Newsgroups: sci.physics.plasma
From news@pppl.gov Thu Jan 4 21:08:38 1996
From: rhawryluk@pppl.gov (Rich Hawryluk)
Organization: Princeton Plasma Physics Laboratory
Subject: TFTR Update January 4, 1996

Status January 4, 1996:

Since the last update on September 29th, 1995, further analysis of data
from the previous run was performed, a campaign to remove tritium from the
vessel was conducted, a run assessment and planning meeting held, and the
machine and beams were conditioned in preparation for a resumption of high
power experiments.

Highlights of Recent Physics Results:

At the Third International Workshop on Helium Transport and Exhaust
(Charleston, SC) Sept. 25th to 29th, four papers were presented from TFTR.
Ed Synakowski spoke on "Overview of Alpha Particle Studies During D-T
Operation on TFTR" and "Measurements of the Production and Transport of
Helium Ash on the TFTR Tokamak". Martha Redi spoke on "Effect of TF Ripple
Loss of Alpha Particles on Helium Ash Accumulation in TFTR and ITER" and
also presented Robert Budny's talk on "TRANSP Simulations of Alpha
Parameters in ITER". In addition George McKee spoke on "Alpha Distribution
Measurements and Transport Behavior in D-T Plasmas on TFTR".

The 37th annual meeting, APS Division of Plasma Physics was held the week
of Nov 6th in Louisville, Kentucky. TFTR had four invited papers, 14 oral
and about 70 contributed papers at the conference. Some of the
contributed and invited talks from the meeting are posted on the Web:
http://w3.pppl.gov/tftr/aps95/
This is a sub page of the TFTR Web page which has an increasing amount of
information about TFTR:
http://www.pppl.gov/TFTR.
In this update, it is not possible to summarize the large number of
presentations from the meeting. Only some selected ( and perhaps somewhat
arbitrary) topics will be noted.

Six physicists from TFTR gave talks at the 7th International Toki
Conference on Plasma Physics and Controlled Nuclear Fusion, "Fusion Plasma
Diagnostics" in Japan during the week of 28 November - 1 December. K.M.
Young gave a keynote talk on "Advanced tokamak diagnostics"; J.D. Strachan
gave a review talk on "Diagnostics for confinement study of DT plasma in
TFTR"; D. Darrow, H. Takahashi and S. von Goeler gave topical talks on
"Alpha Particle Loss Measurement in TFTR", "Interpretability of magnetic
diagnostics in tokamaks- Search for a locked mode in TFTR" and "Measurement
of electron energy distribution from X-ray diagnostics - foil techniques
used with the hard X-ray camera on PBX-M" respectively and A. Janos gave an
oral presentation on "Bursts of electron cyclotron emission during
disruptions of high beta TFTR tokamak discharges". This was an excellent
meeting, where there was a good chance to share information about tokamak
and stellarator diagnostics. The generosity of the organizers in funding
the travel of the TFTR participants is greatly appreciated. Young also
attended an ITER Progress Meeting on Collective Scattering for Alpha
Particle Measurement and the 7th International Symposium on Laser-Aided
Plasma Diagnostics in the following week.

Alpha Physics Results:

S. Zweben reports that further analysis of the alpha ripple loss experiment
(experimental proposal DT-51) is in progress. There were 43 low-power DT
shots in this experiment, which was designed to test the stochastic TF
ripple diffusion model by measuring alpha loss to a movable midplane alpha
detector for a variety of different q(r) profiles. The main results of the
analysis to
date are: (1) the alpha loss to the outer midplane increases with a
broader j(r) profile, as expected (at least qualitatively) from White's
guiding center ORBIT code; and (2) the alpha loss is strongly affected by
the presence of locked modes, which was not expected.

More recently, an analysis of the other DT shots in this experiment shows
the alpha loss to decrease strongly with decreased TF at a fixed q(a), and
to also decrease with decreasing TF at a fixed plasma current. Further
work is needed to separate the measured TF ripple loss from the first-orbit
loss, and to compare these results with ORBIT code runs.

Another outcome of this experiment is a very interesting set of data on the
effect of sawteeth on alpha loss. Many of these low-powered DT shots had
large sawteeth during NBI, and the sawtooth-induced alpha loss varied
considerably verses the q(r) profile, although the total loss due to
sawteeth was always a small fraction of the TF ripple loss. This
sawtooth-induced alpha loss will also be analyzed using the ORBIT code.

Confinement in D-T Discharges:

Transport analysis of isotope scaling of particle, momentum and energy
confinement in L-mode plasmas continues. Because the transition from
L-mode to supershot confinement in TFTR is a continuous transition
(strongly correlated with edge influx of hydrogen and carbon), it is
important to establish that the improved confinement observed in the DT
L-mode shots compared to the D-only L-mode shots is not an artifact of
imperfect matching of the plasma edge conditions. Database analysis by S.
Scott of the entire dataset collected in the DT19 experiment, which spans
a range of hydrogenic influx, has demonstrated conclusively that there
is an intrinsic, favorable isotope effect on L-mode energy confinement.

