J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 206-210

Feedback Control Achievements and Endeavours in ToreSupra Plasma Wall Interactions Control
GROSMAN Andr6*, GHENDRIH Philippe, GRISOLIA Christian, GUNN Jamie, LOARER Thierry,
MONIER-GARBET Pascale and TORE SUPRA team
Association EUMTOM-CEA sur la fusion control4e,
CEA Cadarache, F- I 3 I 08 Saint- Paul-ldz-Durance Cedex, France(Received: 18 January 2000 / Accepted: 18 April 2000)

Abstract
Feedback control on the power exhaust repartition will be required in any reactor like tokamak
experiments. A review of the experimental studies in Tore Supra is given, where many feedback options
have been empirically tried to optimise the edge radiation while staying very close to detachment so as tostill insure good ICRF waves coupling.

Keywords:
ergodic divertor, radiative edge, detachment, plasma edge measurement, feedback control

1. Introduction
Control of plasma wall interaction is one of theIn Tore Supra, a major effort has been concentrated
on the control of the ergodic divertor conditions. The
feedback control systems are essential in providing

most difficult challenges of controlled fusion research[]. The constraints imposed on the wall components by
the requirement of steady state operation are so severe(i.e. at the feasibility limit) that dedicated edge plasma
conditions have to be provided. These conditions are
subject to modifications that may stem from internallyor externally imposed physical configurations. The time

highly radiating layers at the very threshold ofdetachment, so as to ensure the coupling of ICRH, andgood screening of the impurity influx. This paper will
review various employed techniques, from deduction of
the radiation front by bolometers, to monitoring of theedge electron temperature or the total radiation.

constants involved in the many phenomena display aGenerally, the abrupt transition to plasma detachment
should be prevented in Tore Supra as the coupling of
ICRF waves cannot be achieved anv more in suchwide spectrum from those originating from paralleltransport in the edge (10 3 s) to those linked to the wall
physical conditions, that are generally considered asconditions.
The main action remains the gas injection, either oflarge (10-t-102 s). The necessity to introduce feedback

controls derives from the definition of target edge
conditions of particle and heat exhaust, RF wave
coupling, etc. The major difficulties to be encountered
are the determination of the physics at stake, thecapability to provide an adequate monitoring of thephysics criteria, and finally the implementation of andeuterium or impurities. The actual injection time
constant cannot be decreased below 100 ms, which,appears somewhat marginal and needs a degree ofanticipation in the analysis. The ed,ge T" evolution,measured by Langmuir probe has proven to be the more

efficient, due to a good definition of the "detachment

effective action in due time.

*diagram" knowledge.
@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchC o rre sp o ndin g autho r' s e - mail : a. g ro s man @ c e a.fr

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Grosman A. et al., Feedback control achievements and endeavours in Tore Supra Plasma Wall Interactions Control

decrease observed at detachment prevents theachievement of good ICRF waves coupling, whereas
ICRH is the main auxiliary heating system in ToreSupra and the antennas are unavoidably within the2. Physics Basis of The Feedback Gontrol
Extensive studies in Tore Supra proved the ergodic
divertor to operate in a very similar manner as the X
point divertor l2l. A remarkable finding is that in thelaminar zone, the existence of distinguishable flux tubes
allows to apply the so called 2 points model [3] to relate

divertor region in the ergodic divertor configuration.
On the opposite, the high recycling regime inducesconditions which are very favourable to enhance theradiation losses inside the divertor region. It was shownin [5] that the radiation losses could be written:upstream and downstream plasma parameters. Verysimilar "density regimes" have been found in ToreSupra ergodic divertor experiments [4]. Noteworthy, astrong non linear evolution of the electron density at thetarget nt as a function of the upstream density nu(nr-nu3), while the target temperature displays an inverseevolution (Tr-nu-2), both at least in the high recycling

P'^o = I f ,,,""n(Z"u- l)n,7, R r(7,)(l)

where Pru6 is the total radiated power, Z"n the effectiveregime. Any attempt to decrease too much thecharge of the plasma bulkrf,".""n the ratio of the impurity

temperature will lead to detachment, where the targetdensity decreases. Such an evolution is shown in Fig. I
for one Tore Supra shot, in which the target parameters
are measured with Langmuir probes, inserted within thecontent in the edge and in the bulk and R,, the radiation

function as defined in [6]. The latter function varying
rather slowly except very close to the radiation
maximum (at about 7 to 20 eV, depending on theimpurity species), it appears that the enhancement is ingreat part due to the increase of the screening ofdivertor target plate.
It should be stressed here that the edee densitv

incoming impurities (itself a linear function of n,).

