From: WOLFE@CMOD.PFC.MIT.EDU
Date: Tue, 20 Jan 1998 12:28:20 -0500
Message-ID: <980120122820.3a600e28@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT Plasma Fusion Center
Newsgroups: sci.physics.plasma

Alcator C-Mod Weekly Highlights
Jan 20, 1998

Plasma operations continued on Alcator C-Mod last week. Four run days were
scheduled and completed. Over eighty plasma shots were produced.

Tuesday's run was in support of MP#154, Helium H-modes, and concerned the role
of neutrals and charge exchange in the development of the H-mode edge. The
machine was prepared by helium ECDC over the weekend. H-modes were obtained in
helium plasmas at 5.4T, using H-minority ICRF heating. The threshold edge
electron temperature was found to be similar to, but slightly higher, than for
equivalent deuterium discharges. The low-density boundary, below which no
H-modes were obtained, appeared at higher density than in deuterium. The
H-modes were either ELM-free or exhibited type-III ELMs; no EDA-like H-modes
were obtained in helium. At the end of the day, deuterium was introduced
through one gas valve, and EDA H-modes were recovered. Radiation was
relatively low, and the 2pi photodiode and main bolometers agreed on the total
radiated power level, indicating negligible charge exchange losses. RF loading
was low for the helium discharges, indicating that the a shorter scrape-off
length.

Wednesday's run was devoted to reverse shear startup optimization
(MP#149A). The initial (first 100msec) current ramp rate was systematically
varied between 3 and 5 MA/sec, and the final ramp between 1.5 and
3.6MA/sec. At the slower ramp rates, less current rise MHD activity was
observed, but the onset of sawtooth activity came earlier. ICRF was
successfully coupled to the plasma as early as 50msec into the
pulse. Decreasing the outer gap early in the pulse facilitated early RF
heating. Increased gas puffing caused the sawtooth initialization time to
decrease, but the central temperature and line density at 0.1 sec were nearly
identical. However, Zeff was significantly reduced. Li pellets were
successfully shot into the early ramp, but did not penetrate deep into the
plasma and did not trigger PEP mode. The plasma temperature was too high for
deep penetration. PEP and P modes were achieved in the flat top part of the
discharge.

TAE modes were observed on several of these discharges. King-Lap Wong from
PPPL visited MIT from 1/14/98 - 1/16/98 to participate in the Reversed Shear
experiment MP149A and analyze TAE mode data. A number of shots exhibited TAE
modes during the reversed shear phase of the current rise with ICRF heating.
The modes lasted from 10 msec to as much as 60 msec. The rotation direction
was found to be in the ion direction, as expected for TAE modes. The measured
frequencies varied from 380 - 480 kHz, usually increasing in time. The
estimated toroidal mode numbers lie between 4 and 5. The longest TAE mode was
obtained with the fastest current rise and was interrupted by a Li pellet. If
the mode is assumed to lie in the center of the TAE gap, the frequency
variation would imply a variation in q, at the radius with the steepest
gradient in the fast ion pressure, from about 2.5 to 1.8 over 60 msec. Since
sawtoothing did not begin until about 50 - 60 msec after the end of the TAE
mode, this variation in the central q value appears to be reasonable.

Thursday's run (MP#166) was aimed toward investigating the H/L and disruptive
density limits in C-Mod. The approach was to establish a high density H-mode
and slowly ramp down the plasma current thereby raising the density limit
paramter n/(I/a^2). Bt was not ramped down until late in the current ramp in
order to allow ECE electron temperature profile measurements. In general we
found strong deterioration in H-mode confinement at the higher densities; both
global and local parameters deteriorated toward those of L-mode. Still, clear
transitions could be seen on the edge Te and xray measurements. H/L
transitions occurred at n/nlimit = 0.75-0.8. Temperatures and stored energy
continued to fall in the subsequent L-mode phase, which were terminated by
disruptions at n/nlimit = 0.9-1.1. On one shot, only 1 RF transmitter
operated and the plasma stayed in L-mode. The density apparently ran up to
5.7e20/m3 which is the highest density gas fueled discharge we have seen.
Strong marfing was detected so the interferometer density measurement might be
suspect; this shot will be examined in more detail.

