Gas Fueling (Gas Puff) System
J. Miyazawa, K. Yasui, K. Nagahara, G. Motojima, H. Kasahara, and H. Funabae-mail: miyazawa@nifs.ac.jp

1.

Objective(1) Supply hydrogen (H 2 , D 2 ) and impurity gasses (He, N 2 , Ne, Ar, Kr, Xe, etc.) into the LHD vacuum
vessel for the plasma experiment.
(2) Supply hydrogen (H 2 , D 2 ) and impurity gasses (He, N 2 , Ne, Ar, Kr, Xe, etc.) into the LHD vacuum
vessel for the discharge cleaning.
(3) Supply a large amount of H 2 , He, and Ar, at an emergency, e.g, quench of superconducting magnet coils
or earthquakes, to stop or prevent the plasma discharge.
(4) Serve low-pressure (0.1 MPa in gauge) gasses of H 2 , D 2 , He, N 2 , Ne, Ar, Kr, Xe, etc., andhigh-pressure (8 MPa in gauge, at the maximum) gasses of H 2 , D 2 , He, N 2 , Ne, Ar, as a gas utility
system in the LHD hall for users.

2. Apparatus
2.1. Piezo-valves- Three types of piezo-valves with the maximum flow rate of ~100 Pa⋅m3/s, ~50 Pa⋅m3/s, and ~5 Pa⋅m3/s, inFig. 1. Distribution of piezo-valves, mass-flow controllers, and supersonic gas puff (SSGP) in the LHD.

the case of H 2 , are distributed at 3.5L,
5.5L, and 9.5L ports (see Fig. 1) [1].
- The gas species are basically fixed foreach piezo-valve, since 2014FY.
- The working gasses of H 2 , D 2 , He, N 2 ,Ne, Ar are always available, while otherimpurity gasses as Kr, Xe, are available
according to the request from users.
- The working gas pressure is basically set
to 0.1 MPa in gauge, for all of the gasspecies.

Fig. 2. A photo of the manifold at 3.5L.

- The flow rate of each piezo-valves has
been calibrated in situ, i.e., by measuring the pressure decrease in the reservoir tank after puffing thecorresponding gas into the LHD vacuum vessel. Examples of calibrated flow rate are shown in Fig. 3.
Note that not necessarily all of the piezo-valves are calibrated yet, at this moment.
- At an emergency, H 2 and Ar are puffed from the 3.5L port, while He and Ar are puffed from the 9.5L port,
through large piezo-valves, to increase the pressure in the LHD vacuum vessel to a few to ~10 Pa. Thisemergency gas puff is triggered by the signal from the LHD central control system and/or the manualswitch on the gas puff control desk.

Fig. 3. Calibrated flow rates of typical piezo-valves. Calibration has been done in situ in LHD, using thecorresponding gas. Horizontal axis is the control voltage, which is applied to the piezo-valve after
amplification for 30 times.

2.2.

Mass-flow controllers- Two types of mass-flow controllers withthe maximum flow rate of 3,000 sccm
(standard cubic centimeter per minutes)
and 300 sccm, in the case of H 2 , aredistributed at 3.5L and 9.5L ports,
respectively (see Fig. 1). At each port, gas
species of H 2 , D 2 , He, N 2 , Ne, and Ar,
are available.
- The gas species are fixed for eachmass-flow controllers, since 2014FY.
- These mass-flow controllers are alsoavailable in the plasma experiment.

2.3.

Supersonic gas puff (SSGP)- For supersonic gas puff (SSGP [2]), alarge Laval nozzle is installed inside the
3.5L port (Fig. 4).
- A solenoid-valve is connected to the Laval
nozzle (Fig. 5).
- The working gas can be chosen from H 2 ,
D 2 , He, N 2 , Ne, and Ar.
- The working gas pressure is adjustable inthe range of 0.5 – 8 MPa in gauge.

2.4.

Gas utility- Low-pressure (0.1 MPa in gauge) gassesof H 2 , D 2 , He, N 2 , Ne, Ar, Kr, Xe, etc.,
and high-pressure (8 MPa in gauge, at themaximum) gasses of H 2 , D 2 , He, N 2 , Ne,Ar, are available at A-stage, B-stage,Fig. 4. A schematic view of the large Laval nozzle for

C-stage, D-stage, and B2-floor.

SSGP installed inside the 3.5L port.

3. Operation
3.1. Normal gas puff for plasma experiment- All of the piezo-valves and mass-flow controllers are operated in the pre-programmed mode or various
feedback modes [1].
- In the pre-programmed mode, the control voltage is applied to a selected piezo-valve or mass-flow
controller, according to the arbitrary waveform programmed in advance. Several piezo-valves can be usedat the same time with different waveforms.
- In the feedback mode, which starts at a pre-programmed timing, the control voltage is basicallydetermined from the difference between thearbitrary pre-programmed target density signal andSolenoid-valve

the measured real-time density signal. Both of the
proportional control and PID control are available.

3.2.

SSGP

- The solenoid-valve for SSGP is also operated in the
pre-programmed mode or the feedback mode.
- In the pre-programmed mode, the solenoid-valve isoperated with the programmed pulses, where thestart timing, the pulse width, the frequency, and thenumber of pulses can be arbitrarily determined.

Pressure gauge- In the feedback mode, which starts at a given
timing, the solenoid valve becomes open when thedensity signal is lower than the target value.

4.

Available data by “Retrieve”

4.1. LABCOM
The gas puff data can be retrieved with the signal
names of “GASPUFF” and “DGASPUFF”.

4.2.

Fig. 5. A photo of the solenoid-valve for SSGP set
just below the 3.5L port. Both of the magnetic
shield and the neutron shield are not installed yet.

Kaiseki-data server

The gas puff data can be obtained with the signal names of “gas_puf” and “gas_ssgp”.

5.

Remarks- To protect the piezo-valves from the neutron and/or gamma ray irradiation during deuterium experiments,
the manifolds at 3.5L (Fig. 2), 5.5L, and 9.5L are moved from just below the ports, i.e, R ~ 4 m, where R
is the major radius from the LHD center, to R ~ 8 m, since 2014FY. Because of the additional
conductance between the manifolds and the ports, the response of the plasma after gas puffing tend to be
slower compered with that before 2014FY.

References[1] J. Miyazawa, K. Yasui, H. Yamada, LHD Experimental Group, Fusion Engineering and Design 83(2008) 265.
[2] A. Murakami, J. Miyazawa, et al., Plasma Physics and Controlled Fusion 54 (2012) 055006.