Plasma and Fusion Research: Regular Articles

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Extension of Operation Region for Steady State Operation on
QUEST by Integrated Control with Hot Walls∗)
Makoto HASEGAWA, Kazuaki HANADA, Naoaki YOSHIDA, Hiroshi IDEI, Takeshi IDO,
Yoshihiko NAGASHIMA, Ryuya IKEZOE, Takumi ONCHI, Kengoh KURODA, Shoji KAWASAKI,
Aki HIGASHIJIMA, Takahiro NAGATA, Shun SHIMABUKURO and Kazuo NAKAMURA
Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan(Received 16 November 2020 / Accepted 24 January 2021)
The controllability of particle supply during long-term discharge in a high-temperature environment wasinvestigated at the Q-shu University Experiment with steady state spherical tokamak (QUEST). QUEST has
a high-temperature wall capable of active heating and cooling as a plasma-facing wall. With this hot wall, atemperature rise test was conducted with 673 K as the target temperature. It was confirmed that the hot wall
could maintain the temperature above 600 K. Feedback control of particle fueling was conducted to control Hαemission, which is closely related to influx to the wall. Using this particle fueling control and setting the hot wall
temperature to 473 K, it was possible to obtain a discharge of more than 6 h. In this discharge, the fueling rateof particles decreased with time, and finally became zero, losing the particle fueling controllability. However, assoon as the cooling water started to flow through the hot wall, particles could be supplied again, and controllability
was restored. Thus, indicating that temperature control of the plasma first wall is important even in the hightemperature environment of 473 K to control particle retention of the wall.
c 2021 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: steady state operation, long pulse operation, high-temperature wall, spherical tokamak, particle fueling, temperature control, feedback control, QUESTDOI: 10.1585/pfr.16.2402034

1. Introduction
Particle recycling is an essential thing in a long-term
discharge of tokamak. However, the controllability of particle fueling becomes diﬃcult in the later phase of longterm discharge. This is because, when discharging for along time, the temperature rises and thermal desorption occurs from the first wall or other components of the vacuum
vessel, and the fueling of unintended particles gradually increases. The eﬀects of particle recycling on long-term discharge have been investigated in many fusion experimental devices [1–6]. Although it is a relatively low parameter(plasma current: 20 kA, density: 1018 m−3 , and RF power:
7 kW), TRIAM-1 M achieved a discharge of 3 h 10 min. It
was also shown that the first wall shifts from particle storage to release as the wall temperature rises during longterm discharge [1]. In LHD, plasma discharge with helium
was performed for more than 40 min, and changes in wall
pumping in a discharge were observed with global particle balance analysis [2]. According to JET ITER-like wall
(ILW) operation, the temperature of the tungsten (W) surface divertor plate, which is not actively cooled, rises withplasma exposure, and the desorption of deuterium atoms
or molecules occurs significantly [3]. Alcator C-Mod [4]and ASDEX-U [5] showed that recycling not only divertor
author’s e-mail: hasegawa@triam.kyushu-u.ac.jp∗) This article is based on the presentation at the 29th International Toki
Conference on Plasma and Fusion Research (ITC29).

but also the main chamber plays a significant role in global
particle balance. In Tore Supra, an unintended increase indensity was observed with a long discharge of 2 min. It isbecause the wall of the part far from the plasma and not
suﬃciently baked was heated by a heat radiation from theplasma and water desorption occurred [6]. Thus, particle
desorption occurs at various locations in the device, and
it is necessary to actively control so as not to cause unintended fueling recycling.
Q-shu University Experiment with steady state spherical tokamak (QUEST) has a high-temperature wall called
a hot wall as a plasma-facing wall, and can actively heatand cool the hot wall [7]. Long-term discharge using
a high-temperature wall can reduce the particle retentionof the first wall and suppress an unexpected increase indensity. Future fusion power plants will operate in hightemperature environment of around 773 K owing to required power generation eﬃciency. Thus, it is essential
to investigate particle recycling in high-temperature environment using a hot wall of QUEST.
The operation of QUEST began in 2008, and noninductive plasma ramp-up experiments using electron cyclotron waves, and steady state operation experiments wereconducted [7, 8]. Since 2010, long-term discharges havebeen attempted at several magnetic field configurations andwall temperatures. Besides, the experiment using the hotwall started in 2014. The discharge time is increasing

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c 2021 The Japan Society of Plasma
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yearly, and the discharge of more than 1 h was obtainedin 2016 [7]. From 2018, the center stack (CS) has beenchanged from a stainless plate sprayed with APS-W to asmall tile made of stainless steel type 316 L to improve
maintainability.

