements performed during
high-current pulses (>50kA) showed no unusual features. Initial low-current
measurements on Monday were nominal, as were those during high-current
pulses. A modification to the normal TF current waveform was adopted which
provides a 50KA plateau at the end of the shot to allow continued monitoring
of the resistances during plasma operations.

The J-port antenna was operated with the new feed configuration, which permits
adjacent current straps to be driven 180 degrees out of phase. Both 0,pi,pi,0
and 0,pi,0,pi phasing were used in these experiments. A phase feedback circuit
operated successfully to maintain the programmed phasing during plasma
operation. The cavity modifications permitted increased power from FMIT#1 & 2
(D- and E-port antennas), with a maximum of about 1.8MW injected from each
system; the limit on coupled power from these systems is now set by the
voltage at the antennas, rather than the source. With all transmitters
operating simultaneously, a total of over 5MW was injected for a short time.

On Thursday a study of heating and impurity generation from each antenna was
carried out. For the same power levels, the radiated power associated with the
J-port antenna is higher than for either D- of E-port. Central heating
appeared to be similar, but the temperature rise in the edge region is smaller
with J-port, and it was more difficult to access the H-mode and EDA
regimes. The impurity influx from J-port appears to be dominated by titanium,
which presumably comes from the TiC coating on the Faraday screens and/or
antenna protection tiles. Further aggressive antenna conditioning in plasma
might help to alleviate this problem.

On Friday we carried out a systematic study of the ICRF heating efficiency as
a function of minority (H) concentration. The H/D ratio was determined
spectroscopically using Balmer alpha emission, which provides a measure of the
concentration in the plasma edge and SOL. D- and E-port antennas were used to
inject about 2.5MW into the plasma, with the power waveforms modulated to
allow the absorbed power to be determined by the break-in-slope
technique. Best results were obtained at the lowest H concentration obtainable
with no H2-puff, following overnight ECDC in D2; this condition corresponded
to a measured H/D of 2.5%. H2 gas puffing was used to increase the
concentration up to >20% during the course of these experiments.


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

Preliminary analysis of fluctuation data recorded during a recent ohmic H-mode
run by the A-port and F-port scanning probes (ASP, FSP) indicates the presence
of a coherent ~100 kHz mode near the separatrix during EDA H-modes. This mode
is seen on both the ASP and FSP probes over approximately 1 mm of their final
travel when they are scanned up to or slightly inside the last closed flux
surface. At the same time that the probes detect a coherent oscillation, the
reflectometer and phase-contrast imaging systems also see a coherent
oscillation at the same frequency (+/- 10%). The oscillation is most
noticeable on ion saturation current signals but is also seen on floating
potential signals. The mode appears to propagate in the electron diagmagnetic
direction. Fluctuation-induced particle fluxes are estimated from the FSP
probe set, using two probes as plasma potential fluctuation monitors and one
probe as a density fluctuation monitor. Cross-field particle transport is
clearly enhanced by the presence of the ~100 kHz coherent fluctuation,
suggesting that this mode is responsible for the reduced particle confinement
time and enhanced D-alpha seen in EDA H-modes. In contrast, no coherent
oscillations are seen in ELM-free H-modes. During the ELM-free phase, ion
saturation current fluctuation levels normalized to the mean value decrease
dramatically as the probe traverses the scrape-off layer towards the
separatrix. Fluctuation-induced particle fluxes correspondingly decrease as
the separatrix is approached. The magnitude of the fluctuation-induced
particle transport in the scrape-off layer is consistent with simple estimates
from ionization measurements and flows to limiter/divertor surfaces.

A new edge fluctuation diagnostic is operational this run campaign. It relies
on visible light emitted at the plasma boundary and detected by sensitive
diodes, whose photo-current is amplified by a circuit with high-gain out to
high frequencies (~250 kHz). The detection system is similar to the U. of
Texas BES system also operating on C-Mod and described in earlier Weekly
Reports. The experimental design is similar to the BES experiment in its
attempt to relate fluctuations in light emission to fluctuations in electron
density. Like the BES system, this new diagnostic has essentially-toroidal
views of intrinsic emission. However it can also view a gas jet near the
plasma boundary with extremely high radial (~3 mm) resolution. There are three
detection systems allowing for three views at any one time. The gas jet and
the fiber optic views are part of another experiment. The gas jet allows for
a local, non-chord-averaged measurement. By puffing D2 from the jet, we
observe that the relative fluctuation amplitude from the local measurement is
increased by a factor of two compared to the chord-averaged measurement. (The
D_alpha brightness was increased by a factor of four as well.) The observed
RMS variation in the D_alpha emission from a D2 puff is typically 15-20% of
the average emission in L-mode plasmas. The fluctuations are NOT a result of
photon-counting statistics. The frequency spectrum of the D_alpha emission
fluctuations from the puff is broadband out to about 100 kHz, dropping
strongly at frequencies > 100 kHz. Since there is typically no intrinsic He
in C-Mod, all of the measured signal is from the local He emission when He is
puffed from the jet. The RMS fluctuation in the intensity of the neutral He
5876 A line is also measured to be 15-20% in L-mode.

By looking at cross-correlations in the fluctuations from radially separated
views we plan to measure the radial size of the fluctuations and to see if
they propagate radially. These experiments have benefited from help and
guidance from Stewart Zweben at PPPL and are part of an MIT/PPPL
collaboration. Dominque Heubner, a summer student from the Univ. of Wurzburg
in Germany, was instrumental in the experimental construction.

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

Miklos Porkolab attended the Fusion Power Associates Meeting in
Washington, Oct. 19-21, and gave the talk "Advanced Tokamak: Promise and
Near Term Plans". The topic of the meeting was : Fusion Power for the 21st
Century: Science and Technology for the New Millennium".

Last week C S Pitcher attended the Fourth International IEA Workshop on Helium
Transport and Exhaust in Fusion Devices in Georgia. The workshop brought
together approximately 40 researchers from the US, EU and Japan. Pitcher
presented preliminary results from the divertor bypass experiments on Alcator
C-Mod. The topics of the workshop included RI discharges, the behavior of
helium and other recycling gases in divertors, edge fluid and Monte Carlo
modelling, core helium transport, helium exhaust in ST's and stellarators,
ergodic divertors and means of enhancing helium core transport using RF
techniques. The following programs were represented, ITER, JT-60U,
ASDEX-Upgrade, DIII-D, TFTR, JET, TEXTOR and C-Mod.

Dave Mikkelsen of PPPL was on-site C-Mod last week to work on transport
modeling.