Alcator C-Mod Weekly Highlights
			August 6, 2001

Plasma operations continued at Alcator C-Mod last week, the final week of the
2001 Experimental Campaign. Five run days were scheduled and completed. All
runs were extended by an extra two hours in order to maximize the data
obtained from each experiment. A total of 107 plasma shots were produced with
a startup reliability of over 87%. Experiments carried out included two MP's
related to double barrier/ITB formation, a study of pedestal parameters and
the QC mode in EDA H-mode discharges, exploitation of C-Mod's long-pulse
capabilities, and plasma operation at 8T for the first time since 1998. 

Having completed the 2001 experimental campaign, C-Mod is now entering an
extended period of maintenance, inspections, and facility upgrades. 

Physics
-------

This week was extremely successful from the standpoint of learning more about
plasma conditions under which the density barrier and internal transport
barrier (ITB) modes can be produced in C-Mod. The density barrier mode was
produced with off-axis ICRF heating (70 MHz) on BOTH the tokamak high field
side (HFS) at 3.9 T and the low field side (LFS) at 5.4 T. Preliminary
attempts were also made at heating the LFS density barrier with on-axis ICRF
power at 80 MHz.

The density barrier mode was first produced at lower B_t = 3.9 T using 1.5 MW
of off-axis ICRF heating power from the J-port antenna, driven at 70 MHz. The
central density was observed to rise to ne(0) = 6 X 10^20 m^{-3} and the
toroidal rotation velocity (v_tor) from HIREX decreased to zero and reversed
direction, becoming counter-current.  Interestingly, the value of B_t where
the barrier formed at 70 MHz, scales nicely from the critical B_t for barrier
formation at 80 MHz, that is:

     B_t = (70 MHz / 80 Mhz) * 4.5 T = 3.93 T.

This places the minority hydrogen cyclotron resonance at the same minor radius
on the high field side (12 cm) as it was at 4.5 T and 80 MHz.

A density barrier was also produced with the 70 MHz ICRF resonance on the low
field side (LFS) at 5.4 T. Only 1.35 MW of ICRF power from J-port was
necessary. ITB formation was again characterized by peaking density profile
with ne_TS(0) = 6X10^20m^{-3} and ne(0) / ne(ave) > 2.  Formation of the
density barrier at both 3.9T and 5.4T was preceeded by a transition to an
H-mode phase, and the H-mode edge is maintained throughout the ITB evolution.

We also attempted to heat the LFS density barrier mode at 5.4 T using the
available 80 MHz ICRF power from D & E ports. There was some evidence of
heating within the barrier from the 80 MHz power (at the 0.5 MW level). The
neutron rate more than tripled and the central density increase was arrested
at ne_TS(0) = 3.5 X 10^20 m^{-3} during application of the on-axis ICRF
power. These results are not conclusive, however, because there were two H->L
back transitions which prevented the initial barrier mode from becoming well -
developed.

Operations
----------

The runs on Thursday and part of Friday were devoted to extending the plasma
pulse lengths in C-Mod. These experiments succeeded in extending the
end-of-flattop (EOF) from 1.5sec to 3.0sec (3.2 sec for the TF), more than
doubling the flattop time in these inductively-driven discharges. The
long-pulse experiments were carried out at 5 tesla, with line-averaged density
in the range 0.3e20<nebar<1.2e20 m^(-3), and plasma currents of 800kA. The
longest discharges had current flattops of approximately two L/R times. ICRF
heating was applied to several of these discharges, with power levels of ~1MW
sustained for up to 2.4sec, using all three ICRF antennas run
sequentially. The plasma pulse lengths were limited by administrative
restrictions on the currents in the PF coils, especially the EF1 coils which
are mainly used for X-point control. All power supplies functioned as
expected, and magnet and bus heating during the long pulses was measured and
found to be consistent with calculations carried out earlier. The maximum rise
in TF magnet temperature was 39C. The between-shot cool-down time was only
slightly longer than for normal length discharges, and in practice the
between-shot time was dominated by setup time for the next shot and special
measurements and inspections associated with monitoring the operation.

