P. K. Chattopadhyay

Cyclotron resonance heating systems for SST-1

RF systems in the ion cyclotron resonance frequency (ICRF) range and electron cyclotron resonance frequency (ECRF) range are in an advanced stage of commissioning, to carry out pre-ionization, breakdown, heating and current drive experiments on the steady-state superconducting tokamak SST-1. Initially the 1.5 MW continuous wave ICRF system would be used to heat the SST-1 plasma to 1.0 keV during a pulse length of 1000 s. For different heating scenarios at 1.5 and 3.0 T, a wide band of operating frequencies (20–92 MHz) is required. To meet this requirement two CW 1.5 MW rf generators are being developed in-house. A pressurized as well as vacuum transmission line and launcher for the SST-1–ICRF system has been commissioned and tested successfully. A gyrotron for the 82.6 GHz ECRF system has been tested for a 200 kW/1000 s operation on a water dummy load with 17% duty cycle. High power tests of the transmission line have been carried out and the burn pattern at the exit of transmission line shows a gaussian nature. Launchers used to focus and steer the microwave beam in plasma volume are characterized by a low power microwave source and tested for UHV compatibility. Long pulse operation has been made feasible by actively cooling both the systems. In this paper detailed test results and the present status of both the systems are reported.

Test and Commissioning of 82.6 GHz ECRH system on SST-1

Electron Cyclotron Resonance Heating (ECRH) system will play an important role in plasma formation, heating and current drive in the Superconducting Steady state Tokamak (SST-1). Commissioning activity of the machine has been initiated. Many of the sub-systems have been prepared for the first plasma discharge. A radial and a top port have been allotted for low field side (LFS) and high field side (HFS) launch of O and X- modes in the plasma. The system is based on a gyrotron source operating at a frequency of 82.6±0.1GHz (GLGD-82.6/0.2) and capable of delivering 0.2 MW / 1000s with 17% duty cycle. The transmission line consisting of ~15 meters length 63.5mm corrugated wave guide, DC break, wave guide switch, mitre bend, polariser, bellows that terminates with a vacuum barrier CVD window. A beam launching system used to steer the microwave beam in the plasma volume is connected between the end of the transmission line and the tokamak radial and top ports. A VME based real time data acquisition and control (DAC) system is used for monitoring, acquisition and control. Hard-wired interlock operates a rail-gap based crowbar system in less than 10µs under any fault condition. Burn patterns are recorded at various stages in the transmission line. The gyrotron is tested for ~200 kW / 1000s operation on a water dummy load. Transmission line is tested at various power levels for long pulse operation. The paper highlights the experimental results of successful commissioning of the system.

A linear helicon plasma device with controllable magnetic field gradient

Current free double layers (CFDLs) are localized potential structures having spatial dimensions - Debye lengths and potential drops of more than local electron temperature across them. CFDLs do not need a current for them to be sustained and hence they differ from the current driven double layers. Helicon antenna produced plasmas in an expanded chamber along with an expanding magnetic field have shown the existence of CFDL near the expansion region. A helicon plasma device has been designed, fabricated, and installed in the Institute for Plasma Research, India to study the role of maximum magnetic field gradient as well as its location with respect to the geometrical expansion region of the chamber in CFDL formation. The special feature of this machine consisting of two chambers of different radii is its capability of producing different magnetic field gradients near the physical boundary between the two chambers either by changing current in one particular coil in the direction opposite to that in other coils and/or by varying the position of this particular coil. Although, the machine is primarily designed for CFDL experiments, it is also capable of carrying out many basic plasma physics experiments such as wave propagation, wave coupling, and plasma instabilities in a varying magnetic field topology. In this paper, we will present the details of the machine construction, its specialties, and some preliminary results about the production and characterization of helicon plasma in this machine.

42-GHz 0.5-MW ECRH System for Tokamaks SST-1 and Aditya

A 42-GHz electron cyclotron resonance heating (ECRH) system will be used to carry out preionization and start-up experiments on tokamaks SST-1 and Aditya. The system would give reliable start-up in the SST-1 tokamak at 1.5-T operating toroidal magnetic field. The fundamental O-mode will be launched from the low-field side of the tokamak. The same system will be used in tokamak Aditya to carry out second-harmonic ECRH-assisted breakdown experiments at 0.75-T operation. The gyrotron capable of delivering 500-kW power will be installed such that it will deliver power to both the tokamaks without dismantling any component. An approximately 50-m-long transmission line will be used to transmit power from the gyrotron to each tokamak. The total transmission loss in the line is less than 20%; in this case, we can launch 400-kW power to carry out reliable ECRH-assisted breakdown experiments at the fundamental and second harmonics. The launcher design is different for both the tokamaks. In Aditya, due to space restriction, a simple waveguide-type launcher is used to launch ECRH power in X-mode at the second harmonic. In the SST-1 tokamak, there are two options to launch ECRH power: 1) from the radial port and 2) from the top port. From the radial port, a conventional ECRH launcher consisting of two mirrors (one focusing and one plane) would be used; however, from the top port, one mirror would be used along with a corrugated waveguide. The VME-based data acquisition and control system will be used for the 42-GHz ECRH system. The slow interlocks would be activated through software while the fast interlocks would be hardwired to remove the high voltage within 10 . This paper discusses the physics and technical aspect of the 42-GHz ECRH system and preliminary design of launchers for SST-1 and Aditya.

