The steadystate superconducting tokamak (SST1) is aimed to demonstrate long pulse plasma discharges employing non-inductive current drive by means of lower hybrid current drive (LHCD) system. The major and minor radius of the machine is 1.1 m and 0.2 m, respectively. The LHCD system for SST1 comprises of klystrons, each rated for 0.5 MW-CW rf power at a frequency of 3.7 GHz. The grill antenna comprises of two rows, each row accommodating 32 waveguide elements. Electron cyclotron resonance breakdown assisted Ohmic plasma is formed in SST1 to overcome the issues associated with low loop voltage start-ups. With recent modifications in the poloidal coils configuration, even with narrow EC pulse (∼50 ms), good repeatable and consistent Ohmic plasmas could be produced which helped in carrying out LHCD current drive experiments on SST1. These experiments demonstrated both fully as well as partially driven non-inductive plasma current in SST1 tokamak. Discharges with zero loop voltages were obtained. The interaction of lower hybrid waves with plasma and generation of suprathermal electrons could be established using energy spectra measured by CdTe detectors. Various other signatures like drop in loop voltages, negative loop voltages, spikes in hard x-rays and increase in second harmonic ECE signal, further confirmed the current drive by LHW’s. The beneficial effect of LHW’s in suppressing hard x-rays was also demonstrated in these experiments. The longest discharge of ∼650 ms could be obtained in SST1 with the help of LHW’s. In this paper, the experimental results obtained with LHCD experiments on SST1 is reported and discussed in more details.

A new passive active multijunction antenna (PAM) has been designed and fabricated for ADITYA-U tokamak [1]. The PAM antenna has the ability to couple lower hybrid waves (LHW’s) into the plasmas near cut-off densities. The coupling of LHW’s depends on plasma density and its profile near the mouth of the antenna. To determine these plasma parameters, experimentally, an X-mode reflectometry system has been designed. The reflectometery system is designed to operate in the frequency range from 26 GHz to 36 GHz and would cover a density range from ~1x10 m (i.e. Scrap Of Layer (SOL) density ) to 5x10 m with a toroidal magnetic field of 1.5 Tesla at major radius (R0) of the tokamak. The total frequency band is swept in 100 microsecond, as a trade-off between technological difficulties and to avoid fast density fluctuation for better phase tracking. The ADITYA reflectometer is built to operate in frequency modulation continuous mode (FW-CM) or at a fix frequency mode for density fluctuation study. The reflectometery consists of two parts, i.e., the transmitter and the receiver. The transmitter mainly consists of microwave source, amplifier, a single sideband modulator (SSBM), frequency multiplier and a horn antenna to launch X-mode into the plasma. Similarly the receiver end consists of horn antenna, amplifier, mixer and de-modulator. In the de-modulator section, a quadratic demodulation (IQ) is used to extract in-phase and quadraturephase information from the reflected signal. Finally, an ADC with 12 bit resolution will convert the analog signal in to a digital signal which will be processed through a FPGA based data acquisition system. Two sectorial E plane horn antennas are chosen so that one may be used for launching microwave power into the plasma and the other for receiving the reflected signal from the plasma. The limited space available near the radial port which houses both, the PAM antenna and the horn antenna, accounts for the choice of sectorial E-plane horn antenna. A microwave broad band window is designed to interface the horn antennas and the microwave hardware as the antennas will be placed inside vacuum vessel of ADITYA-U tokamak.

The successful operation of superconducting tokamaks is very challenging because of limitations arising out of a complex geometry, configuration and construction. The slow penetration of loop voltage because a continuous plasma chamber requires low loop voltage plasma initiation. Apart from these operational challenges, SST1 also has technical issues related to cooling down of the superconducting poloidal field (PF) coils, for obtaining advanced plasma operation and control. Furthermore, limits are imposed on the loop voltage by magnet power supplies and PF coil insulation. To improve our understanding of these issues, operations in SST-1 have been attempted with low loop voltages and will be discussed in this paper. It is envisaged that designing and carrying out experiments in SST1, to understand above issues, may be very complex, time consuming and may not be feasible due to complexities in SST1 machine operational scenario. Hence Aditya machine upgradation was initiated with the understanding that solution to some of these problems may be obtained through experiments specially designed for this purpose in ADITYA-U, in parallel. Thus some of these experiments are also being planned to carry out in tokamak ADITYA-U, in particular, non-inductive current drive using lower hybrid waves, employing passive active multi-junction (PAM) antenna. The performance of the lower hybrid waves launched by the grill antenna is presented and the need for PAM antenna is presented. The design of the PAM antenna for ADITYA-U machine is also discussed. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research

