From: IRBY@CMOD.PFC.MIT.EDU
Date: Tue, 2 Jun 1998 9:46:19 -0400
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980602094619.25a00663@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT

Alcator C-Mod Weekly Highlights
June 1, 1998


Engineering:

Work on the refurbishing of the TF magnet continued as our
main focus last week. Sealing of all finger joints on the TF core
was completed. Some rough sanding of the core fingers was started as we
begin to clean up the areas that will be replated. Sealing of the TF arm
finger joints has also been completed.
Inductive heating tests at Pillar Industries went well last week.
The new transformer and coil greatly improved the coupling efficiency
to the inductive heating supply. Visual inspections of the soldered
feltmetal pads looked very good, but analysis of the quality of the
solder joint is still ongoing. The inductive heating unit will be
in-house this week, so technique development will soon proceed at a much
faster pace.
The new instrumentation rack needed for the full scale,
full current, feltmetal test has been completed. This rack includes the
isolation amplifiers and integrators needed for temperature, voltage, and
current measurements. The CAMAC and serial highway hardware is also ready,
and the data system tree has been set up. A Rogowski coil capable
of measuring total current and current density through a single feltmetal
pad has been designed and is on order. This coil must pass through a slot
between finger joint fingers only 0.025" wide. Printed circuit board
fabrication techniques are being used to produce this new device. Operation
of the TF supply into the low impedance test joint is being carefully
reviewed.
We continued to make progress on the DNB. The required communication
between the VAX control and timing software and the PLC was finalized. The
interface between CAMAC and the PLC I/O modules was designed and
construction was begun. Compensation of the high voltage dividers for
measurement of voltages at the beamline were completed. Some modifications
were added to prevent high voltage problems should the dividers become
disconnected. Major elements of the Kirk Key safety interlock system were
completed. The high voltage dividers for the mod/reg were assembled,
installed, and anode and cathode connections made. Work continued on the
PLC programming, detailed design changes and associated documentation for
the Master Control Logic chassis, and conditioning of the oil in the tank
shared by the arc/filament/snubber supplies.
FMIT#4's input cavity was disassembled to locate and repair a
suspected arc. Several arc tracks were found in the vicinity of the
load isolation capacitor. The most significant track was located under the
the collar which makes up part of the isolation capacitor. To fix the
problem, some teflon tubing will have to be replaced. In discussing
the problem with the PPPL RF group, we found they had not attempted to fix an
isolation capacitor before, but they did suggest a vendor from which to
buy the teflon tubing (a specialty item).

Physics:

Analysis has recently been carried out of Te pedestal and L-H
temperature thresholds in a current scan from 450 kA to 1.4 MA (run 980203).
Te pedestals in H-mode were measured using a technique in which small ramps
of Bt sweep the ECE channels a few cm. It was found that the Te pedestal
width did not show a simple scaling with current or rho_pol, but increased
with Ip up to 800 kA and then decreased at higher current. This indicates
that other variables must be important. The best correlation found was
with the pedestal height; higher pedestals had larger widths and similar
Te gradients. In contrast, the x-ray pedestal had increasing amplitude and
decreasing width as the current increased, consistent with the x-ray
pedestal parameters previously measured during current ramps in a single
discharge.
Temperatures just prior to the L-H transition were also analysed.
Te at the 95% poloidal flux surface showed no significant current dependence.
However, the temperature gradient at this location, and Te at locations
further in the plasma, did increase (less than linearly) with Ip.
Recent RF analysis effort has centered upon a hydrogen concentration
scan in deuterium plasmas. The goal was to document the transition from
minority to mode conversion heating in D(H) plasmas. We mixed various D/H
concentrations in the gas fueling plenum in an attempt to control the hydrogen
concentration in the plasma. PCX measured H/H+D ratios were 2.5%-30%. At low
concentrations (H/H+D ~ 2.5%) the total central absorbed power normalized to
injected power (Pabs/Pinj) was about ~65% in H-mode. Increasing the measured
H/H+D to ~10%, Pabs/Pinj increased to ~85% which is in qualitative
agreement with the single pass absorption minority concentration
scaling. Using sawtooth reheat analysis, the central electron heating
power was greatest for H/H+D ratios between 5-10%. For H/H+D ~ 15%,
the central electron heating power decreased, although Pabs/Pinj
remained constant within the error bars. This scaling suggests the heating
power is mostly to ions at this concentration. The observed electron
heating power scaling appears to be in agreement with theory although
detailed study is still ongoing. At the moment, the modelling is
shackled by problems with the SPRUCE (RF) module in FPPRF. Dan Clark
under the supervision of Cynthia Phillips (PPPL) is working to replace the
SPRUCE module with TORIC. At the highest concentrations, mode
conversion was hoped to be observed, but the experimental evidence is
inconclusive. This result may be because of the single pass absorption
density dependence. These discharges were performed at typical
C-Mod densities, ne_av~3x10^20 m^-3, which even at the highest H/H+D
ratio results in poor transmission to the mode conversion layer.
Further analysis and modelling is ongoing.
Last week, we were successful in running ACCOME on an ALPHA machine
with the equilibrium solver, bootstrap current drive module, and
neutral beam current drive module all iterating properly. Work will
continue to add the lower hybrid module. The major problem in
implementing ACCOME on the ALPHA's has been converting the code from
single to double precision (on the CRAY a single precision roundoff is
~10^-15 and on an ALPHA it is ~10^-7).

Visits and Travel:

Peter Heimann, Karl Behler, Klaus Englehard, and Georg Kuner visited
from IPP Garching to discuss the use of the MDSplus data system on
Asdex-upgrade and W7X. Boyd Blackwell from Australia National University
visited to arrange the installation of MDSplus for use on the H-1NF experiment.
Dr. Hiroshi Tamai from the Large Tokamak Experiment Division II
(JT-60U), JAREI visited the C-Mod facility and met with members of the
C-Mod edge group on Tuesday and Wednesday. This visit was made possible
through ongoing US-Japan exchange agreements. Topics of discussion
included divertor neutral pressure measurements, particle recycling
and control, detached divertor physics, and divertor impurity enrichment
and compression.
John Goetz, Brian LaBombard, Bruce Lipschultz, Dimitrios
Pappas, Spencer Pitcher, Jim Terry, and Maxim Umansky attended the
PSI conference in San Diego, May 16-23. Five posters were presented
(LaBombard, Nachtrieb, Pappas, Pitcher, and Umansky). Also
three orals (Goetz, Lipschultz, W. Wampler - a Sandia collaborator)
and one invited talk (Terry) were presented. Bruce Lipschultz also attended
the ITER edge database expert group meeting following PSI. Miklos Porkolab and
Earl Marmar attended the FESAC meeting on May 27th. Miklos Porkolab attended
a Director's meeting with Anne Davies afterwards in Gaithersburg. William
Burke and Edward Fitzgerald were at Pillar Industries on May 28-29 for
acceptance tests on the induction heating power supply. Jim Irby attended
the Enabling Technologies meeting at DOE, Germantown, May 28 and 29. Bill
Rowan (U. Texas) is in Austin Texas assembling DNB related optical and
vacuum equipment, and directing preparatory machining on F-port horizontal
flange (surface grinding, bolt holes, cowling recess)