The purpose of Friday's run (MP#162A "Volume Recombination in the Alcator
C-Mod Divertor") was to explore the characteristics of detachment and
recombination. This was addressed successfully in a number of ways: visible
and VUV spectroscopy in the divertor region, divertor and Fast-scanning probes
documenting depth of detachment, plasma TV imaging the divertor using D_Gamma,
bolometry and divertor RGA. Deep detachment was found at high densities.
This is evidenced by movement of the recombination radiation up the divertor
face to above the noses and to the x-point. This was clear from the TV and
from the Reticon arrays and spectrometer views. Strong recombination was
observed on and above the inner nose and along the outer leg. Strong
self-absorption of Lyman-Beta was seen on several shots; strong
self-absorption of Lyman-Alpha was implied on shot 036. A slow detachment was
achieved for several shots. The RF power was ramped up on the final sequence
of shots to reattach the outer divertor. J. Goetz and S. Pitcher did some
experiments relating to filtering the neutral power from reaching the 'Ledge'
bolometers. Gas was puffed into the bolo box to attenuate the neutrals
through scattering. Clear effects were seen on some channels. J. Goetz was
also looking at the compression of Ar into the divertor through this period.
There were significant effects there as well, with more Ar in the
divertor during detachment.

Engineering systems performed well this week, although about four hours were
lost from Wednesday's run due to a problem with one of the toroidal field
supply breakers, which would not close on command. This problem was diagnosed
and repaired, and the run resumed. The TF scanner, which monitors the
resistance of each leg of the toroidal field coil, is being run routinely, in
preparation for 8 Tesla operation next month.

Progress continued on development of the DNB for C-Mod. The required 8 HV
cables were installed in the completed sections of the low capacitance HV
transmission line. The short high capacitance section required at the
cell penetration is under construction. Mechanical design of the kirk key
interlock was completed.

Roger Bengtson and David Winslow UT-FRC visited to continue installation of
the reciprocating Langmuir probe optimized for turbulence measurements.
The probe is now in place at K-port. Installation will be completed during
the week of January 26.

Gary Hallock, UT-FRC, visited to assist with the continued development of
the phase contrast interferometry diagnostic.

Basil Duvall from EPFL in Lausanne visited Tom Fredian and Josh Stillerman to
install remote bitbus communication software. The software developed by Basil
will allow us to perform computationally intensive operations required for the
hybrid plasma control system on faster computers. Previously these operations
had to be done on slower computers equipped with computer-to-bitbus
communication hardware. The new software will be put into service when there is
a sufficent break in the C-Mod operation schedule to provide time for testing of
the software. Basil also discussed MDSplus usage at EPFL and possible interest
in the MDSplus port to UN*X currently underway.

The Plasma Surface Interactions Conference Program Committee met at MIT last
Monday and Tuesday to review submitted abstracts. The conference will be held
in San Diego the week of May 18. The participants who attended were Ali
Mahdavi (chairman - DIII-D), J. Winter (U. Bochum, Germany), G. Matthews
(JET), N. Noda (NIFS-Japan), R. Behrisch (Max Planck, Germany) and Bruce
Lipschultz (MIT).

Dr. Noriyasu Ohno from Nagoya University in Japan has joined the PSFC
as a visiting scientist for a 9 month stay. In Japan he has worked recently on
the linear device NAGDIS-II which was used as a divertor-plasma simulator. At
the PSFC he will work primarily with the Divertor Theory group, but will also
participate in C-Mod divertor experiments.

Ricky Maqueda from Los Alamos Nat. Lab. visited C-Mod last week to work on
the calibration of the IR imaging system.

Pablo Acedo returned last week to continue work on the tangential two color
interferometer. He will be here until mid February as part of the Carlos III
University/MIT collaboration.

A fresh boronization was carried out over the weekend. 137 psi of diborane was
used, for an average coverage of about 2000 Angstroms. This boronization
processed the most diborane ever used during a single C-Mod boronization. In
addition, we scanned the ECDC resonance out past the Faraday shield midplane
location to just behind the current strap. Previous boronizations during this
run campaign have been limited to just up to the Faraday shield. This week
will be another plasma operations week, with four runs scheduled.