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
May 21, 2001

Plasma operation resumed at Alcator C-Mod this week. Run days were scheduled
Tuesday and Wednesday for the purpose of full operations systems testing and
commencement of plasma conditioning. These runs were completed successfully,
with short plasma shots being produced each day. The facility was then
returned to standby status for the remainder of the week, to permit work on
auxiliary systems, particularly the RF, DNB, and diagnostics, to proceed.
Work also continued on the lower hybrid and divertor upgrade projects.

Operations
----------

The Electron Cyclotron Discharge Cleaning (ECDC) conditioning campaign begun
the previous week continued through the weekend, with the vessel held at
60C. On Monday the magnet systems were cooled down to -125C in preparation for
operation. Also on Monday, the alternator, which provides 13.8KV power for the
experiment, was run up to full speed for the first time this year.

On Tuesday all magnet power systems were tested under PLC control. All systems
performed nominally, except for a spurious fault indication in one of the OH2
supplies which was traced to a defective solid-state relay, which was
replaced. The power testing took most of the day. The final shot was a plasma
attempt, which was successful in producing a short (170msec) 500kA plasma at
our standard field of 5.3 tesla. The discharge was quite resistive and dominated
by wall-influx and impurities, as is typical for first operation following a
major vent. Several weeks of tokamak operation and continued ECDC are normally
required to produce high-quality discharges for physics studies.

Following overnight ECDC, we began operation on Wednesday with a repeat of the
plasma shot from Tuesday, with quite similar results. The next three plasma
attempts resulted in fizzles. This behavior is also typical of initial plasma
operation, as each full plasma evolves dirt from the walls which must be
cleaned away before another successful current rise can be produced. During
this sequence it was noted that the resistance of the coaxial joint connection
to the OH2L coil was increasing from a fraction of a uOhm to the 2uOhm
level. We have observed this behavior, attributed to reseating of the joint
connection, at the start of all recent campaigns; after additional machine
cycles and thermal cycling the resistance returns to its nominal value. A
check of the resistance on Thursday morning indicated it was indeed back below
1 uOhm. The last part of the run day Tuesday was devoted to a series of
beam-into-gas shots, with 5 tesla toroidal fields, in support of testing and
calibration of the diagnostic neutral beam diagnostics.

Plasma operation will resume next week, with the primary goal of machine
clean-up and conditioning. Additional diagnostic systems will also be brought
back on-line during this period.

ICRF Systems
----------

Work continued this week to bring the ICRF system back online after the
shutdown. XMTR 1 and 2 (D and E antennas) were successfully tested in vacuum.
Their instrumentation is fully functional and tested. These systems were
vacuum conditioned up to 30 kV each. Further conditioning is still required.
XMTR 3 and 4 are ready for operation (J antenna,4 strap). The additional
instrumentation for J-port, which includes additional directional couplers,
voltage probes, B dot probes and faster phase measurements, is being brought
online and calibrated. Upon completion of the instrumentation checkout, the
J-port system will be ready for vacuum conditioning as well.

The J-Port MKS pressure gauge operated properly during plasma discharges with
a very low noise level. This gauge will be used to monitor the pressure
behind the J-Port antenna.

DNB Diagnostics
---------------

Beam conditioning continued this week. Early in the week, a low
voltage control system power supply was repaired: Its overvoltage
protection latched up during a HV supply crowbar. Some time was lost
due to problems with a CAMAC digitizer: The software driver was
modified and caused a digitizer malfunction.

On the beam into gas shots, some BES data was acquired. First data was
acquired with the h-alpha duct monitors. An MSE calibration was also
attempted. Although some signal was observed, the MSE S/N was not adequate
for the gas pressures available, which were limited by concerns regarding DNB
duct damage. The calibrations were deferred until later in the campaign.

By noon on Friday, the beam had operated to 49 kV and 5 A.

Power Systems
--------------

Additional capacitors have been received and are being stored in the power
room. This gives us a total of four spare capacitors for the commutation
banks.

Testing and conditioning of the reworked commutation capacitor banks was
completed during the power systems tests on Tuesday. The new cabling
connections show little or no movement as viewed from the monitoring camera
during operations.

A calibration error on the recently installed 4160v experimental power camac
signal was traced to the tolerance value of the voltage divider resistors,
which are now being replaced. Work continues on calibration checks of the 4160
three phase current signals.