SST-1 Tokamak has recently completed the 1st phase of up-gradation with successful installation and integration of all its First Wall components. The First Wall of SST-1 comprises of similar to 3800 high heat flux compatible graphite tiles being assembled and installed on 132 CuCrZr heat sink back plates engraved with similar to 4 km of leak tight baking and cooling channels in five major sub groups equipped with similar to 400 sensors and weighing similar to 6000 kg in total in thirteen isolated galvanic and six isolated hydraulic circuits. The phase-1 up-gradation spectrum also includes addition of Supersonic Molecular Beam Injection (SMBI) both on the in-board and out-board side, installation of fast reciprocating probes, adding some edge plasma probe diagnostics in the SOL region, installation and integration of segmented and up-down symmetric radial coils aiding/ controlling plasma rotations, introduction of plasma position feedback and density controls etc. Post phase-I up-gradation spanning from Nov 2014 till June 2016, initial plasma experiments in up-graded SST-1 have begun since Aug 2016 after a brief engineering validation period in SST-1. The first experiments in SST-1 have revealed interesting aspects on the 'eddy currents in the First Wall support structures' influencing the 'magnetic Null evolution dynamics' and the subsequent plasma start-up characteristics after the ECH pre-ionization, the influence of the first walls on the 'field errors' and the resulting locked modes observed, the magnetic index influencing the evolution of the equilibrium of the plasma column, low density supra-thermal electron induced discharges and normal ohmic discharges etc. Presently; repeatable ohmic discharges regimes in SST-1 having plasma currents in excess of 65 KA (q(a) similar to 3.8, B-T = 1.5 T) with a current ramp rates similar to 1.2 MA/s over a duration of similar to 300 ms with line averaged densities similar to 0.8 x 10(19) and temperatures similar to 200 eV with copious MHD signatures have been experimentally established. Further elongation of the plasma duration up to one second or more with position and density feedback as well as coupling of Lower Hybrid waves are currently being persuaded in SST-1 apart from increasing the core plasma parameters with further optimizations and with wall conditioning.

Steady State Superconducting Tokamak (SST-1) is a `working‟ experimental superconducting device since late 2013. Referring to the last IAEA-FEC; SST-1 has been upgraded with Plasma Facing Components (PFC) with an objective towards long pulse operations in both circular and elongated configurations. The PFC integration has been completed in August 2015 and initial experiments have begun in SST-1 with circular plasma configurations. SST-1 offers a unique possibility of investigating long pulse discharges with large aspect ratio (> 5.5) compared to contemporary devices. Presently, SST-1 standard Ohmic discharges are in excess of 100 kA with typical core density ~ 1 × 10 19 m -3 and core electron temperatures ~ 200-300 eV having duration in excess of 300 ms. A 42 GHz ECR pre-ionization source at ~ 150 kW in 1.5 T central toroidal field breaks down the hydrogen gas, the current starts up at ~ 1.3 MA/s in 60-80 ms in an induced electric field of ~ 0.3 V/m. These standard discharges demonstrate copious saw teething and MHD activities as the pulse progresses including NTM, mode locking and MHD characteristics. Studies on eddy currents influencing the NULL dynamics, field errors, equilibrium index evolutions, wall influencing plasma characteristics, plasma positions, plasma rotational andTearing Mode characteristics including the island width and growth etc. have been carried out. Presently, SST-1 is attempting at multi-second long high aspect ratio plasma discharges by coupling the Lower Hybrid with the Ohmic plasma as well as with robust real time position and density controls. SST-1 device has also been upgraded with a pair of internal coil aimed at effective fast plasma control and a pair of segmented coil aimed at controlling some of the rotational aspects of plasma including the RMPs and ELMs. Supersonic Molecular Beam Injection (SMBI) from both high field and low field sides and Pellets Injection Systems have also been added with several edge plasma diagnostics aimed at both density control and edge plasma turbulence studies. This paper elaborates some of these up-gradation and results from the initial experiments.

