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
June 28, 1999

The maintenance interval continued last week at Alcator C-Mod. No plasma
operation was scheduled. The tokamak continues in a standby mode, with
cryogenic cooling and vessel heating turned off. The maintenance period will
continue this week.

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

Observations of pedestal positions from the edge x-ray arrays and the
visible continuum array indicate that the location of the outboard midplane
x-ray pedestal foot (position plus half-width) is closely correlated with the
location of the visible bremsstrahlung plateau (position minus half-width).
This may be evidence for the effects of neoclassical impurity transport on the
outgoing fully stripped fluorine ions. The dominant contribution to the edge
x-ray signals is believed to be due to the radiative recombination of these
ions. Using the bremsstrahlung profiles to construct density profiles, fluorine
transport modeling, including the neoclassical inward pinch terms, which is
dominated by the strong density gradient, yields profile shapes which are in
excellent agreement with those seen experimentally near the outboard midplane.
This model cannot explain the x-ray profiles which are observed near the top
edge of the plasma, where the emissivity pedestal is typically located much
closer to the last closed flux surface. Vertical impurity drift, as has been
documented for highly stripped argon ions [Rice, et al., "X Ray Observations of
Up-Down Impurity Density Asymmetries in Alcator C-Mod Plasmas", Nuclear Fusion
37(1997)241], is a likely mechanism influencing the location of the recombining
fully stripped fluorine at the top of the plasma. Two dimensional impurity
transport modeling will be required to sort out these effects quantitatively.


Scrape-off layer (SOL) profiles of density and temperature from a number of
high spatial resolution edge diagnostics have been compared. Cross-field
density and temperature profiles from the outer divertor probe array (ODIV),
the F-port vertical scanning probe (FSP), the A-port horizontal scanning probe
(ASP) and the edge Thomson scattering system (ET) were found to be in
reasonable agreement. All these diagnostics showed sharp gradients in electron
pressure on open field lines which could be made to overlay with small
adjustments to their midplane-mapped major radius coordinate (Rho) determined
by the EFIT magnetic surface reconstruction. Data from the visible
bremsstrahlung camera (NROOTZ) and the helium gas puff diagnostic (Kaiser)
were also considered for a limited set of shots. Presently, data from these
diagnostics do not appear to follow the sharp gradients in the SOL as seen by
the probe or ET systems. However, both of these systems are still under
development. Data from the reflectometer system does show sharp density
gradient features in the SOL, but was not included in the survey since these
data are also preliminary.

Cross-field density and temperature profiles from over 120 time slices were
included in the survey, spanning a variety of discharge conditions. Assuming
that the electron stagnation pressure should be roughly constant on open flux
surfaces in the SOL, the mapping between diagnostics via the EFIT magnetic
surface reconstruction was tested. As expected, random fluctuations in the
mapping on the order of 2 mm in Rho were evident. These may be a consequence
of fluctuations in the measured n, Te values as well as the EFIT-derived Rho
coordinates. However, some persistent systematic offsets and trends were also
identified, some lying well outside the 2 mm random error range. For
discharges with low stored energy (typically L-mode cases) systematic
adjustments were required to bring the observed profiles into alignment; the
largest of these is approximately 4mm for the ASP location. Discharges with
higher stored energy, including H-modes, exhibited trends in the mapped
location which varied linearly with stored energy for ODIV, ASP and ET; the
FSP data did not indicate any additional shift correlating with stored energy.
Offsets which are independent of plasma conditions may be due in part to
errors in diagnostic placement. However, offsets which appear to depend on
plasma stored energy may result from errors in the EFIT reconstruction.

For a small set of H-mode discharges in this survey, the visible
bremsstrahlung camera recorded the major radius location of the pedestal in
continuum emission. It was noted that the location of the pedestal also showed
an offset and trend in major radius relative to the EFIT-derived LCFS, similar
to that found for the ASP diagnostic. Adjusting the LCFS location to match
the trend of the ASP data, one concludes that the visible continuum pedestal may
in fact be close to the LCFS, independent of stored energy.

It was also noted that in some H-mode discharges, the peak electron pressure
detected on the outer divertor probes could reach 1/2 of the electron pressure
recorded by the edge Thomson system at the top of the density pedestal. Since
the divertor probes are necessarily on open field lines, this also implies
that the sharp gradient (L ~ 2 mm) associated with the density pedestal must
be quite close to the LCFS.


Results from the (LANL) IR imaging system have been analyzed for recent shots.
The surface heating in the divertor region below the outer nose is dominated
by hot spots with typical sizes of 1-2 cm^2 and discontinuous toroidal bands.
The position of these hot spots are consistent with EFIT equilibria
reconstructions and are observed to reach ~500 deg. C, corresponding to an
energy flux of ~10 MW/m2. Other surfaces reach typically the 150-250 deg. C
range. Data analysis continues as well as preparations for the oncoming runs.

