Estimation of Electron Temperature on Glass Spherical Tokamak (GLAST)

Glass Spherical Tokamak (GLAST) is a small spherical tokamak indigenously developed in Pakistan with an insulating vacuum vessel. A commercially available 2.45 GHz magnetron is used as pre-ionization source for plasma current startup. Different diagnostic systems like Rogowski coils, magnetic probes, flux loops, Langmuir probe, fast imaging and emission spectroscopy are installed on the device. The plasma temperature inside of GLAST, at the time of maxima of plasma current, is estimated by taking into account the Spitzer resistivity calculations with some experimentally determined plasma parameters. The plasma resistance is calculated by using Ohm's law with plasma current and loop voltage as experimentally determined inputs. The plasma resistivity is then determined by using length and area of the plasma column. Finally, the average plasma electron temperature is predicted to be 12.65eV for taking neon (Ne) as a working gas.


Introduction
Spherical tokamak is an evolutionary extension of conventional tokamak with additive advantages of low aspect ratio, low field, and tokamak confinement to achieve high beta. Glass Spherical Tokamak (GLAST) is a small spherical tokamak indigenously developed in Pakistan with an insulating vacuum vessel (Pyrex glass). The design parameters of GLAST are R= 15cm, a= 9 cm, κ =2.5, I p =50kA, B T = (0.1-0.4) Tesla,  p =10ms, and T e = (300-400) eV.
There are two tokamak devices (Fig.1c, 1d) currently operating at NTFP named GLAST-1 (having central tube of steel) and GLAST-2 (having central tube of glass). The toroidal field coil system consists of sixteen coils made from 1.5mm thick copper strip. In GLAST-1, a special ring (Fig.2) is used to join the coils in series, keeping inner legs straight while in GLAST-2, the inner legs are tilted at certain angle to join in series. This scheme provides an effect of additional pair of coils that helps to stabilize the plasma. The ohmic heating system consists of a central solenoid and two pairs of compensation coils connected in series. This system provides the necessary loop voltage for the startup and also deflects the unnecessary magnetic flux (due to the solenoid) out of the vacuum vessel. The size and the number of turns in these coils were determined experimentally with the help of double flux loops. The vertical field coil system, consisting of three pairs of coils, is currently being used to provide small vertical magnetic field to support the plasma current startup. The magnetic diagnostics such as induction coils, flux loops and Rogowski coils (Fig.3) are installed on the devices for the measurement of magnetic flux and the plasma current.
The plasma diagnostics installed on GLAST (Fig.4) are two high speed cameras, fast phtodiode, Langmuir probe, Ocean Optics spectrometer HR4000 and a high resolution spectrometer JOBIN YVON THR1000.
A commercially available 2.45 GHz (0.8kW) magnetron source was modified for generating a microwave pulse of 4msec with enhanced output power of about 1.5kW. This pulsed waveguide microwave system is used as a source of pre-ionization for plasma startup in GLAST. In our initial studies neon was used as working gas with optimum pressure ranging from 1.0 mTorr to 0.5 mTorr. A plasma current of 5 kA for 0.5 msec have been generated in GLAST-2. The recorded experimental signals of microwave pulse, loop voltage, photodiode and of Rogowski coils are shown in Fig.5.
An image of the device recorded with high speed camera (5000fps) at the time of maxima of the plasma current is shown in Fig.5a. The intense glow has filled the whole space of the tokamak vessel  giving effect of shining star. The temporal evolution of the plasma current, light intensity and the corresponding variations in the loop voltage and in the microwave pulse are clearly evident.

Estimation of Electron Temperature
In this work, Spitzer resistivity formula is exercised for estimating the plasma electron temperature using the measured values of plasma current and the variations in the loop voltage. The plasma inside GLAST-2 is assumed to be in Coulomb phase.