29SI-2
球状トカマクに於ける粒子制御と定常プラズマ放電
Control of wall recycling and Steady State Tokamak Operation in QUEST
図子秀樹

H. Zushi

九州大学応用力学研究所 RIAM
Steady State operation
The steady state operation of ST plasma is not
only one of the missions of the QUEST projects,
but also is required as the minimum
performance of a key device in the frame work of
the bilateral collaboration. At least three
independent but inter-related subjects must beresolved; (1) the current drive performance, (2)heat balance on the PFCs and (3) global particle
balance in the system.
In the typical discharge (#19674) Prf of
50 kW was used to drive high p plasma at Ip of 10kA, as shown in Fig.1. Hard X-rays measured along theco(anti parallel to the Ip direction), counter, andperpendicular directions show the steady statesustainment of the energetic electrons. Although theHX fluxes are remained almost constant, energy
spectra are quite different (not shown). The Energy of
X-rays emitted from the co-moving electrons reaches ~1MeV, but that from counter-moving
ones islimited below 0.4 MeV. Two perpendicular ones arebetween them. Unfortunately the density could not bemeasured. The series of puff were used to keep thepressure a certain value (3 × 10-6 Torr) inside thechamber, but it stopped at 90 sec. If the gas puff wascontinued further, the pressure increased and then Ip
was reduced below 10 kA. Hincreased until 90 sec,
and then remained a certain value, indicating constantglobal PWI. However, a small change in the surface
temperature of the movable forced cooled guard limiter
made of W was found at 90 sec, which indicated a local
change in PWI. On the horizontal plane four guard
limiters are located at 65 mm from the wall(R=1400mm). Since these Mo plates (W=50 mm and H=220 Fig.1 Time evolution of SST plasma; (a) Ip, (b)mm) are not cooled, the surface temperature limits the HX(co-red,vertical-blue,vertical-green,present SSTO duration. Local power deposition, ctr-black), (c) temerature of the movable limiter,
which is considered as a consequence of the local (d) Hfrom the CS region.
loss of energetic electrons on particular PFCs,
causes an abrupt surface temperature rise or melts them. In the next campaign cooling performance will
be upgrading.
Global particle balance
In order to demonstration SSTO, the global particle balance must be well understood and controlled.
Two subjects for working gas H2 have been investigated in relatively low density plasma; (1) globalbalance between fueling and external pumping, and (2) wall pumping and release. The volume of
the chamber and extension pumping port is 12.8 m3 and the surface area of the plasma facing

components is ~ 35 m2. The one third of thePFC is tungsten and the rest is SS. No active
coating of light materials is done. Twocryogenic pump and one TMP are used toexhaust H2 at pumping speed of 2.7 m3/sec. The
gas is fuelled by a piezo unit whose fueling rateis controlled by the width and the number ofvoltage pulses. The pressure inside thechamber is measured by an ionization gauge
and the partial gas components are measured
by a residual gas analyzer near the TMP head.
The recycling is monitored by several
spectroscopic instruments.
The global particle balance in SN 19674, asshown in Fig. 2, is analyzed as follows;1) Qfuel: total fueled particles at almost
constant fueling rate of 2.5×1017 H2/s for 90s~2.2 ×1019 H22) Qpumped(Td): pumped particles for thedischarge duration of Td=180s is 1.3×1019 H2~0.6×Qfuel3) At 34 sec after the discharge has been
terminated, Qpumped(Td+34s) equals to Qfuel, andFig. 2 Global gas balance for #19674(180sdischarge). The puff was stopped at 90 sec.

then Qpumped(4Td=720 s) reaches 4.5× Qfuel.
Thus, QUEST results show that 1) the metal
wall pumping dominates the global particle
balance during the discharge, and then 2)
enhanced particle release just after thetermination of the discharge recovers Qfuelwithin ~ 30 sec. In order to understand theunbalanced condition that Qpumped is 4.5×
Qfuel during the interval duration, thedischarge history has to be investigatedfurther.
For
previous
longpulsedischarges(total 400s duration in SN
19668-19673) the total sum of Qfuel is only 5.8×Qfuel(SN19674).
Pumping of He
In addition to the H2 global balance, thewall pumping and release of He is also
examined. The total pumping speed is ~ 1.9
m3/sec. The He signal of RGA with(solid) andwithout plasma(dotted) are shown in Fig.3.
The valves are open at 80 s and the discharge
duration is 20 s. The difference between twois also shown in the lower figure. The He iswall pumped at the constant rate for 20s
and then He is released for 40 s. After that
release rate is much reduced. Thus evenduring the interval, 2/3 of Qfuel is retained in
the metallic wall.

Fig. 3 Global gas balance for #19637(20s duration).
The He puff was done during the pulse. All valves
are closed.