Enhanced Reverse Shear Experiments:

Analysis of turbulent electron density fluctuations by the TFTR correlation
reflectometer by E. Mazzucato has revealed intriguing behavior in enhanced
reversed-shear (ERS) plasmas: the level of turbulent fluctuations within
the reversed-shear region is very small. Qualitatively, this is the result
one would expect if the fluctuations observed by the reflectometer were
responsible for cross-field transport. The reflectometer also observes an
MHD mode, possibly an infernal mode, which is localized to the region of
minimum q. The existence of this mode does not appear to be related to the
ERS phenomenon.

In our continuing effort to understand the enhanced reverse shear on TFTR,
E. Synakowski has studied the role of ExB flow shear in reducing the
transport in reversed shear (RS) and enhanced reversed shear (ERS) plasmas.
According to theory by T.S. Hahm, the important figure of merit should be
the relative sizes of the plasma turbulence growth rates and the ExB flow
shear (=Bpol/Bd/dr(Er/Bpol)). As a result of the prompt pressure increase
after the transition, the ExB flow shear increases markedly in ERS plasmas
as compared to RS plasmas of similar beam powers. Prior to the transition,
the ExB shear in RS and ERS plasmas is quite similar, indicating that, if
ExB shear is related to the transition mechanism, the threshold is sharp.

This leads to an outline for a possible mechanism for the transition to the
ERS mode. A decrease in the calculated linear growth rate (M. Beer) of the
dominant modes accompanies the increase in the plasma pressure after the
onset of high power neutral beam injection. It is proposed that the
transition to the ERS mode may occur when the growth rate falls below some
critical value relative to the ExB shearing rate. The enhanced confinement
is maintained through positive feedback between steeping pressure gradient,
which increases the ExB shear, and the further decreasing of the turbulence
growth rates. Also, the peaking of the density profiles provides decreased
drive of the ITG modes (S. Parker, G. Rewoldt), providing a further
reduction in the plasma turbulence.

Comparisons of TFTR ERS discharges with gyrofluid simulations are underway
and initial results were reported at APS [Beer & Hammett]. Linear
gyrofluid calculations indicate that at the ERS transition, the longer
wavelength microinstabilities are suppressed, but a shorter wavelength
trapped electron mode remains unstable, which may account for the anomalous
electron heat flux after transition. (If all microinstabilities were
suppressed the heat flux should be neoclassical, with corrections for the
large banana-width.) Nonlinear gyrofluid simulations show that these
shorter wavelength trapped electron modes produce significantly less
transport than before the transition, by about a factor of 40. In
addition, gyrofluid calculations find that the stabilization due to
reversed magnetic shear alone is not as dramatic as the stabilization due
to the large Shafranov shift in ERS discharges. Further analysis of the
TFTR ERS data is underway to try to identify the trigger for the ERS
transition.

MHD Stability:

Analysis was carried out on the experiments examining the scaling of high
beta disruptions with toroidal field, this is an important issue for ITER
disruption physics. E. Fredrickson reports that an important result of
these experiments was that the beta limit was found to be significantly
(30%) higher at low toroidal field (2T) than at high field (5T) for
discharges with standard monotonically increasing q-profiles. Several
initial TRANSP runs have been
finished and analysis of the MSE data for the current profile measurement
has begun. Studies of MHD activity in these low field plasmas has
identified n=1 fishbone-like activity and sawtooth-like events prior to the
onset of the locked mode which precedes the disruption. Measurements of
the electron temperature profiles from ECE suggest that during the locked
mode phase a thermal collapse similar to that seen in high density
disruptions on TFTR is occurring.

Dr. L.E. Zakharov from the theory division has been working with the TFTR
MHD group in order to understand the triggering of high beta internal
disruptions in supershots. He has developed a new Sweeping Equilibrium and
Stability Code (SESC). This code uses new advanced technique for solving
Grad-Shafranov equation. It has an interface with PEST and
DCON and provides accuracy in the first derivatives and can automatically
scan experimental databases. It also facilitates studies of effects of
error bars in data and facilitates implementation of theoretical models.
The code is well suited for massive processing experimental data and
comparing with the theory.
The results from this code which has been applied to TFTR are: A model for
triggering the internal disruptions revealing the positive feedback between
the m=1 and ballooning modes is found for TFTR supershots. The two-fluid
model describes well the stability of both the ideal and KBM (or local)
ballooning modes. The SESC code is being designed as a new, easily
updatable tool for the massive processing of experimental data and
comparing with the theory. Active control of stability seems to be
possible for high-performance supershots in TFTR as well as for ITER.
Further development of this model of disruption triggering in tokamaks
involves: Clarification of the role of the m=1 mode, i.e.: 1. evaluation
of ballooning stability in the presence of a finite m=1 perturbation; 2.
theory of the saturated m=1 mode itself; and 3. study the disruptions in
regimes with no q(0)=1 in the critical zone (including reverse shear).
More extensive comparison between the theory and experimental observation
of excitation of local ballooning modes is being carried out on TFTR. Long
term goal is to test this model on other machines and apply it to ITER
profiles.