3. Radiative Power Control in Tore Supra
This paragraph will review the various methods

lo=1,4 MA

2 [A ilCRH

10

used in Tore supra to optimise the radiative losses at theIedge. From the previous paragraph, it stems that themajor actions to be promoted are an increase of thedensity and of the edge impurity content. Both of themcan be achieved at the time only by gas injection. ThisIIEbE
atrkind of action proved to work efficiently; Yet, twoquestions were raised:- the injection time constant (about 0.3 s due the4030

injection tubes conductance) is rather long comparedto some of the major physics constant for the plasmaedge such as Til (= L"/C", the parallel time constanttro- the wall evolution with much longer time constant(10 s at least) will in the long run plays a dominant

about l0-2s).

F'role through the recycling process, unless a efficient
100pumping is provided.
Concerning the criteria, 3 absolute measurements
were used, the average density <n">, the total radiation
P136 ord the target electron density. Unfortunately, therelation of the former to edge parameters cannot be
unequivocally deduced; the second proved to be difficult1234<n.> llore m{)

Fig. 1 Evolution of the target density and temperature as

to deduce as radiation is non axisymmetric. The latter issubject to the well known difficulties encountered with
Langmuir probes.
It is the reason why two relative criteria were alsoused. The ratio of the two outer bolometer line of sieht

a function of the plasma average density for 3conditions (neon injection: below and lefq
deuterium injection: up and middle. and with theadjunction of 2MW RF power: points with errorbars)
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Grosman A. et al., Feedback control achievements and endeavours in Tore Supra Plasma Wall Interactions Control

could be used to monitor detachment processes [7],whereas, the degree of detachment parameter, deduced
from the deviation of the edge density to the high

3.2 Feedbacks with security relying on therecycling evolution [8] proved to be even more efficient[9]: in fact, the edge bolometers were affected by strayeven more accurate to use a criterion actually related tosignals in the presence of ICRH.

dynamic close to the detachment limit. Practically, theDoD is defined during the initial density ramp up. The
fitted ion current: ,/sot,"rr = f (n) is calculated using

DoD
Instead of using a threshold value of ?", it appeared

the detachment process. The DoD exhibits a large

3.1 Feedback on <n; and control on I
A simple modification of the usual feedback

data points whose corresponding temperature lies
between two predefined values, 13 eV and 18 eV for thealgorithm allows to insure good RF coupling:-tlr@rrr = GAIN,[r*r**(t)example shown here. The DoD is undefined until thetemperature falls into this range. Once it is defined, thesecurity routine is activated and attenuation of the gas
flow begins at lower threshold DoDl and is cut off at
upper threshold DoD2, similar to the security on edge
temperature described in the previous section. A linear
fit of the DoD on <n"> proved to be sufficient.

- n."ro, (t)l x S ( 4 (tr. (2)Here, S(?i) is a factor that attenuates the gas flow in realtime based on the instantaneous measurement of thedivertor electron temperature. Above an upper threshold
temperature 2"1 the factor equals unity, so the gas
injection proceeds normally. Between 7"1 and a lower
threshold temperature 7"2, the factor decreases linearlyaccording toS0l)=

T'- T"z.
T"r - T,zThis successful attempt allowed to achieve
experiments in which optimum values of Te weresearched. The DoD was efficient in avoiding to reachtoo low values and detachment. An example is given inFig. 3. The reference value of temperature was set to 5
eV at the end of the ramp. The gas flow was attenuated(3)with the gas flow being completely cut off when T" <7.2. Here, the upper threshold was chosen to be T"r=!Q

eV, and the lower value 7"2= 14 eV. This guarantees
that the edge will not fall out of the high recycling
regime, and even with a disastrous event such as50oe3i4

coupling failure, or excessive gas injection, there will besome margin for the edge to warm up and recover.

