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 10, 2000

Operations
----------

Alcator C-Mod is now in a scheduled maintenance period, which will last until
September. Following the last plasma discharge on June 30, the C-Mod Cryo
System was shut down, and the warm up of the machine was begun. By Thursday
of last week all machine components were well above 0 C and preparation for
invessel work was begun with a backfill of nitrogen gas and D2O. The heavy
water is expected to react with the boron hydrides on the vessel wall and
allow safe manned entry into the vacuum vessel without adding large amounts of
hydrogen to the wall inventory, as would occur when venting to room air. The
removal of the hydrogen and the reduction of the H to D ratio to a point
suitable for ICRF heating experiments has been a major delay in obtaining
research grade plasma discharges in the past. We hope this new technique will
shorten the startup process somewhat. Entry into the vessel is expected on
Wednesday of this week following two days of Thomson scattering calibrations.

Preparations for invessel work are proceeding well as we are nearing the
short machine vent scheduled for this week. The work will focus on the
modification of the 4 strap antenna, and consists mainly of enclosing the
sides of the antenna in order to increase its heating efficiency.
In addition, we will substitute all molybdenum protective tiles with boron
nitride ones on all antennas (including the two 2 straps). The new tiles are
ready to be installed after completion of a high temperature bake early last
week. A final MIT local review of the changes to the j-port antenna was
held last Thursday in which all aspects of the invessel changes were
discussed. Disruption forces, new plate design, location of and hold down
hardware for the BN tiles, capacitor layout and soldering techniques, and
plasma shields were all discussed.

As time allows, small invessel tasks, such as light calibrations for BES, MSE,
and camera views, will be completed in parallel with the antenna work.
Flux loops will be examined, connections on the second Lyman alpha detector
will be checked, as well as for a emissive probe, and a flux probe. Finally
we will try to install the new fluctuation imaging system built
by S. Zweben (PPPL).

Preparations for our first period of long pulse operation continues.
Simulation of coil temperature rises during the planned 3.5 second flattops
indicate a large safety margin. Calculations of power supply component
junction temperatures are proceeding and have indicated which components will
need to be monitored. The design and procurement of instrumentation needed to
monitor these critical components is proceeding.

Upgrades to the C-Mod glow discharge system are underway. Fast IGBT switches
are being installed to allow for current rollback control during arcs. This
change will eliminate damage to invessel components during glow discharge
campaigns. In addition, improvements to the gas control system will allow
more automatic operation of the glow system, and the addition of RF chokes to
the leads to the invessel glow paddles will eliminate RF pickup by diagnostics
near the glow system electronics rack.

Progess is also being made in the upgrades to the cryostat and igloo we plan
to make during the machine inspection late next Fall. Preliminary designs
have been completed. These changes will allow for the easy removal and
installation of the horizontal ports to fix leaks, much better thermal
insulation of the ports from liquid nitrogen, and new instrumentation ports
through which to bring cabling out of the igloo.


ICRF System:
------------

We began an inspection of D-port coax for evidence of arcing. This antenna
did not condition well this past campaign and would trip consistently on a
specific pair of faults (DC0 arc detect and phase balance). The resonant
loops and a portion of the 9" coax have been inspected without finding any
significantly damaged component. The inspection will continue this week.

The J-port antenna modifications were reviewed last week. The plan is to RF
ground the antenna structure, ground the end front tile and the side plate,
and replace the Mo protection tiles with BN. We will begin with an inspection
of the antennas at first invessel opportunity. Analysis of heating
experiments with the J-port antenna configured as a dipole (center strap pair)
show heating efficiencies ~70-75% (similar to D and E-port antennas). This
suggests that proper RF grounding of the antenna should improve its 4-strap
performance.


