Gas Fuelling System for SST-1Tokamak

SST-1 Tokamak, the first Indian Steady-state Superconducting experimental device is at present under operation in the Institute for Plasma Research. For plasma break down & initiation, piezoelectric valve based gas feed system is implemented as a primary requirement due to its precise control, easy handling, low construction and maintenance cost and its flexibility in the selection of the working gas. Hydrogen gas feeding with piezoelectric valve is used in the SST-1 plasma experiments. The piezoelectric valves used in SST-1 are remotely driven by a PXI based platform and are calibrated before each SST-1 plasma operation with precise control. This paper will present the technical development and the results of the gas fuelling system of SST-1.


Introduction
Magnetic fusion devices require fast-responding, versatile and accurate gas injection system in order to initiate the plasma breakdown and subsequently to maintain and control the plasma density. A number of techniques like external gas feed, neutral beam injection, pellet injection and their combinations are employed in fusion devices to maintain the plasma density at desired levels for long periods. Main functions of the gas fuelling system are (a) pre-filling of the vessel to the required pressure, (b) feeding of required amount of gas during plasma current and density build up phase, (c) feeding of required amount of gas to maintain plasma density at the desired value, (d) feeding of gases into the vessel during wall conditioning and (e) feeding of required amount of gases in the divertor region to remove the heat and exhausted particles. For all these operations, a very efficient gas fuelling system is essential and therefore developments of effective fuelling methods has been one of the key issues in the plasma study. Hydrogen gas feed, Supersonic molecular beam injection and pellet injection are the techniques implemented for the SST-1 tokamak.
The steady-state superconducting tokamak (SST-1) has a minor radius of 0.2 m and a major radius of 1.   . DC and RF assisted glow discharge cleaning using H 2 /He gas and their mixture was carried out both before and in between plasma shots. For this purpose, two rectangular SS electrodes mounted at diagonally opposite ports and two power supplies each having a rating of 0-1000 V, 0-15 A and an RF supply of 300 W at 13.56 MHz were used.

SST-1 Gas Fuelling System
Three types of fuelling systems have been installed on SST-1 vacuum vessel such as (a) Supersonic molecular beam injection system (SMBI) (b) Pellet injection system (PI) and (c) Fast gas injection system (FGI).
the injection points of these three fuelling systems are

Supersonic Molecular Beam Injection system (SMBI)
The SMBI was designed for feedback based density control. Main advantage is that the neutrals are fed at supersonic velocity and therefore have higher probability of penetrating into the core as compared to ordinary gas puffing. Two (02) numbers of M/s Parker make solenoid valves have been installed on the high field side at radial port nos. R-7 & R-10. An inlet pressure of 10 bar(g) was maintained at the valve inlet and a rectangular voltage pulse of 20 VDC with varying time duration was applied to feed the desired number of neutrals into the plasma chamber. Each of these two valves has a molecular injection rate of 2.8  10 22 H-atoms/s.

Fast Gas Injection Fuelling system (FGI)
Out of the three fuelling systems installed on SST-1 vessel, only the fast gas injection method has been used extensively in SST-1 plasma campaigns. The objective of fast gas injection system is to feed the desired amount of hydrogen gas during pre-filling and density build-up phase during the plasma operation as well as to feed fuel gas during the wall conditioning, Lower Hybrid Current Drive (LHCD) and Ion Cyclotron Resonance Heating (ICRH) assisted plasma shots. The key element of this system is piezoelectric valve which was chosen due to its fast response time of ~ 2.0 ms, capability to work in the presence of magnetic field, low cost and reliable performance. This valve has a maximum throughput of 625 sccm at 100 V, 25 °C and 1.0 bar(g) inlet pressure. The throughput of this piezo valve is linear with respect to its inlet pressure and has a capacity to withstand a maximum inlet pressure of 3 bar(g). FGI system consists of reliable hardware and software necessary to control and monitor the flow of gases required for the plasma experiments remotely from the control room. Four (04) numbers of M/s. Maxtek make MV-112 piezo valves (02 nos. on the high field and 02 nos. on the low field side) are used to feed gas into SST-1 vessel during pre-filling. Another two (02) numbers of valves were used during the density build-up phase. These valves were mounted in radial ports diagonally opposite to each other to maintain the overall balance of gas distribution inside the vessel. In addition, two (02) more numbers of valves were mounted on either side of LHCD antenna and one (01) number of valves near ICRH antenna to feed the desired amount of neutrals during LHCD and ICRH assisted plasma shots. For Glow discharge cleaning (GDC) and boronization experiments, two (02) numbers of valves were mounted at diagonally opposite radial ports R-3 and R-11. Research grade ultra-high pure hydrogen, helium and their mixtures were used in SST-1 plasma experiments. A palladium based hydrogen purifier was connected in hydrogen inlet line which provides 99.99995% pure hydrogen gas at its outlet. Since the variation in inlet pressure to the valve