B. K. Shukla

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.

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.

42-GHz/500-kW Electron Cyclotron Resonance Heating System on Tokamak SST-1

The 42-GHz electron cyclotron resonance heating (ECRH) system on SST-1 is used to carry out ECRH-assisted preionization, breakdown, start-up, and heating experiments at 0.75-T (second harmonic) and 1.5-T (fundamental harmonic) operation. The gyrotron delivers 500-kW power at 42-GHz frequency at -50-kV beam voltage, 20-A beam current, and 19-kV anode voltage. The gyrotron has been commissioned successfully on dummy load for full parameters (500 kW/500 ms). The transmission line consists of matching optic unit, circular corrugated waveguide, miter bend with bidirectional coupler, waveguide switches, polarizer, bellows, dc breaks, and an uptaper. Approximately 20-m-long transmission line is used to launch the power from gyrotron to tokamak, and the burn pattern at the exit of line near the tokamak ensures a good Gaussian beam. A composite launcher [consisting of four mirrors (two profiled and two plane), two gate valves, and two vacuum barrier windows] is used to connect two ECRH systems (42 and 82.6 GHz). The 82.6-GHz/200-kW ECRH system is also planned for SST-1 to carry out experiments at 3-T magnetic field. The 42-GHz ECRH system has been commissioned with tokamak SST-1, ECRH power has been launched in tokamak, and successful ECRH-assisted breakdown is achieved at second harmonic.

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.

Data Acquisition and Control System for ECRH Systems on SST-1

In steady-state superconducting tokamak-1, two electron cyclotron resonance heating systems have been installed to carry out breakdown and heating experiments. The 82.6-GHz gyrotron delivers 200-kW continuous wave power (1000 s), while the 42-GHz gyrotron delivers 500-kW power for a 500-ms duration. The gyrotron system consists of various auxiliary power supplies, the crowbar unit, and the water cooling system. The Versa Module Europe-based data acquisition and control system has been designed, developed, and implemented for safe and reliable operation of the gyrotron. The control and the monitoring softwares have been developed using the VxWorks real-time operating system and open source softwares like Tcl-Tk tool kit, MDSplus, and the GNU C compiler on Linux.

VME based data acquisition and control system for Gyrotron based ECRH system on SST-1

A Gyrotron capable of delivering 200kW power at 82.6 GHz has been installed for pre-ionization and heating experiments on SST1 tokamak. Gyrotron based Electron Cyclotron Resonance Heating (ECRH) system involves systematic operation of different auxiliary power supplies, crowbar unit and water cooling system. Versa Module Europe (VME) based Data Acquisition and Control (DAC) system has been designed and implemented for the gyrotron operation. Softwares have been developed using VxWorks RTOS and open source Tcl-Tk tool-kit, GNU C/C++ compiler on Linux.

ECRH assisted plasma experiments on tokamaks SST-1 and Aditya

The 42GHz Electron Cyclotron resonance Heating (ECRH) system is used in tokamaks Aditya and SST-1 to carry out ECRH related experiments at fundamental and second harmonic. The Gyrotron capable to deliver 500kW power for 500ms duration is commissioned in SST-1 tokamak hall and ~75m long transmission line is laid to connect both the tokamaks. The corrugated waveguide (φ63.5mm) based transmission line at normal atmospheric pressure is used to transmit power in HE11 mode. The special feature of this line is that it consists of two waveguide switches, which facilitates to transmit the power in either tokamak without changing any hardware. The total loss in the line is less than 1.1dB. In SST-1, the 42GHz ECRH system is used for tokamak start-up at fundamental (1.5T) and second harmonic (0.75T). Since the loop voltage of SST-1 is low (~3.5V), the ECRH assisted start-up is mandatory for reliable plasma discharges. Since the first pass absorption of ECRH is not 100% and in this case the reflected beam is used to drive the plasma current from high field side. In order to achieve this, a profiled reflector is installed on the inboard side wall of the tokamak. This reflector is adjusted such that ECRH beam is reflected to ~17°, which helps in driving the ECCD. The ECH power (~180kW to 300kW) is launched and pulse duration is varied from 100ms to 400ms. The systematic experiments are carried out to monitor the effect of ECCD in SST-1 plasma at fundamental and second harmonic. It is observed that IECCD contribution is ~ 5 to 10kA, which is a significant contribution as it supports the plasma current (IP) and long pulse shot with higher plasma current are achieved with longer ECRH pulse. In Aditya, the second harmonic ECRH assisted low loopvoltage plasma startup experiments have been carried out systematically. The ECRH power in extraordinary (X2) mode is launched from low field side of tokamak. The ECH power is launched in tokamak around 25ms before the loop_voltage and pulse duration is varied from 75ms to 125ms. The normal operating loop voltage of Aditya is ~22V and with ECRH assisted breakdown, the normal discharges (Ip~100kA, plasma duration ~150ms) are achieved at loop voltage as low as 7V, which shows significant (~70%) reduction in the loop_voltage.

Commissioning of 42GHz/500kW ECRH system on tokamak SST-1

The 42GHz ECRH system on SST-1 will be used for ECRH assisted pre-ionization, startup and heating experiments at 0.75T (second harmonic) and 1.5T (fundamental harmonic) operation. The Gyrotron delivers 500kW power at 42GHz frequency at -50kV beam voltage, 20A beam current and +19kV anode voltage. The Gyrotron has been tested successfully on dummy load for full parameters. The good gaussian burn pattern at the exit of matching optic unit ensures required mode purity. The circular corrugated waveguide based transmission line consists of matching optic unit, mitre-bend with bi-directional coupler, waveguide switches, polarizer, bellows, DC breaks and an uptaper at the exit of line. Approximately 20 meter long transmission line is used to launch the power from Gyrotron to tokamak. The transmission line has been commissioned on tokamakSST1 using a switch which gives flexibility to operate the Gyrotron either on dummy load or on tokamak. The high power test of transmission line is done and burn pattern at the exit of transmission line near SST1 tokamak ensures good Gaussian beam. A composite launcher consisting of four mirrors (two profiled and two plane), two gate valves and two vacuum barrier windows is used for two ECRH systems (42GHz and 82.6GHz). The distance between plasma centre and plane mirror is 900mm. The mirror size for 42GHz ECRH systems are 170mm × 240mm, the focal length of mirror for 42GHz system is 353mm. The mirrors size for 82.6GHz launcher is 140mm × 200mm and focal length for mirror is 481mm. The beam size at plasma centre for 42GHz launcher is 35mm (1/e beam radius) and for 82.6GHz it is 20mm. The complete launcher system with low power testing has been commissioned on tokamak SST-1.

High-Power Test of Chemical Vapor Deposited Diamond Window for an ECRH System in SST-1

High-power tests on chemical vapor deposited (CVD) diamond window are carried out using 82.6-GHz Gyrotron. To test CVD diamond window with calorimetric dummy load, a matching optic unit is designed and fabricated with special profiled mirrors. The mirrors are fabricated by Gycom Russia and a mirror box assembly is fabricated at Institute for Plasma Research. The mirrors and mirror box are cooled with water. The mirror box consists of four arc detectors for the protection of window. The CVD diamond window is connected to a dummy load with this mirror box. The alignment of mirror is done during the testing of CVD window in pulsed condition. After achieving the desired Gaussian pattern at the exit of mirror box, system is connected to a dummy load for continuous wave test. The CVD window is tested maximum power up to 60 kW in pulse duration of 600 s.