A steady state superconducting tokamak (SST-1) has been commissioned after the successful experimental and engineering validations of its critical sub-systems. During the 'engineering validation phase' of SST-1; the cryostat was demonstrated to be leak-tight in all operational scenarios, 80 K thermal shields were demonstrated to be uniformly cooled without regions of 'thermal runaway and hot spots', the superconducting toroidal field magnets were demonstrated to be cooled to their nominal operational conditions and charged up to 1.5 T of the field at the major radius. The engineering validations further demonstrated the assembled SST-1 machine shell to be a graded, stress-strain optimized and distributed thermo-mechanical device, apart from the integrated vacuum vessel being validated to be UHV compatible etc. Subsequently, 'field error components' in SST-1 were measured to be acceptable towards plasma discharges. A successful breakdown in SST-1 was obtained in SST-1 in June 2013 assisted with electron cyclotron pre-ionization in the second harmonic mode, thus marking the 'first plasma' in SST-1 and the arrival of SST-1 into the league of contemporary steady state devices.Subsequent to the first plasma, successful repeatable plasma start-ups with E similar to 0.4 V m(-1), and plasma current in excess of 70 kA for 400 ms assisted with electron cyclotron heating pre-ionization at a field of 1.5 T have so far been achieved in SST-1. Lengthening the plasma pulse duration with lower hybrid current drive, confinement and transport in SST-1 plasmas and magnetohydrodynamic activities typical to large aspect ratio SST-1 discharges are presently being investigated in SST-1. In parallel, SST-1 has uniquely demonstrated reliable cryo-stable high field operation of superconducting TF magnets in the two-phase cooling mode, operation of vapour-cooled current leads with cold gas instead of liquid helium and an order less dc joint resistance in superconducting magnet winding packs with high transport currents. In parallel, SST-1 is also continually getting up-graded with first wall integration, superconducting central solenoid installation and over-loaded MgB2-brass based current leads etc. Phase-1 of SST-1 up-gradation is scheduled by the first half of 2015, after which long pulse plasma experiments in both circular and elongated configurations have been planned in SST-1.

A 2.0 MW CW lower hybrid current drive (LHCD) system based on 3.7 GHz klystron sources, is in advanced stage of commissioning, which would drive and sustain plasma current, non-inductively, in superconducting steady state tokamak (SST1) for long pulse operation. Four klystrons, each rated for 0.5 MW CW rf power, delivers 2.0 MW of rf power to four layer of the LHCD system, which finally feeds the rf power to grill antenna. The antenna system along with vacuum window and vacuum transmission line is successfully integrated on the machine. Its vacuum and pressurization compatibility has been successfully established. To validate the high power performance of LHCD system for SST1 machine, stage-wise commissioning of LHCD system in staggered manner is planned. It has been envisaged that LHCD power may be gradually increased initially, since full power may not be required during the initial phases of SST1 plasma operation. Also if the system is integrated in steps or in phases, then integration issues, as well as high power operational issues, if any, can be addressed, attended and handled in a simpler way before integrating all the layers to the grill antenna. To begin with, one klystron is connected to one layer, out of four layers, which energizes a quarter of the grill antenna. Gradually, the rf power and its pulse length is increased to validate high power performance of the system. Arcing and reflections are observed as rf power is gradually increased. The problems are analysed and after taking appropriate remedial action the system performance is improved for operation up to 160kW. Several trains of short pulses are launched in SST1 vacuum vessel for rf conditioning of the LHCD system. Normally, reflections are high when power is launched in vacuum; therefore the pulse length is restricted up to 100 milliseconds. The high power performance of this layer, connected with grill antenna is validated by launching high power microwaves in vacuum vessel of SST1 machine and experimental results obtained during the validation is presented in this paper. Based on this experience, the rest of the layers would be connected to the grill antenna to validate the LHCD system for full rated power.