2. Experimental Apparatus
2.1

Configuration of hot wall

The hot wall is installed in a conical shape inside thevacuum vessel (Fig. 1), and is attached using a leaf spring
to avoid thermal elongation. Its material is stainless steel
type 316 L coated with 0.1-mm-thick APS-W. The hot wall
structure is described in detail in [9]. The hot wall is divided into 24 sections in the toroidal direction. One section consists of a panel with an electric heater and a thermocouple embedded in it. An oxygen-free copper plateis also sandwiched so that the temperature distribution becomes uniform. In addition, two cooling water pipes are incontact with the hot wall via stainless steel plates, each ofwhich has diﬀerent thermal resistance. This is because if
the cooling water pipe contacts directly with hot walls, itwill be overcooled. Radiation shields are installed between
the hot wall and the vacuum vessel wall to avoid an excessive temperature rise of the vacuum vessel wall. The radiation shield is made by stacking thin stainless steel plates inlayers at intervals. The radiation shield prevents the radiantheat from the hot wall from going directly to the vacuum
vessel wall.

Fig. 1 Poloidal cross section of QUEST.

2.2 Heater system and temperature rise test
Figure 2 shows the toroidal section view of lower hot
walls. There are 24 sections of hot walls in the toroidal direction. It is grouped into one for every four sections andhas six groups in the toroidal direction. Since it is further
divided into a conical part, a horizontal part, and an upper and lower part, it is divided into 24 groups (= 6 in thetoroidal direction × 2 in conical and horizontal part × 2 inthe upper and lower part) in total. All groups have thermocouples for heater control. Feedback controls of heater
powers are conducted based on these temperatures. A totalof 200 thermocouples are installed on the hot wall. Besides, many thermocouples are installed in other places toinvestigate the temperature profile in detail. For example,
more than 80 and 50 thermocouples are installed in the vacuum vessel and radiation shield, respectively.
Using such a temperature feedback control system anda measurement function; a temperature rise test was performed. The set temperature of the hot wall and the vacuum vessel were 673 K and 373 K, respectively. Figure 3
shows the results. The test was conducted over 4 d while
confirming the safety, such as thermal strain. The vacuum
vessel elongation due to thermal strain was also measured
using a laser displacement meter. In the temperature rise
test, the preset temperature was raised steps by step, suchas 473 K, 523 K, and 573 K. It was held at each temperature for about 1 d. Finally, the set temperature was set
at 673 K. As a result, most parts of the hot wall exceeded
600 K. The radiation shield plate was heated to 450 K or
more by radiant heat from the hot wall.
The temperature rise test at a set temperature of 673 K
is only once this time so far. A typical set temperature forlong-term discharge using a hot wall is 473 K, and coolingFig. 2 Toroidal section view of lower hot walls. (Dot: Thermocouples (TC). Cross: TCs for heater control).

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Fig. 3 Hot wall temperature rise test at set temperature 673 K.

water is passed through the CS. Here, the temperature distribution can be divided into three groups: a vacuum vessel, radiation shields, and hot walls, and the temperatures
are about 300 K, 400 K, and 450 K, respectively.

2.3

Feedback control of particle fueling

Since plasma emission Hα is closely related to the particle influx to the first wall [10], it is essential to control
the intensity of Hα. Therefore, the Hα increases with anincrease in particle fueling. Thus, a feedback loop can beconstructed with input as the deviation of Hα emission andoutput as the flow rate (Fig. 4). The flow rate can be controlled by a mass flow controller (MFC). The MFC is a
device used to measure and control the flow of liquids andgases. In this experiment, MFC capable of controlling theflow rate of 0.2 to 20.0 mL/min of H2 gas was used.
First, the particle supply by MFC was given in a stepwise manner to investigate the response of Hα Figure 5
shows its result. In Fig. 5, particles are supplied by gas puﬀ
only once at the start of plasma discharge. The plasma wasmaintained by continuing to apply the RF of 8.2 GHz, andparticles were supplied by MFC from 15 s in a stepwisemanner. The result showed that the Hα emission increases
gradually with a delay of several seconds. One of the reasons for this delay seems to be that the MFC installation
position is several meters away from the vacuum vessel.
This is because there is a solenoid valve inside the MFC,
and it is necessary to avoid the influence of the magnetic
field generated by the tokamak device. Other causes of thedelay may include particle adsorption by wall pumping. It
was obtained that this system is represented by the generaltransfer function G(s) of eq. 1. It means that this system
consists of a delay element τ and a first-order lag element
T.
K exp(−τs).
(1)G(s) =
1 + sT
From Fig. 5, the delay element τ and the first-order lag
element T are on the order of a few seconds, which is sig-

Fig. 4 Particle fueling feed-back control with Hα signal.

