From: WOLFE@CMOD.PFC.MIT.EDU
Date: Tue, 17 Feb 1998 18:58:58 -0500 (EST)
Reply-To: WOLFE@PSFC.MIT.EDU
Message-ID: <980217185858.2440059a@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT Plasma Fusion Center


Alcator C-Mod Weekly Highlights
Feb. 17, 1998

Physics operations continued at Alcator C-Mod last week. Four run-days were
scheduled and completed. Approximately 95 plasma discharges were produced.

A fresh boronization was carried out on Monday night; a total of 133psi of
diborne mixture was used. Tuesday's run began with a series of fiducial shots
to evaluate wall conditions and establish reproducible operations. As usual,
startup reliability after a boronization was not good, but improved through
the day. Following a series of 800kA fiducial discharges, with RF power up to
2MW, the current was increased to 1MA and the RF power to 2.7MW. This resulted
in two shots with stored energy above 210kJ and neutron rates over 10^14/sec,
which were the highest performance shots obtained in the current campaign.

Having established satisfactory operational reliability, we proceeded to carry
out an experiment (MP#135) to investigate the role of surface voltage on SOL
flows and recycling asymmetry. The goal of these experiments was to look for
any influence that the surface loop voltage might have on the up/down
asymmetry in recycling light on the inner wall during limiter discharges. The
approach was to program 0.1 MA steps in 0.8 MA inner-wall limited plasmas,
thus imposing approximately a 2.5V peak-to-peak modulation on the loop
voltage. The symmetry of the recycling light on the inner wall was monitored
using the B-side and F-side D-alpha reticon arrays. Also, D-Balmer series
emission from the inner wall was monitored with the Chromex spectrometer to
determine the relative importance of volume versus surface recombination. The
symmetry of the inner-wall recycling light did appear to be affected by the
perturbations. For moderate density discharges a large up-down asymmetry was
generally observed when the loop voltage was high. This asymmetry went away
when the loop voltage dropped to zero. Since we were not able to reverse the
loop voltage (minimum loop voltage was ~ 0) we could not determine whether the
asymmetry actually followed the sign of the voltage, which would have
strengthened the evidence for the proposed effect. Nevertheless, we have some
interesting data to sift through.

Wednesday's run was a continuation of our investigations of reverse shear
operation. Current ramp rates up to 5.7MA/sec were tried. Toroidal field was
also varied between 4.9 and 5.7T. RF power in the current ramp was varied,
with up to 2.7MW injected before 0.1 sec. Electron temperatures approaching 5
keV were obtained at densities around 1.1e20/m3. We found that a ramp rate of
> 5.5 MA/sec resulted in MHD modes which were related to impurity injections.
Scanning the magnetic field to move the heating off-axis was not beneficial;
the time to onset of the first sawtooth crash was reduced considerably even
though the central temperatures were similar. Li pellets were injected both
in the flat top and ramp up. The plasma accepted three big pellets readily
even at 0.07 sec into the discharge. We found no enhanced confinement with
the pellets early in the ramp up (0.07 and 0.12 sec), but PEP mode behavior
was observed in discharges where the pellet was injected at 0.15 or 0.18 sec.
This suggests that deeply reversed shear profiles do not help in obtaining PEP
mode. The sawtooth onset was delayed in the latter (PEP) cases to ~.26sec.
These shots also had significant increases in toroidal rotation after the
pellet injection. PEP modes were readily obtained in the flat top.

Thursday's run was devoted to studies of D(He3) ICRF heating at Bt~8 Tesla
(MP#150A). This run was significantly better than last week's. A few H-modes
were obtained, but did not develop steady state character nor particularly
good quality. From the RF perspective, the optimum He3 gas puff for good
central ion and electron heating via He3 minority heating was identified.
Central ion temperatures reached 4 keV and central Te at the top of the
sawteeth reached 5 keV. The multiple H-mode transitions during the discharge
make determining the total absorbed power difficult; however, absorbed power
appears to vary strongly with He3 concentration with a maximum ~75% . By
varying the He3 concentration, the split between minority and mode conversion
heating was explored. Lowering the toroidal field to move the D resonance and
Shear Alfven resonance out of the plasma did not significantly improve total
absorbed power. A more detailed analysis will be required to confirm this.
Over the last few discharges, the density, current, and RF waveform were
varied to explore their effects on obtaining H-modes. Lowering the density
improved H-mode accessibility, but not the duration of the H-modes.

Friday's run was in support of MP#163 "High q|| dissipative divertor". This
proposal is an attempt to achieve simultaneously a good confinement
H-mode with low Zeff and a detached divertor. A previous run was successful
in obtaining a detached divertor H-mode with good confinement. However,
the Zeff was rather high (although the change in Zeff due to impurity
puffing was acceptable). Friday's run was an attempt to improve the
plasma cleanliness and to diagnose well the divertor plasma.

