From: rhawryluk@pppl.gov (Rich Hawryluk)
Newsgroups: sci.physics.plasma
Subject: TFTR Update March 17, 1997


Status (March 17, 1997):

During the past three run days, the emphasis has been the exploration and
utilization of radiative mantles and the laser-based DOLLOP system which
injects lithium to coat the vessel walls during a plasma pulse. These new
techniques were used both to extend the operating range of TFTR and perform
transport studies.

On Wednesday 12 March, further experiments were conducted with the DOLLOP
apparatus for lithium coating of the limiter to modify the plasma-wall
interaction and thereby enhance plasma confinement and fusion performance.
Since the initial experiments in the previous week, the collimating
aperture had increased in diameter to allow a higher rate of introduction
of lithium into the plasma edge. Using just the DOLLOP apparatus, higher
confinement was produced and sustained than in comparison supershots
without lithium but the gains were not as great as those produced by
combining DOLLOP with the injection of lithium pellets and the lithium
"painting" technique. In the second shift, high current, high toroidal
field were produced again in preparation for an experiment being planned to
integrate the DOLLOP technique to enhance confinement with the "radiating
mantle" technique to control the plasma-limiter interaction. Through this
combination of techniques, it was hoped to produce a plasma scenario which
would be capable of sustained high fusion performance in DT operation.

In addition to its use as a tool for modifying the plasma-wall interaction,
the DOLLOP system has been used to make tests of electron transport and to
modify the plasma current distribution. The injection of several tens of
Li micro-particles per laser pulse into the edge of 1.6 MA discharges has
led to an immediate increase in the core electron temperature of up to 1
keV. The temperature increases have been sustained by a laser repetition
rate of 30 Hz for up to 500 ms. This core temperature increase has been
accompanied by an increase in the sawtooth period only during the time when
the DOLLOP laser is turned on. Further, a reproducible rise in the plasma
internal inductance begins when the laser is turned on and ends when the
laser is turned off. These observations are perhaps related to the
non-local nature of electron transport as has been studied by K. Gentle at
Texas and M. Kissick at Wisconsin. Initial assessments of the data are
underway.

On the first shift of Thursday 13 March, further tests of the radiating
mantle scheme were performed. Feedback controlled injection of krypton and
xenon was used to sustain radiated power fractions up to about 55% in the
same plasma conditions to determine which impurity was more likely to
produce beneficial effects on the limiter interaction without adversely
affecting confinement. Krypton was judged more appropriate in the
conditions of this experiment. In the second shift, the radiating mantle
and limiter coating by both DOLLOP and the lithium pellet injector were
combined in high-current (2.7MA), high-field (5.5T) plasmas with up to 1.7s
of high-power NBI heating, including two shots with D-T NBI. The second of
these produced a total of 7.6MJ of D-T fusion energy, a new record for
TFTR. Time did not permit the optimization of the fusion energy within the
calculated stability constraints of these plasmas.

On Friday 14 March, it had been planned to combine the radiating mantle and
DOLLOP techniques in high-li plasmas (XP801). However, difficulties were
encountered in producing the low-q plasma startup on the outboard RF
limiter which was an integral part of this experiment. Subsequent
inspection of the vessel interior revealed some damage to the screen of the
Bay-N ICRF antenna which was introducing inconel impurities into the
plasmas run on the RF limiter and causing the startup to fail. In view of
these difficulaties, the experiment was changed to a further investigation
of the underlying transport effects of the radiating mantle (XP702).

A perturbative study of local transport was carried out using krypton and
xenon puffing to vary the net heating power to electrons. A major
difference from previous enhanced-radiation experiments on TFTR is that
these target plasmas were prepared without "growth" in minor radius during
startup to provide a monotonic q-profile with q(0)>1 that eliminated
sawteeth. A second difference was the study of helium L-mode plasmas in
addition to supershot plasmas, to assess whether the unexpectedly small
change in electron temperature observed previously in supershots also
occurs when electrons get most of the heating power directly, i.e. in
plasma conditions more similar to ITER. The fraction of power radiated was
typically 50-70% versus ~25% with no impurity puffing. Only a small
reduction in Te of order 10% was observed in the L-modes with xenon puffing
that doubled the radiated power fraction from 25% to 50% of the beam power.
Even in a plasma where the radiated power fraction was deliberately
increased towards 100%, the electron temperature responded very weakly to
the radiative loss, falling only ~10% compared to a shot without xenon
puffing, and changing on a timescale of ~600ms, much longer than an energy
confinement time. This suggests that the electron temperature is very
insensitive to the net delivered heating power, although a detailed
time-dependent transport analysis will be required to account for the
details of ion-electron power coupling, ohmic heating, and other effects.
TFTR has now generated discharges with radiative collapse using impurity
gas puffing of argon, krypton, and xenon. Analysis of these discharges may
be useful in assessing stability of "radiative mantle" scenarios being
considered for ITER.

Future Plans

The experimental campaign will continue through April 3, 1997. There are
eleven remaining run days left on TFTR along with two days for maintenance.

R. J. Hawryluk
609-243-3306
e-mail rhawryluk@pppl.gov

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_________________________________________________________________________
R. J. Hawryluk
rhawryluk@pppl.gov
PPPL - LOB 325
Phone: (609) 243-3306
Fax: (609) 243-3248

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