Fig. 5 Step response of Hα by mass flow meter. (a) Fueling rateof MFC, (b) Hα signal of plasma.

nificantly larger than other parameters that require control
on the order of milliseconds, such as the coil current. The
plasma control system (PCS) that controls each parameter operates at 4 kHz, and various parameters, such as thecoil current, are controlled by a 4 kHz feedback loop by
PCS. For simplicity, the feedback control cycle by MFC is
set to every several seconds. It means that when the feedback control changes the flow rate, the feedback control is
performed again after waiting for a few seconds until theplasma responds. Figure 6 shows the test result of feedback
control of Hα emission intensity by MFC. ProportionalIntegral-Diﬀerential (PID) control starts from 10 s, and thetarget value of Hα is fixed at 0.5 (a.u.). Calculation of PID
control loop is conducted every 15 s; meaning that the particle supply changes every 15 s. Thus, the waveform of

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Fig. 7 Inside view of vacuum vessel of 6 h plasma discharge.

Fig. 6 Test result of feedback control of Hα emission intensity
by MFC. (a) Plasma current, (b) Hα emission intensity,and (c) fueling rate by MFC.

Fig. 6 (c) is not smooth. The supply of MFC increases ordecreases to keep the Hα level constant at 0.5 at the initialstage of discharge. After that, the fueling supply rate canbe gradually reduced for 1000 s, and the Hα level is kept
to a constant value of 0.5 and is well controlled.

3. Experimental Results
3.1

Long duration discharge

Using the hot wall and feedback control of fuel supply by MFC described so far, long-term discharge was attempted. In this discharge, the heater-heating system tries
maintaining the hot wall at 473 K and the vacuum vessel
at 373 K. The toroidal and poloidal magnetic fields at the
major radius R = 0.6 m were set to 0.14 T and 8 Gauss,
respectively. RF input power is about 25 kW at 8.2 GHz.
In this setting, the resonance layer is at R = 0.3 m and themagnetic configuration is a limiter configuration (Fig. 1).
With this setting, plasma discharge over 6 h was obtained with a plasma density of less than 1018 m−3 anda plasma current of about 2 kA. Figure 7 shows the limiter configuration plasma in the vacuum vessel of this discharge. The discharge is manually stopped in 6 h. Figure 8 shows the typical wave forms of discharge. In thisdischarge, feedback control of particle fueling was performed so that the signal level of Hα became constant at
0.35 (a.u.). The total pressure increases slightly from the
beginning of discharge (Fig. 8 (d)). In Figs. 8 (b) and (c),Hα level is well controlled and constant at 0.35 (a.u.) by
decreasing particle fueling rate gradually in the initial stageof discharge. Besides, Hα level gradually increases afterparticle fueling rate reaches zero. It means that even ina high-temperature environment of 473 K, particle fueling
control cannot be performed in the latter phase of the discharge owing to uncontrollable density rise.

Fig. 8 Wave forms of 6 h long plasma discharge. (a) Plasma
current, (b) Hα emission, (c) fueling rate by MFC, and(d) total pressure.

Fig. 9 Stacked line chart of mass spectrometer.

Figure 9 shows the changes in the mass spectrometer during long-term discharge. Although the resolution
is not high in these mass spectrometer measurements, theamount that can be caused by hydrogen is constant until
the first half of the discharge and gradually increases in
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Fig. 10 Temperature changes in each part of (a) hot walls, (b) radiation shields, and (c) vacuum vessel during long discharge. The averagetemperature (solid line), minimum and maximum temperature (dash dot lines) of the area are shown.

so that the signal Hα level becomes constant. Thus, thepartial pressure of hydrogen was kept constant in the firsthalf of the discharge. However, in the latter half of thedischarge, it was shown that hydrogen was supplied fromother than MFC. Besides, the partial pressure of H2 O has
been increasing from the beginning of discharge, which isthe same tendency of total pressure of Fig. 8 (d).
Figure 10 shows the temperature changes in each partof QUEST during long discharge. The locations are mainlyclassified into three groups: hot walls, radiation shields,
and vacuum vessel. It can be seen that there is a temperature diﬀerence of about several tens of kelvin in every part of each group. The rapid temperature rise in several minutes occurs at radiation shields in the initial phase(Fig. 10 (b)). Since the radiation shields are made by assembling thin stainless steel plates in layers, the heat capacity is small. The small heat capacity can cause a rapid
temperature rise. The radiation shield is installed between
the hot wall and the vacuum vessel and does not directly
face the plasma. Thus, this temperature rise seems to be
not direct heating from plasma but resistant heating owing
to high frequency multi-reflection of RF. It is worth notingthat this increase in temperature does not aﬀect the degreeof total pressure. Furthermore, the temperature of the hotwalls and the radiation shields gradually rises during thedischarge. Some parts of the vacuum vessel are kept at a

substantially constant temperature since the cooling wateris passed through them.

