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
July 29, 2002

Plasma operations continued at Alcator C-Mod last week. Four run days were
scheduled and completed. Conditioning of the ICRF Antennas continued, and
Physics Operation began with one run day devoted to a Divertor/Edge Group
experiment. Work also continued on the Lower Hybrid and Diagnostic Neutral
Beam Systems.

Plasma operation are scheduled to continue this week.

Operations
----------

Plasma operations were carried out Tuesday-Friday last week. Startup
reliability for the week was about 70%, with a total of 64 successful
discharges being produced. Following Tuesday's run, a small leak was
identified at one of the Thomson Scattering laser windows. This window was
replaced on Wednesday; at the same time a failed ratiomatic ion gauge at
G-side was also replaced. Since these components are located behind isolating
valves, it was not necessary to break main vacuum to carry out these repairs.

Three of the week's runs were dedicated to conditioning of the ICRF
antennas. The target plasmas for these runs were mainly inner-wall limited
equilibria, in order to remain in L-mode as the power coupled to the plasma
increased above the MW level. The loading changes associated with L-H
transitions can lead to increased faulting which detracts from the
conditioning process.

Thursday's run was in support of MP#313, submitted by the Divertor/Edge Group
and Density Control Task Force. The goal of this experiment was to quantify
the effective conductances of neutral gas through various pathways in and
around the lower and upper divertor structures during plasma discharges. This
information will help reconcile some of the puzzles with respect to divertor
neutral leakage, and will also help us understand the behavior of gas
compression in the upper chamber and allow us to anticipate pressure changes
there in response to a future cryopump system. Both standard lower single
null and upper single null equilibria were investigated in this experiment.

The primary tools were the NINJA capillary gas puff system and divertor
flappers, in conjunction with pressure gauges (in-vessel and ex-vessel) and
the divertor TV camera system. The new miniature Penning gauge under the B-C
divertor module ran better than expected, recording fast pressure changes in
this region as the B-C flapper was opened or the B-C Bottom capillary was used
to puff gas. The divertor camera (DIV2, viewing D-alpha) observed puffs from
the B-C and C-D capillaries located above the B-C and C-D flappers. The
camera also was able to record changes in local D-alpha light when the
flappers were opened, giving us the possibly to 'calibrate' the flow through
the flapper. In addition, the pressure under the B-C flap was recorded by the
Penning gauge there, which should allow us to estimate the effective
conductance through the open flap in the presence of plasma.


Diagnostics
-----------

The new tangential HIREX (High Resolution X-ray) spectrometer systems are
operational, and observations of toroidal rotation profiles during ohmic and
ICRF operation have begun.

Following repair of digitizers in the core system, and replacement of the
Thomson laser window on Wednesday, the Edge and Core Thomson Scattering
diagnostics began taking data last week.

The number of FRCECE radiometer channels was increased to the full
complement of 32 this week with installation of the detectors that
were recently repaired by the manufacturer. The detector problems
were due to faulty manufacture. Calibration for recent shots was
completed.


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

In this week, we ran RF conditioning on Tuesday, Wednesday and Friday. The
D-port antenna has been conditioned up to 1.2MW and the E-port antenna to
1.6MW. The D and E modulator boards were modified to accommodate a soft-start
after faulting, This modification has been shown to provide more reliable
operation.

We also changed the phase feedback input system for the four-strap J-port
antenna. We added 8309 limiter boards in the feedback path of the controllers
to improve the dynamic range of these inputs. We have also developed an
operating strategy which allows the phase to lock on startup at low power and
then ramp up to higher power. Using this scheme, the J-port antenna has been
conditioned up to the 0.5MW level.

Diagnostic Neutral Beam System
------------------------------

Work continued to identify the source of the arc current stability problems
that have shown up since the power supply arcdown a few weeks ago. Since
then, damaged components in several subsystems have been replaced.
including fuses, voltage divider resistors, pulldown resistors, bleed
resistors, protection diodes, and one arc charging SCR. The final repair was
to replace a feedback resistor in the control circuitry for the arc
supply. This appears to have remedied the instability in the arc current. The
DNB was operated for approximately 10 shots over the weekend, with no faults
beyond those normally encountered during conditioning. Additional conditioning
shots will be run this week, with beam-into-plasma operation anticipated
during C-Mod operation.

Work continues on the upgrade to the beam cryo system. The cryo distribution
panel for the cell end of the vacuum line has been assembled, and the fill
station end to supply the vacuum line has been re-plumbed. Remaining work
includes installation of the line through the last diagnostic lab, mounting
the vapor vent in the diagnostic lab, and piping the vapor vent through the
back wall to the outside.


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

The back panels for the EMI/RFI cabinets were modified to accept the
coax filtering boards. Previously only the cabinet for Cart #3 had this
board installed, but there were clearance issues with this panel. The new
panel was retrofitted on the equipment rack for Cart #3. The new
mechanical arrangement has no clearance issues. The old panel which was
removed from Cart #3's cabinet is being modified to address the clearance
issues.

The 24VDC power supplies for the water system flowmeters arrived this
week and are being installed. The schematic for the water interface is being
modified to include the new power supplies, and a tech request was generated
to provide 120VAC to these supplies.

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

Phil Ryan of Oak Ridge visited the RF Group on Tuesday to Thursday. He
participated in the ICRF conditioning activities during C-Mod operations.

Ricky Maqueda from LANL visited C-Mod during the week and re-installed the
infrared imaging system. This system is now operational.





-----------------------------------------------------------------------

The contents of this message have been scanned for viruses by
the Clearswift ES Policy Server, and no viruses were found.