From: irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT
Newsgroups: sci.physics.plasma



Alcator C-Mod Weekly Highlights
March 23, 2001


Fabrication of the J-port antenna stripline components is nearly complete.
We continued work on the ICRF control system, the DNB, inner divertor,
and the Lower Hybrid MIE Project.


Physics
-------

Double transport barrier plasmas have been observed by operating at 4.5 T so
that the 78 MHz ICRF resonance is located near r/a = -0.5. These plasmas have
a strong gradient in the electron density profile, and the core density is
seen to increase continuously in time until radiative collapse. The barrier
is also seen in the ion temperature profile, indicating that this is a true
energy barrier. Application of additional ICRF power at 70 MHz to these
plasmas is seen to arrest the increase of the electron density, to heat the
ions at the plasma center, and to hold the double barrier in steady state
for nearly 10 energy confinement times. The bootstrap current is estimated
to be of the order of 100 A/cm^2 at r/a = 0.5 for these plasmas.


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

All inconel components arrived in-house this week and are undergoing final
machining, polishing, and fitup procedures. Test inconel pieces have been
successfully e-beam welded, and welding of the stripline components is now
planned for early next week. Copper plating of the antenna backplates
has been completed, and the backplates are back on the antenna test stand
to aid fitup tests of the striplines. Machining of the J-port flange
for installation of the new diagnostics has begun. Four windows and
a port for a pressure measurement will to added to the flange. A
dB/dt probe has been tested on the antenna mockup and shown to have
a more than adequate rejection of electrostatically coupled signal. Six
of these probes will be installed on the antenna.

Following replacement of the intermediate power amplifier tube, FMIT#3
was brought up successfully to full power into a dummy load. Both
FMIT#3 and FMIT#4 are now ready for operation. A matching transformer
similar to the one installed in FMIT#4 was installed in FMIT#3 this week
to improve the drive to the IPA stage. Similar improvements in the standing
wave ratio and drive efficiency were found. We continue to work on the
rf control system.


DNB
---

Resumption of arc conditioning was delayed by a failure of the arc and
snubber power supplies. The failure was traced to two 15V, 2A supplies
which failed when the house power was cycled. The 15 V supplies
have been replaced and work on the arc and snubber supplies was resumed
on Friday. Machining of the new CXRS shutter components has begun.

At the weekly videoconference meeting on the C-Mod diagnostic neutral
beam, Paul LaMarche (PPPL) presented his redesign of the MSE lens mounts.
This upgrade is planned to be installed during C-Mod's upcoming long opening,
and should prevent any further lens and mirror damage from disruption shocks.


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

All modifications to, and new wiring for, the klystron carts will be
completed in the next few days. The design of the klystron equipment rack
component layout is also nearly complete. Wiring tasks for these racks have
have been released. The klystron equipment racks will primarily contain the
tube protection boards and associated fault logic. Work continues on the
klystron phase and amplitude control system. A complete feedback loop has
been tested and results indicate a very good response time. A test of a
new arc detection technique that relies on the detection of the rf produced
by the arc well above the 4.6 GHz drive frequency looked very promising.
This technique may allow arcs on the plasma side of the vacuum windows to be
seen in sections of waveguide well before the launcher.

Work on the launcher continued at PPPL this week. PPPL is finalizing
the design of a fixture for testing the water loads designed here at
MIT that are needed to dump the reflected power from the launcher.
Four water pipes installed in the waveguide will conduct away the power.
Another fixture for calibrating the alumina windows is being fabricated.
The alumina bricks for the launcher's air/vacuum interface have been ordered.
Preparations for the Lower Hybrid final design review are also underway
at PPPL.


Inner Divertor
--------------

Machining continues on the divertor backing and C plates. Vendors
continue to produce tiles. All tiles for the divertor are expected
to be in-house within the next 3 to 4 weeks.


Travel and Visits
-----------------

Martha Redi from PPPL visited on Tuesday, 3/20 to discuss plans and early
results of gs2 simulations of particle transport in ITB regimes. She
had discussions with Catherine Fiore and Martin Greenwald about the
experimental measurements and accessing C-Mod data.