One experiment which was very recently carried out during the weekend of
Dec. 16 and 17 was an ITER R&D high priority issue, item 1.3 Pre-emptive
disruptions: Demonstration of impurity pellets for controlled disruptions.
The experiment consisted of injection of krypton doped (< 2%) deuterium
pellets into ohmic and neutral beam heated plasmas. Measurements of the
krypton spectra were made including fast time dependence. Good radiation
bolometric measurements were made of the radiated power. Various current
quench time were caused with current decay times from 400 msec to about 20
msec. No runaway generation has been identified as yet. A full report will
be made available to the ITER central team when all the data is analyzed.

Tritium Removal from the Vacuum Vessel:

Several techniques were used to remove tritium from the vacuum vessel:
deuterium glow discharge cleaning, He/O2 glow discharge cleaning, and a
purge of the vessel and beamlines with moist air. The results from this are
still being analyzed. However, initial results indicate that the tritium
removal rate with deuterium glow discharge cleaning is comparable to He/O2
glow discharge cleaning. The purge of the vessel indicated that >80% of
the tritium in the vessel is not released during a room temperature vent of
the vessel and for TFTR operating conditions the tritium is tenaciously
held up. These results demonstrate that it is possible to maintain the
in-vessel inventory in TFTR below regulatory requirements. Note this
planned vent of the TFTR vessel was the first vent in over two years of
operation.

Run Assessment and Planning Meeting:

On Tuesday and Wednesday November 28 and 29, 1995 the TFTR group had a run
assessment and planning meeting with participation from many outside
collaborators. Topics discussed were, Run Schedule, Machine Conditioning,
Experimental Proposals, Diagnostic Upgrades, Communication of Scientific
Results, Collaborations, and Programmatic Elements for '96 Run. R.
Hawryluk opened the discussions with comments on the requirements for
success which included 1) Safe D-T operation of TFTR without impacting
the environment. 2) Experimental program which realizes the full potential
of fusion energy and 3) Communication of our results.

J. Hosea summarized the TFTR operational plans that were developed during
this assessment. TFTR will operate for a 5 weeks run beginning in January
1996 and then have a 3 weeks maintenance period. The weekly schedule will
be a 4 day run week, Monday - Thursday. Friday maintenance and planning
will be adopted to allow neutral beam regeneration on Friday followed by
tritium processing over the weekend. The daily schedule will be 7 a.m. -
11 p.m. operations with shutdown by 12 midnight. Opening for upgrade
installations will begin in mid- April.

Schedule for TFTR meetings: Operation coordination meetings will continue
to be held each day at 8:30 a.m. in the control room annex. The daily
physics meeting will be at 9:00am in B318 and a summary of this Physics
meeting will be put on Internet and E-mail for our collaborators. A Friday
operational/experimental planning meeting will be held at 1 p.m. to outline
the plans for the next week and to assure support readiness for the
following Monday. Friday Physics Staff meeting will be held at 2:00 pm to
discuss summary of the weeks results and plans for following the week. To
facilitate collaborations Cu-SeeMe or MBONE will be available for 2:00 pm
Physics meeting on Fridays in room B318. This will begin with this
Friday's meeting.

K. McGuire and R. Hawryluk discussed programmatic elements of the '96 run.
With limited run time, we will concentrate on experiments with the highest
potential for major advances. About half the run time to be devoted to
RS/ERS - the rest of the run time divided between high li, wall coatings,
MCCD and others.

The presentations at the run assessment and planning meeting can be found
on the Web:
http://www.pppl.gov/TFTR/TFTR_RA95/ .
The viewgraphs were posted and the meeting was transmitted by MBONE and
Cu-SeeME to our collaborators.

Machine Status:

As a result of the reduced funding level during November and December,
operations were performed on Saturday and Sunday to avoid an electrical
demand charge. The focus of the operations was to recondition the vessel
and beams after their respective vents, check out diagnostics, and perform
ITER R&D experiments. The machine has undergone bakeout, glow discharge
cleaning, boronization, pulse discharge cleaning and disruptive discharge
cleaning in December. High current operation has been reestablished. The
beams have been reconditioned to about 28 MW.

During the week of Jan 2 1996, high power TFTR machine and NBI
conditioning is taking place in preparation for the January run campaign.

Future Plans

For the week of the Jan 8th, the plan is to run high li plasmas with q and
current ramps and to start the enhanced reverse shear work the week of Jan.
15th .

R. J. Hawryluk
609-243-3306
Fax: (609) 243-3248
e-mail rhawryluk@pppl.gov