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

The primary focus on FMIT#3 and #4 was on preparing the RF system for vacuum
conditioning of the J-port antenna. Initial conditioning was started but
unanticiptated control and protection characteristics allowed excessive power
during vacuum conditioning with FMIT#4, resulting in a transmission line arc.
A series of tests were done to investigate the antenna protection (arc
detection and over voltage) and rf control (power feedback) characteristics.
Each transmitter's low power RF drive up to the input of the IPA was tested
into dummy load. System tests of the antenna fault protection were also
conducted. A power-limiting drive will now be utilized during vacuum
conditioning. At present, 100 kW, corresponding to 20 kV at the antenna, has
been injected with FMIT#3 during vacuum conditioning (the target values are
250-300kW, 40kV). Progress has been steady. FMIT#4 had successfully injected
60 kW before the transmission line arc. During testing, FMIT#4's IPA was
observed to be self-oscillating. It was successfully retuned to eliminate the
oscillation and the output power into dummy load was raised to 1.8 MW. The
instrumentation has progressed to the point where the FPA anode current and
voltage are calibrated and have good time response. An RF interrupt test, to
measure the voltage swing resulting from a fast source shutdown, was done. The
remaining issues for FMIT#3 and #4 are to finish vacuum conditioning and
complete the instrumentation.

On FMIT#1 and #2, the crowbar unit testing has been completed. The next step
is to high-pot the tube and tune the transmitters; FMIT#2, which has a higher
power tube, will be the first to be tested. Initial tests of the low power
RF drive have begun. A complete test of the fault and rf control system is in
progress.

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

Rejean Boivin attended a Workshop on radiative mantles, radiative modes,
impurity seeding at Culham Labortory (UK). He presented a paper "Sources and
Radiation Profiles, and their Effects on H-mode plasmas in Alcator C-mod".

Robert Granetz was at JET and Culham last week, attending a tripartite workshop
on issues of steady-state operation with internal transport barriers (ITB's),
as well as a workshop on MHD, Disruptions, and Control. There is considerable
overlap between the two areas because the high beta_N and steep gradients at
the ITB in reversed-shear plasmas tend to give rise to double tearing modes
around the q_min radius, as well as neo-classical tearing modes when the steep
pressure gradient is near the q=3/2 or q=2 radius. These problems are found to
be less of a limitation in the flat (or "optimised") shear regimes (still with
enhanced confinement), but it is expected that further increases in beta_N will
give rise to problems with resistive wall modes.

During the period 21-23 June, Spencer Pitcher visited JET/Culham. This
coincided with a workshop on the RI Mode, which Pitcher attended for part of the
visit. Discussions were had with Culham staff on edge physics on Compass and
MAST, and edge diagnostics (e.g. the Compass/MAST helium beam). With respect
to the helium beam, Compass has been experiencing similar problems as C-Mod
with regard to helium background light levels. Various solutions were
discussed, including chopping the beams. There was some discussion on
informal collaboration in the future.

Ricky Maqueda from Los Alamos visited PSFC during the first days of last
week. In addition to re-calibrating the infrared camera he gave a short
presentation at the Monday Physics Meeting on results from the IR imaging
system.

Perry Phillips, UT-FRC, began a two week visit. He will be developing
software for calibration of the profile data generated by the new 32-channel
ECE system (FRCECE) as well as developing software for profile analysis.

Seven members of FESAC visited MIT this past Thursday, June 24th. (One member
visited a few weeks ago, participating in extensive laboratory tours and
program discussions). FESAC was welcomed by Professor Lawrence Backow, MIT's
Chancellor, emphasizing the importance of the PSFC to MIT's research and
educational activities. There were presentations on the activities of each of
the PSFC's 5 Divisions, consisting of overview talks of recent activities,
longer range plans, and issues. The Alcator program was featured
prominently. Presentations were also given on LDX, Plasma Technology Spinoffs,
and Magnet and Gyrotron Technology Programs. The basic experimental plasma
physics program, as well as progress in theory research, relationship to
astrophysics, and compact ignition experiments (IGNITOR) were also
discussed. The presentations were followed up by laboratory site visits by
committee members.

Mike Roberts and Ray Schwartz of the DoE Office of Fusion Energy Science
visited C-Mod on Thursday and Friday last week. They discussed facility
operations issues with Ian Hutchinson, Earl Marmar, Jim Irby, Steve Wolfe,
Catherine Fiore, and Martin Greenwald.