3.2 Use of active cooling of hot walls
Another long duration discharge over 6 h was obtained. All parameters, such as the preset temperature,poloidal and toroidal magnetic fields, and RF input power
were the same as the previous 6 h discharge. However, cooling water at room temperature was flowing from
300 min to the upper hot wall. Before starting the water
flow, the inside of the stainless steel cooling water pipe wasthe atmosphere, and the temperature of the pipe was about
450 K, which is the same as the hot walls. Since cooling water at room temperature is passed from this state, it
seems that sudden boiling occurs, but the flow path is theopen end. Thus, the increase in pressure inside the pipe
can be suppressed and finally water can flow. Figure 11
shows the waveforms of 6 h long plasma discharge withcooling water. The discharge is also manually stopped after 6 h. In Fig. 11 (c), although the particle supply gradually decreased to zero by 300 s, it is resumed at the sametime as the start of cooling water flowed. This is because
the cooling water was passed through the upper hot walland the temperature of the upper hot wall became lowered. Figure 12 shows the temperature changes of the upper and lower hot wall. It can be seen that the temperature

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Fig. 11 Wave forms of 6 h long plasma discharge with coolingwater. (a) Plasma current, (b) Hα emission, (c) fuelingrate by MFC, and (d) total pressure.

Fig. 13 Waveform at the start of vibrations. (a) Plasma current,(b) Hα emission, (c) fueling rate by MFC, and (d) totalpressure.

Fig. 12 Temperature change of upper and lower hot wall. (a)Conical part and (b) horizontal part of hot wall.
Fig. 14 (a) Fueling rate of two discharge, and (b) total fueled particles.

of the upper hot wall became lowered at a rate of 0.5 K/min
by passing cooling water through the upper hot wall from300 s. It is worth noting that this temperature decrease hasa significant eﬀect on particle retention and controllability,
even in a high-temperature environment of 473 K.
In this discharge, a phenomenon was also observed
where each parameter shown in Fig. 11 oscillates. Figure 13 shows an expanded view of around 71 min when
vibration starts in Fig. 11. The plasma current, Hα signal, fueling rate, and total pressure are vibrating in a cycleof about 1 min. The trigger at which the vibration starts
seems to be a sudden decrease in the Hα signal. Thus, theMFC feedback control increases the particle supply to raise
the Hα signal. However, the Hα signal increases suddenly
since it responds nonlinearly to the particle supply. Therefore, the feedback control by MFC is lowering the particle
supply to lower the Hα signal, and finally makes it zero.

The Hα signal continues to fall, even after the particle supply is reduced to zero, and finally its value drops below thetarget value of Hα, so MFC resumes the particle supply. In
this way, the vibration continues.
The cause of the sudden decrease in the Hα signal andthe nonlinear response of the Hα signal to the particle supply is unknown, but hunting oscillation is occurring. In thisdischarge, feedback control by MFC is performed every
5 s. Vibration may be suppressed by extending the control
interval and waiting for the plasma response. Hunting oscillation is self-excited and is undesirable from the equilibrium viewpoint. However, the situation seems to be a little
diﬀerent regarding this discharge. To show this, the difference in particle supply between the two 6 h discharges
is shown in Fig. 14. As shown in Fig. 14 (b), total fueled

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Fig. 15 Temperature change of 24 sections during 6-hour discharge of discharge number of (a) #42981 without cooling water and (b) #42991 with cooling water.