Observation of low magnetic field density peaks in helicon plasma

Single density peak has been commonly observed in low magnetic field (<100 G) helicon discharges. In this paper, we report the observations of multiple density peaks in low magnetic field (<100 G) helicon discharges produced in the linear helicon plasma device [Barada et al., Rev. Sci. Instrum. 83, 063501 (2012)]. Experiments are carried out using argon gas with m = +1 right helical antenna operating at 13.56 MHz by varying the magnetic field from 0 G to 100 G. The plasma density varies with varying the magnetic field at constant input power and gas pressure and reaches to its peak value at a magnetic field value of ∼25 G. Another peak of smaller magnitude in density has been observed near 50 G. Measurement of amplitude and phase of the axial component of the wave using magnetic probes for two magnetic field values corresponding to the observed density peaks indicated the existence of radial modes. Measured parallel wave number together with the estimated perpendicular wave number suggests oblique mode pr...

Experimental observation of left polarized wave absorption near electron cyclotron resonance frequency in helicon antenna produced plasma

Asymmetry in density peaks on either side of an m = +1 half helical antenna is observed both in terms of peak position and its magnitude with respect to magnetic field variation in a linear helicon plasma device [Barada et al., Rev. Sci. Instrum. 83, 063501 (2012)]. The plasma is produced by powering the m = +1 half helical antenna with a 2.5 kW, 13.56 MHz radio frequency source. During low magnetic field (B < 100 G) operation, plasma density peaks are observed at critical magnetic fields on either side of the antenna. However, the density peaks occurred at different critical magnetic fields on both sides of antenna. Depending upon the direction of the magnetic field, in the m = +1 propagation side, the main density peak has been observed around 30 G of magnetic field. On this side, the density peak around 5 G corresponding to electron cyclotron resonance (ECR) is not very pronounced, whereas in the m = −1 propagation side, very pronounced ECR peak has been observed around 5 G. Another prominent density p...

42GHz 0.5MW ECRH system for Tokamaks SST-1 and Aditya

A 42GHz ECRH system would be used to carry out pre-ionization and start-up experiments on Tokamaks SST-1 and Aditya. The system would give reliable start-up in SST-1 Tokamak at 1.5T operating toroidal magnetic field. Fundamental O-mode would be launched from low field side of tokamak. The same system would also be used in Tokamak Aditya to carry out second harmonic ECRH assisted breakdown experiments at 0.75T operation. The Gyrotron capable to deliver 500kW power would be installed such that it will deliver power to both the tokamaks without dismantling any component. It will be achieved by using two waveguide switches in the transmission line. First switch will divert power either to dummy load for Gyrotron testing or to launch power in the tokamaks. The second switch will give the option to transmit power either to tokamak Aditya or SST1. Approximately 50-meter long transmission line will be used to transmit power from Gyrotron to each tokamak. The transmission line consists of a matching optic unit, DC break, mitre-bends, polarizer, 63.5mm ID circular corrugated waveguide and bellows. The total transmission loss in the line is less than 20%, in this case we can launch ∼ 400kW power to carry out reliable ECRH assisted breakdown experiments at fundamental and second harmonic.

Confinement time of electron plasma approaching magnetic pumping transport limit in small aspect ratio C-shaped torus

A long confinement time of electron plasma, approaching magnetic pumping transport limit, has been observed in SMARTEX-C (a small aspect ratio partial torus with Ro/a∼1.59). Investigations of the growth rate reveal that they are governed by instabilities like resistive wall destabilization, ion driven instabilities, and electron-neutral collisions. Successful confinement of electron plasmas exceeding >1×105 poloidal E→×B→ rotations lasting for nearly 2.1±0.1 s is achieved by suppressing these instabilities. The confinement time has been estimated in two ways: (a) from the frequency scaling of the linear diocotron mode launched from sections of the wall that are also used as capacitive probes and (b) by dumping the plasma onto a charge collector at different hold times.

Study of density peaking in a diverging magnetic field helicon experiment

Density peaking phenomena have been studied in different magnetic field configurations in a low field (< 100 Gauss) helicon discharge. The study has been carried out in the linear helicon plasma device (Barada et. al., Rev. Sci. Instrum. 83, 063501, 2012) using argon gas with m = +1 right helical antenna operating at 13.56 MHz by varying the magnetic field from 0 Gauss to 100 Gauss (G) with two different magnetic field geometry. The plasma density varies with varying the magnetic field at constant input power and gas pressure and reaches to its peak value at a critical magnetic field value (s). For a magnetic field of 88 G near the antenna the density rises at an axial location away from the antenna in the diffusion chamber having a diverging magnetic field. On the m=−1 propagation side of the antenna, the density peak vanishes around 30 G which is well evident on the m=+1 propagation side. The results are explained on the basis of resonance cone propagation of right circularly polarized helicon waves and...

Investigation of diocotron modes in toroidally trapped electron plasmas using non-destructive method

Experiments with trapped electron plasmas in a SMall Aspect Ratio Toroidal device (SMARTEX-C) have demonstrated a flute-like mode represented by oscillations on capacitive (wall) probes. Although analogous to diocotron mode observed in linear electron traps, the mode evolution in toroids can have interesting consequences due to the presence of in-homogeneous magnetic field. In SMARTEX-C, the probe signals are observed to undergo transition from small, near-sinusoidal oscillations to large amplitude, non-linear “double-peaked” oscillations. To interpret the wall probe signal and bring forth the dynamics, an expression for the induced current on the probe for an oscillating charge is derived, utilizing Greens Reciprocation Theorem. Equilibrium position, poloidal velocity of the charge cloud, and charge content of the cloud, required to compute the induced current, are estimated from the experiments. Signal through capacitive probes is thereby computed numerically for possible charge cloud trajectories. In ...