A regulated high-voltage power supply (RHVPS) and its results are presented for its initial integration and operation with lower hybrid current drive (LHCD) system. Experimental results are shown to show the ability of RHVPS as a practical source of electrical power for LHCD. Klystron is operated at 3.7 GHz. with a continuous wave RF power level of 500 kW with various subsystems. RHVPS has proven its capability for feeding power to klystron with a safe and good level of required quality. Up to 65 kV and a load current in excess of 18 A was imparted to klystron with a trapezoidal pulse of voltage. Repeatability of operation is shown by successive shot results. RHVPS is explained with its scheme and control system. Basic parameters are fulfilled with a dummy load testing of RHVPS. Test results are illustrated for basic parameters with resistive dummy load. LHCD system operation parameters and experimental parameters of RHVPS for its operation are discussed. A regulation on flat top level of voltage is as per the requirement of LHCD system. Initial integration of RHVPS has been successful and future plans are mentioned for full-fledged SST-1 operation.

A 3.7-GHz lower hybrid current drive (LHCD) system has been designed and installed for driving noninductive plasma current for steady-state operation of steady-state super-conducting tokamak-1 machine. Currently, its capability has been enhanced up to 2 MW by adding two additional klystrons, each capable of providing 500 kW, continuous wave (CW) power, to LHCD system. After performing factory acceptance test of these klystrons, they are installed and commissioned at site, at rated power, for more than 1000 s, before connecting them to main LHCD system. The auxiliary systems like supporting power supply system (magnet, filament, ion pump, etc.), active heat management system, protection system, transmission line pressurization system, low power RF drive system, and so on are interconnected with klystron system through versa module europa (VME)-based data acquisition and control system for remote CW operation of klystron at rated power. The klystron is protected by fast interlock protection in an event of arc or an electrical parameters excursion beyond a set window. The slow interlock is invoked in case of cooling failures, pressurization of waveguides, and so on. The klystron has two output arms and a recombiner is used to recombine power coming out from both the arms. The output power from the recombiner of the klystron is split into two arms, employing 3-dB power divider. This reduces the high power requirement of water dummy loads (DLs) to 250-kW CW. A directional coupler is included in both the arms for measuring forward and reflected power. The reflected power signal is fed to low power RF drive control, which removes the RF drive once reflected power beyond a threshold power is detected within 5 μs. The forward power signal is used for monitoring RF power being dumped into the water loads. The calorimetric measurements, employing Pt-100 sensors, are also carried out on water dummy loads. Our measurements suggest that the maximum RF power ( ~500-kW CW) extracted from klystron is dissipated on water cooled DLs. The unspent dc power ( ~800-kW CW) is dissipated in the collector that is heavily cooled with water flowing at ~1300 L/min. The power loss in the klystron body remained within 15 kW. The cavity temperature, measured using J-type thermocouple, remained below 150 °C. The output RF power, sampled through directional couplers and measured by RF detectors shows good agreement with calorimetric measurements. A detailed description of the klystron test setup and the test results obtained during its commissioning is presented in this paper.

A 3.7 GHz, LHCD system aims to drive non inductive plasma current for SST1 machine. Its capability has been enhanced up to 2 MW by including two additional klystrons, each rated for 500kW, CW power. The additional klystrons are installed and commissioned at site, for rated power, for more than 1000 seconds, before connecting them to main LHCD system. The auxiliary systems, like supporting power supply system (magnet, filament, ion pump, etc.), active heat management system, slow and fast interlock system, transmission line pressurization system, low power rf drive system, etc. are inter-connected with klystron system through VME based data acquisition and control system for remote CW operation of klystron at rated power. The calorimetric measurements, employing Pt-100 sensors, suggests that the maximum rf power (~500kW CW) extracted from klystron is dissipated on water cooled dummy loads. The unspent DC power (~800 kW CW) is dissipated in collector which is heavily cooled with water flowing at ~1300 litres/min (lpm). The power loss in the klystron body remained within 20 kW. The cavity temperature, measured using J-type thermocouple, remained below 150°C. The output rf power, sampled through directional couplers and measured by rf detectors shows good agreement with calorimetric measurements. The detailed experimental plan along with results obtained during this campaign is presented in this paper.