Evolution of the wall particle inventory and recycling was investigated using
short gas puffs during the long low-density flattop discharges. We showed that
the wall does NOT saturate at a time <3 seconds, at least at the low densities
run in this experiment. We also showed that the inventory in the walls is not
so large that it will significantly affect our ability to run at low density.
These results are relevant to planning for the C-Mod Lower Hybrid/AT Physics
Program. 

In the course of these experiments we observed H-modes at nebar~0.65e20/m^3
with ICRF power of 1 MW.  We also observed, for L-mode cases, a (poorer)
confinement mode at these low densities, in which temperature drops, sawtooth
amplitude decreases or disappears, and fueling efficiency decreases (as
evidenced by the Ha increase). These poorer confinement conditions tended to
appear between 1.2 and 1.6 seconds into the discharge, and persisted for the
remainder of the pulse.

Divertor heating was observed during the long pulses with the (LANL) IR camera
system. We were able to puff N_2 via feedback in an attempt to reduce the heat
load on the divertor; analysis of this aspect of the experiment remains to be
carried out. 

Ten shots on Friday morning were dedicated to high-field plasma operation. We
operated at toroidal fields up to 8.0T for the first time in several
years. All systems functioned nominally, and several 1MA plasmas were
produced. ICRF heating (D-He3 minority heating scenario) was applied
successfully.

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

All ICRH systems were operational this week. ICRF heating at 80MHz (D and
E-port antennas, transmitters #1 and 2) and 70MHz (J-port antenna,
transmitters #3 and 4) was applied in support of the physics
experiments. System performance was good up to a total power of 3-4MW. At
higher power injections from near the top of the J-port antenna, and also in
some cases from the D antenna, were observed, often leading to discharge
termination.

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

The DNB operated at full current and voltage throughout this last week of
C-Mod operations.  The thermal sensitivity of the suppressor supply appears to
have been remedied.  Reduced efficiency of the cryopumps due to postponed
maintenance did not affect operation.  The DNB will be shut down for
approximately 6 weeks to faciliate tear down of C-Mod.  Operation will then
continue until approximately 8 weeks before the next operational period.
Operation during the shutdown will be directed toward improvement of
conditioning techniques and component mix.  All of the DNB diagnostics were
operated.  In particular, CXRS spectra were consistently observed.  The
diagnostics will remain intact for calibration until the end of next week.
Then we will assess the success of repairs and modifications of the internal
optics which were made prior to the current campaign.


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

Modified C-Plate tile support plates and transition keys are now in-hand.
Several mills of material had to be removed from each side of the tile support
plate to make plates fit more loosely into the pockets.  Modified plates were
assembled into the C-Plate and they fit perfectly in between the ribs.  The
remainder of the Tile Support Plates (220 total) are being modified in-house.

There will be 3 more probes at the bottom portion of the C-Plate than
originally planned.  These probes are being fabricated, and the new probe box
is nearly complete.

Mock-up building is moving forward.  Several long (1.5") studs have been
welded to the Mock-up wall. BP coils and other diagnostic cables have been
added to the Mock-up Wall. Plastic Rear Plates, Inconel C-Plate, Tile Support
Plates, Tiles, Transition Keys, and Old Limiter Backplates with tiles have been
mounted on the Mock-up Wall as well.  Everything fits perfectly in the
Mock-up, and we are waiting for the real Rear Girdle Plates to finalize the
mock-up assembly.
   

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

Tom Fredian and Martin Greenwald travelled to Chicago for a meeting
of the SciDAC funded national fusion collaboratory.  Participants
from MIT, GA, PPPL, ANL and LBNL attended.  This was the first face
to face meeting of the group and concentrated on developing a
detailed work plan for the first year of the project.  MIT work on
the collaboratory will be focussed on the addition of a certificates
based authentication scheme for MDSplus and SQL server access.

Jim Irby, Monty Grimes, Dave Terry, Jim Zaks and Ron Parker attended the Lower
Hybrid Coupler Final Design Review last Tuesday, the 31st of July, at
PPPL. The Review was successful and we concluded that procurements and
fabrication could proceed. While at PPPL for the LH launcher design review,
Terry met with Nevell Greenough and discussed briefly the demodulator and
protection circuitry they are using as well as the new phase feedback system
they are implementing.

Parker stayed at PPPL Wednesday and Thursday to attend the FESAC meeting.