-Flg. 30

The ability of the program to respond toC'It

3Fn 20

unexpected events has been shown and an example isgiven in Fig. 2.

.-f'10002oshot 26696 : disruption preventeddl€,Ez7

Pso!4q2'o--_o1Oao€gooIUo-e=0s6Eoo-.20
F,0
6

10o

2o,,",6I

Et [s]

1oFig. 2 Example of disruption prevention, where density
is adapted to ensure good RF coupling, with a cutoft at 7," = 14 eV.

Fig. 3 Evolution of the linear density, the target electrontemperature, the gas injection and the DoD. Anedge temperature below 1OeV is thus controlled.
208

Grosman A. et al., Feedback control achievements and endeavours in Tore Supra Plasma Wall Interactions Control

starting at 6 s due to the increase of the DoD towards
the cutoff value DoD2 = 1.9. The gas flow was notpower. A gas mixture of about 2Vo of neon in deuterium
was prescribed before the shot. Pru6 increases at 6 s,after a delay (almost I s) due to the time constant of gas

totally stopped because the overall gain of the feedback
loop was well adapted to these particular conditions, andthe detachment was held stable until the end of thecurrent plateau at 8 s, as evidenced by the flat line-At equilibriufir, P.ad stays constant at 6 MW and theradiated power fraction is then equal to 857o. This valueof Pru6 is I MW lower than the target value of Pru6 due
to a low proportionality factor chosen on the feedback

averaged density.

prerequisite. This high value of P."6 induced a3.3 Feedback on bolometric signals
In Fig. 4, the time trace of P.u4, estimated from aneutraliser plates ofthe ergodic divertor. An energy flux

combination of raw data of bolometric arrays, and of thetotal injected power (P.o,) are presented together with thereference feedback value of the radiated power. In thefirst part of the discharge, before 4 s, gas injection isthermographic measurements [0].
Yet, the difficulties related to the determination ofan accurate radiation power prevented a confident use ofsignificant decrease of the conducted power on theas low as 0.4 MW.m-2 could be deduced fromfeedback controlled on the central linear density. Then,at 4 s, ICRH heating is coupled to the plasma leading tosuch techniques. The detachment parameter based on theratio of the two outer bolometer line of sights was alsotested; In that case, the major problem was the too

an immediate increase of P,o, and P,u4. At 5 s, gas
injection control is changed to feedback on radiatedabrupt evolution of this parameter (called hereDETACH). This is well displayed in Fig. 5, where theDETACH parameter evolves significantly only after theedge density already decreases. Thus, the control isobtained at the expense of strong modulations of both
the radiation but also the densitv.

Radiated pow€rreference feedbock

4. Gonclusions
Feedback techniques applied to the complex
question of radiating edge control show that strong

improvements of performance through theirimplementation in spite of:- rather long time constant for the gas injection- difficulty in defining criteria.

Fig. 4 Feedback on the total radiated power.

Yet, absolute measurements can be replaced by ratios,
related to "bifurcation physics" and proved (especiallythe degree of detachment DoD) to improve the control
capability in edge radiation optimisation experiments.

References

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Ph. Ghendrih, A. Grosman and H. Capes, Plasma
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t3l A recent review can be found in: C.S. Pitcher andP.C. Stangeby, Plasma Phys. Control. Fusion 39,779 (1997).

I4l J.P. Gunn et al.,Plasma Phys. Control. Fusion 41,8243 (1999).
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t6l H. Capes, Ph. Ghendrih and A. Samain, Phys.
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Fig. 5 Evolution of the edge and average densities andof the DETACH parameter as a function of time.
209

Grosman A. et al., Feedback control achievements and endeavours in Tore Supra Plasma Wall Interactions Control[10] Tore Supra team, presented by A. GROSMAN,
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