DNB System:
-----------

The DNB was operating most of the last week of plasma runs. However,
a minor fault in the HV system led to an early shutdown in beam operation.
Initial measurements of the magnetic field near the beam were completed and
will be used to verify the effectiveness of the magnetic shielding of the
beam. Preparations are now underway for realignment of the beam, improvement
in monitoring of the beam plasma source, improvements in vacuum gauging,
recalibration of the thermocouple system that monitors beam profile and
energy, tests of the CXRS window transmission at F-top, and tests of CXRS
fiber transmission. These tasks will be completed prior to restart of C-Mod
operations.

The BES diagnostic measured good signal levels from the hydrogen DNB during
the week of 6/12. The signals were up to factors of 2-3 over background. The
absolute intensities of the beam signals were compared with our model and
agree within a factor of two. Matt Sampsell (UT-FRC) was at MIT that week to
coordinate the BES measurements. A poster was delivered at the HTPD
Conference in Tucson describing the BES model used to interpret the data.


Lower Hybrid System
-------------------

Testing of the first seven klystrons (all those presently in-house) has been
completed. Four are in nominal operating condition; two (S/N 112 and 115)
experienced high voltage breakdown in the vicinity of the flying leads; and
one has a shorted filament. Physical inspection of tube 115 revealed that the
cathode and heater leads showed signs of cracking, which could lead to the
observed breakdowns. A plan has been devised to repair these klystrons, and
parts for the repair hve been ordered. We are coordinating with Princeton for
the shipment of the remaining nine klystrons, which are presently at PPPL.


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

David Winslow (UT-FRC) completed installation of the Texas Scanning probe.
It has now been successfully reciprocated both during ECDC and tokamak
discharges. Mechanical limitations in the probe drive system prevented
measurements near the separatrix. These will be corrected during the
current break and it will be ready for the next section of the campaign in
September.

The UT-FRC ECE system was returned to operation. As noted in previous
reports, repairs were required mainly due to blockages of the waveguide and
faulty regulation in a power supply. These repairs have now been completed.
By the end of C-Mod operation in June, 30 of 32 channels were back in
operation and measuring electron temperature. Malfunctioning components in
the remaining two channels have now been replaced.

With the recent installation of the Diagnostic Neutral Beam, the Neutral
Particle Analyzer had to be temporarily removed from the tokamak.
It has now been reinstalled in its new position on B-port. It is an
E||B type analyzer, with 39 channels for each species (H and D). Its
new view will look at the center of the discharge, from slightly above the
midplane, nearly perpendicular to the magnetic field. It will be used for
characterization of the tail population, and help us determine the
hydrogen concentration during ICRF heating.

The Tangential Two Color Interferometer (TTCI) began operation near the end of
the run period. This single chord diagnostic monitors the plasma edge density
at 0.88 m and uses a diode pumped frequency doubled laser to provide the two
colors for vibration subtraction. The transitions into and out of H-mode are
clearly seen, are of the expected magnitude, and correlate well with other
edge diagnostics such as the edge diode bolometer array and visible
bremsstralung. The cause of a low frequency drift in the phase measurement
must still be determined and fixed. Typical C-Mod pedestal densities result
in peak phase shifts of 0.03 to 0.05 fringes, depending on pedestal location
and width, so vibration subtraction requirements are quite severe.


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

Cynthia Phillips was at MIT last week (July 5-7, 2000) working with Paul
Bonoli as part of the C-Mod/PPPL collaboration. We are carrying out TRANSP
modelling of the 4.5 T density barrier discharges that were achieved on C-Mod
(with emphasis on shots 1000607007 and 1000607008). We are using the new
upgraded FPPRF ICRF package in TRANSP, which now includes the TORIC ICRF
code. The results of this transport analysis will be reported as part of an
IAEA poster presentation in Sorrento Italy in October, 2000.

Ricky Maqueda (LANL) was onsite working with the LANL fast camera during the
last week of operation.

Stewart Zweben (PPPL) was at C-Mod during the last week of operation making
preliminary measurements in preparation for his fluctuation imaging
diagnostic.