The goal of simultaneously achieving good confinement, low Zeff, and a
detached divertor was reached. The algorithm developed in the previous run
(980123) to use the edge bolometer signal to feedback on the impurity gas puff
was used to achieve detached divertor H-modes with varying scrape-off layer
power flows. This technique worked well in that varying degrees of detachment
were achieved by varying the requested edge bolometer waveform. The deepest
detachments achieved were up to the nose of the outer divertor. The amount of
nitrogen required for minimal detachment varies with q-parallel; the higher
the q-parallel, the larger the impurity puff needed to see detachment near the
strike point. Also, the nitrogen definitely sticks to some surfaces, causing
a buildup throughout the day. The Zeff began the day near 1.0 and was up to
1.3 at the end of the day (during the H-mode, before the impurity puff). On
the best shot the H-factor was 1.8 and the Zeff was 1.1 as the H-mode
developed. As nitrogen was puffed and the divertor detached the H-factor
decreased to 1.6 and the Zeff increased to 1.4. The impurity confinement time
was measured by injection of a trace amount of niobium into the detached phase
of the H-mode. This time is ~70msec, in line with previous measurements for
an EDA H-mode. The x-ray pedestal width and location do not change upon
divertor detachment and are typical of EDA H-mode. Measurements of the heat
flux using fast thermocouples embedded in the plates (Gangadhara and
LaBombard) show that high heat fluxes (>400 MW-m^-2) have been lowered to
immeasureable levels by detaching the divertor. The heat deposition profile
is very narrow in the attached H-mode compared to that observed in high power
L-mode.

Over the weekend, work on the ICRF transmitter FMIT#3 continued. Preliminary
measurements indicate that the transmitter works well below 70 MHz and will
become more difficult for 70-80 MHz. Currently moderate power has been
obtained at 75 MHz. Dr. Gerd Schilling (PPPL) has indicated that 1.5 MW was
obtained at 80 MHz and 2 MW for 76-78 MHz. Retuning of FMIT#3 to 78 MHz will
continue this week.

Lines from the D_0 Lyman series (n=2,3,...9 to 1) are now routinely observed
in the VUV spectral region when observing the divertor plasma or a MARFE in
the main chamber. In addition, radiative recombination continuum emission
corresponding to photo-recombination directly into the D_0 ground state is
observed. The contiuum emission rises sharply around 92 nm (photon energy=13.6
eV) and decreases with increasing photon energy. The fall-off of this
continuum emission is a strong function of the local electron temperature when
the temperature is less than about 3 eV. In the C-Mod divertor plasmas and in
the MARFE, temperatures from 0.7 to 1.3 eV have been measured using these
continuum sprecta. Studies are underway to model the spectra, including the
Lyman and Balmer series lines and the continuum spectra, for a single
temperature/density region. The modeled spectra will be compared with the
measurements of these features to see if the single temperature/density
assumption is justified, and, if so, to measure the temperature and density.

The DNB vacuum system reached its ultimate pressure of 10**-8 Torr early
this week and remained there. Additional tests of the breakdown voltage of
the oil in the DNB Accel supply indicate that it is holding at an
acceptable level and conditioning is finished. Refurbishment of the
control system for the Accel supply continued.

Dr. Gerd Schilling (PPPL) was visiting last week. He participtated in
experiments and RF operations.

Perry Phillips, UT-FRC, reported on progress at FRC and at UC-Davis on the
high resolution ECE system for C-Mod. Millitech completed construction of
the two second harmonic mixers for the system. These mixers cover the
frequency band from 234-306gHz, and each has a 40+gHz bandwidth. A
prototype IF detector system is under construction at UC-Davis with a
projected completion this month. A page on the FRC WWW site
<http://w3fusion.ph.utexas.edu/frc> was set up to describe the high
resolution ECE system and to chronicle progress.

Ricky Maqueda from Los Alamos visited C-Mod during the weeks of Feb. 2 and
Feb. 9. He installed a set of temperature references on the IR imaging
system to aid in the calibration process and added a water chiller to cool
down the IR camera and telescopic lens so as to reduce the number of
background counts. Unfortunately, the recent boronization campaigns have
apparently coated the sapphire vacuum window reducing substantially the
throughput and yielding results of only qualitative value.

Christopher Watts was up from Auburn University last week to work on the
Temperature fluctuation measurements on the Heterodyne Ordinary mode
radiometer. A different configuration of the Auburn ECE T~ system yielded
preliminary correlation power spectra consistent with earlier results on
TEXT.

Earl Marmar attended two meetings at the San Diego ITER co-center last week: a
progress meeting on the design of optical/spectroscopic diagnostic systems for
ITER; and the Eighth workshop and technical meeting of the ITER expert group
on diagnostics. At the second meeting, he gave a talk entitled: "Alcator
C-Mod: Spectroscopic Diagnostic Development".