Lower Hybrid MIE Project
--------------------

Design activities for the Lower Hybrid waveguide layout are proceeding. Design
of all printed circuit boards for the transmitter equipment rack and coupler
protection system was completed. These will be sent out to multiple PC board
manufacturers for quotation. Brackets for mounting the current transformers
on the klystron carts were redesigned for additional high-voltage clearance.
Fabrication of these brackets is proceeding. One of the klystron cart current
transformers came supplied with a steel insert. This insert was determined to
be unacceptable due to sharp edges which would cause field enhancement. The
modification or redesign of these inserts is being evaluated. Modification
and installation of new sub-chassis on the transmitter equipment racks is
underway.

Measurements are being carried out on the vector monitor attentuation and
phase characteristics, using locally-developed software to set the monitor and
a network analyzer for the measurements. Work continued on the arc detector
system.

A LH project design review with PPPL is scheduled for next week.

Inner Divertor Project
-----------------------

135 more tiles arrived from the vendor this week, and passed fit-up and gage
tests. We now have 1157 tiles in-house. Aluminum prototypes of the probe box
tiles were made for fit-up testing. Molybdenum pins for the probe assembly
were fabricated and fit-up successfully. Machining of the Rear Girdle Plates
is proceeding. Machining of the upper section of the C-plates is also
proceeding; some re-work is required in order to bring the transition portion
of the C-plates into compliance with the spec, which is necessary to insure
good thermal contact with the inner wall.

We are proceeding to build a Mock-up of the New Inner Divertor. This will
require welding long studs, and studs-on-studs on the new Inner Vessel Wall
simulation. The goal is to use the same equipment/fixtures that will be used
to install real parts. We are also simulating in-vessel space constraints
while doing this work. Then all hardware will be mounted on the wall to check
the fit. The technicians will develop the final assembly procedures based on
their experience with the mock-up.

Finite Element Analysis was carried out to evaluate the proposed modification
to one C-plate (at F-port) to accommodate the probe box insertion. Removal of
material from the lower portion of the plate was deemed acceptable, because
that region will still be relatively thick. The top section of the C-plate is
sloped, and thinner than the bottom section. The FEA shows that removal of
material in the vicinity of the probe box did not have much effect on the
stresses, which are highest in the ears of the Rear Girdle plate. However,
because the model did not include all the fine detail of the drilled holes,
counter-bores, etc., a design modification to further reduce the amount of
material removed from the C-plate is being considered.

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

Ed Cecil from the Colorado School of Mines visited the PSFC on Monday and
Tuesday (May 13 and 14) as part of an ongoing collaboration with Rejean Boivin
and Earl Marmar directed at the possible installation of a "Faraday
foil"-based lost ion diagnostic on Alcator C-Mod sometime during the coming
year. As part of this visit he presented a summary of results from comparable
instruments on JET and NSTX. In addition he examined data from a Faraday foil
device which had been installed outside of Alcator last year as a measure of
the response of this type of detector to soft x-rays and as a measure of the
electro-mechanical noise which might be expected for a device on the machine
interior. By subtracting signals from adjacent foils, a noise level of a few
nA/cm2 was found. By comparison, a worst case scenario in which all the fast
ions during 3He ICRH minority heating were lost during a typical slowing down
time of ~100 ms would result in a current of ~40microamps/cm2. The fact that
this projected current is well in excess of the few nA/cm2 noise measured
outside the machine suggests the viability of the proposed diagnostic.
Evaluations taking into account more realistic lost ion fractions are ongoing.

Joydeep Ghosh from the University of Maryland arrived last week for an
extended stay, as part of the on-going U. Md collaboration on C-Mod.

Miklos Porkolab attended the FESAC meeting in Germantown, MD, on May 15,16.
During the Public Comment session he gave a brief presentation on the
status of graduate students at the PSFC. He emphasized the healthy status
of graduate student enrollment at MIT, and the strong job market for recent
Ph.D's graduating in plasma and fusion sciences.

Spencer Pitcher was also at the FESAC meeting. He discussed his "open letter to
FESAC" advocating a burning plasma tokamak experiment, during the public
comment session.

Jim Terry, Brian LaBombard, John Rice, Martin Greenwald, Catherine Fiore,
Jerry Hughes, and Valerie Censabella attended the 14th Transport Task Force
(TTF) meeting in Fairbanks, Alaska, May 16-19. LaBombard, Greenwald and Terry
also participated in a Meeting on Plasma Turbulence and Transport in Edge/SOL
regions, also held in Fairbanks on May 14-15, prior to the TTF meeting.

Larry Guttadora (PPPL)was at IF Engineering in Dudley, Massachusetts, on 5/8
for acceptance tests of part of the PPPL upgrade to the C-Mod reflectometer.
The remaining rf parts will be brought from MIT to PPPL for integrated system
tests prior to installation on C-Mod.