particles are calculated from the time integration of the fueling rate of Fig. 14 (a). In shot number 42981, the particle
supply by MFC became zero by 200 min ago, and the particle supply cannot be controlled after that. With discharge
number 42991, particles can be supplied up to 300 min before. As shown in Fig. 14 (b), the total number of supplied
particles between these two discharges is about twice aslarge by 300 min. It is worth noting that the particle supply rate of the two discharges is almost the same during
the time when vibration is not occurring, as can be seenaround 50 min, for example.
Figure 15 shows one of the causes of the diﬀerence inparticle fueling between these two discharges. Figure 15
shows the temperature change of 24 sections during 6 hdischarge of shot number #42981 (Fig. 15 (a)) and #42991(Fig. 15 (b)). As shown in Fig. 15 (a), most of the sections
have a temperature rise of about several tens of kelvin during discharge, or the temperature is kept almost constant.
In Fig. 15 (b), the temperature of three of the 24 sections
rises before the start of discharge. This is because theheater that heats the area around the thermocouple for control does not operate normally owing to disconnection or
ground fault. To raise the temperature of those thermocouples, the output of the peripheral heater also increased,
and the temperature of the peripheral part increased. The
surface area of the upper hot wall is 4.7 m2 , and the three
sections of this are 0.59 m2 . The area of the three sections
is less than 3% of the total since the total surface area of theplasma-facing wall is about 23 m2 . In the actual discharge,
this abnormal operation was noticed before discharge, andthose heaters were stopped manually. Thus, the temperature of the corresponding three sections started decreasing
before the start of discharge. Besides, the temperature decreases even during discharge. The decrease in tempera-

Fig. 16 Particle fueling at diﬀerent CS temperatures. (a) Hαemission controlled to 0.35 (a.u.), (b) fueling rate by
MFC, (c) total fueled particle.

ture during discharge may aﬀect particle retention, leading
to improved controllability of particle fueling. This partial
modification of wall temperature is a candidate for controlling the fuel particle balance since the discharge #42991
could be maintained without density raise unlike the discharge #42981.

3.3 Temperature dependence of CS
It has been stated that the preset temperature in a typical long-term discharge is 473 K, and the temperature of
CS at this time is about 300 K with cooling water. In
the previous CS, APS-W with a thickness of 0.1 mm was
sprayed on the base material of stainless steel type 316L,
but it was changed to tile-shaped stainless steel type 316 Lfrom 2018. Figure16 shows the diﬀerence in particle fueling when the temperature of CS is changed. The temperature was adjusted by stopping the passage of coolingwater, and the temperatures at the start of discharge were42◦ C (315 K), 86◦ C (359 K), and 102◦ C (375 K), respectively. The target value of Hα signal was set to 0.35 (a.u.),and the plasma was discharged for 1000 s (Fig. 16 (a)). The
particles can be supplied until the last 1000 s with all threedischarges. Although, the surface area of CS is smaller
than the surface area of the hot wall, it can be seen thatthe number of fueled particles decreases with an increasein temperature (Fig. 16 (b)). Owing to the temperature difference of 60◦ C, the particle supply amount is 1.5 times
diﬀerent by 1000 s (Fig. 16 (c)). Thus, the diﬀerence in CS
temperature greatly aﬀects the particles’ controllability. It
indicates that we had better control the CS temperature andthe hot wall, and it is a future work.

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4. Discussions and Summary
When cooling water was passed through the upper hotwall at a set temperature of 473 K, the temperature of the
hot wall dropped and particle fueling was immediately resumed. It indicates that particle retention occurs even ina high-temperature environment of 473 K; it is essential
to control the temperature of the hot wall. The radiation
shield, which is not the first wall, showed a sudden temperature rise at the initial stage of discharge, but the totalpressure did not change accordingly.
The total surface of the plasma-facing wall in QUEST
is about 23 m2 , and the surfaces of the upper hot wall andCS are 4.7 m2 (9.4 m2 in the upper and lower hot wall) and2.7 m2 , respectively. As shown in Fig. 1, the discharge isa limiter configuration plasma, and while the central partof CS strongly interacts with plasma, wall pumping seems
to occur in the upper and lower parts of CS. Even if thesurface area of CS is about 10% of the total, the temperature change at that location has a great influence on the
total particle supply. It is considered that this is because
the temperature of the CS is lower than that of other parts,
or that the CS is in a solid stainless steel state without a deposition layer owing to sputtering, and these cases promotewall pumping [10].
It is of great engineering significance to raise andmaintain the temperature of the hot wall of QUEST above

600 K against the operating temperature of 773 K of thefusion reactor in the future. Thus, it is possible to discharge in a situation closer to the operating temperature of
the future fusion reactor. It is essential to perform a longterm discharge at a set temperature of 673 K and perform acomparative study with particle recycling at a discharge of
473 K.

Acknowledgment
This work was supported by a Grant-in-Aid for
JSPS Fellows (KAKENHI Grant Number 16H02441,
24656559) and the NIFS Collaboration Research Program(NIFS19KUTR136). This work was also supported in partby the Collaborative Research Program of the Research Institute for Applied Mechanics, Kyushu University.
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