To operate superconducting steadystate tokamak (SST1) for 1000 seconds, lower hybrid current drive (LHCD) system has been designed at a frequency of 3.7 GHz., which would couple 1.0 MW CW of microwave power to the shaped plasma. The system consists of various rf passive components and transmission line, employing which the rf power from the source is transported to the antenna. During calibration of transmission line, it was observed that the losses in the transmission line is substantial and eventually would lead to less coupled power to the plasma. Further it is anticipated that more LH power would be required for advanced operation of SST1 machine. Thus it is decided to upgrade the existing LHCD system to 2 MW CW power level. The proposed up gradation would demand several infra structural changes and needs to be addressed. Due to lack of space, we have proposed a scheme in which additional two klystrons, along with existing two klystrons would be accommodated in the existing space. The low rf power requirements have also been increased to cater the new needs. Accordingly additional cooling requirements have been proposed to accommodate the two new klystrons. The DAC and auxiliary power supplies have been also designed. The new up graded LHCD system would address several key technological issues. Firstly it would establish the operation of four klystrons at rated power in parallel employing single RHVPS (80kV, 70A). Secondly it would establish the operation of two high power klystrons operation at rated power when their collectors are cooled in series. In this paper we would present the various requirements for up-gradation of LHCD system to 2MW. The main requirements like high power rf source, along with modified support structure, low power rf systems to drive the high power rf source, auxiliary power supplies required for high power rf source, DAC system improvement, cooling improvements, etc. would be discussed.

A 3.7 GHz., 120 kW (pulsed), lower hybrid current drive (LHCD) system is employed to drive non-inductive plasma current in ADITYA tokamak. The rf power is coupled to the plasma through grill antenna and is placed in vacuum environment. A vacuum break between the pressurized transmission line and the grill antenna is achieved with the help of a multi (eight) channel rectangular RF vacuum window.The phasing between adjacent channels of 8-channel window (arranged in two rows) is important for launching lower hybrid waves and each channel should have independent vacuum window so that phase information is retained. The geometrical parameter of the grill antenna, like periodicity (9mm), channel dimensions (cross sectional dimension of 76mm x 7mm), etc. is to be maintained. These design constraint demanded a development of a multi channel rectangular RF vacuum window. To handle rf losses and thermal effects, high temperature vacuum brazing techniques is desired. Based on the above requirements we have successfully developed a multi channel rectangular rf vacuum window employing high temperature vacuum brazing technique. During the development process we could optimize the chemical processing parameters, brazing process parameters, jigs and fixtures for high temperature brazing and leak testing, etc. Finally the window is tested for low power rf performance using VNA. In this paper we would present the development of the said window in detail along with its mechanical, vacuum and rf performances.

To drive plasma current non-inductively in ADITYA tokamak, 120 kW pulsed Lower Hybrid Current Drive (LHCD) system at 3.7 GHz has been designed, fabricated and installed on ADITYA tokamak. In this system, the antenna consists of a grill structure, having two rows, each row comprising of four sub-waveguides. The coupling of LHCD power to the plasma strongly depends on the plasma density near the mouth of grill antenna. Thus the grill antenna has to be precisely positioned for efficient coupling. The movement of mechanical bellow, which contracts or expands up to 50mm, governs the movement of antenna.In order to monitor the position of the antenna precisely, the reference position of the antenna with respect to the machine/plasma position has to be accurately determined. Further a mechanical system or an electronic system to measure the relative movement of the antenna with respect to the reference position is also desired. Also due to poor accessibility inside the ADITYA machine, it is impossible to measure physically the reference position of the grill antenna with respect to machine wall, taken as reference position and hence an alternative method has to be adopted to establish these measurements reliably.In this paper we report the design and development of a mechanism, using which the antenna position measurements are made. It also describes a unique method employing which the measurements of the reference position of the antenna with respect to the inner edge of the tokamak wall is carried out, which otherwise was impossible due to poor accessibility and physical constraints. The position of the antenna is monitored using an electronic scale, which is developed and installed on the bellow. Once the reference position is derived, the linear potentiometer, attached to the bellow, measures the linear distance using position transmitter. The accuracy of measurement obtained in our setup is within +/- 0.5 % and the linearity, along with repeatability is excellent.