Gourab Bansal

Design Progress on the High-Temperature Superconducting Coil Option for the Heliotron-Type Fusion Energy Reactor FFHR

Abstract Feasibility studies on applying high-temperature superconductors (HTS) to the heliotron-type fusion energy reactor FFHR are being carried out. Using HTS, we consider that the three-dimensional helical coils with a ~40 m diameter can be constructed without preparing a huge winding machine. A practical method for realizing this concept is proposed. The electromagnetic stress inside the helical coil packs is examined using an FEM analysis for double-pancake windings. The effect of error magnetic field generated by the shielding currents in HTS tapes is also examined.

Online tuning of impedance matching circuit for long pulse inductively coupled plasma source operation—An alternate approach

Impedance matching circuit between radio frequency (RF) generator and the plasma load, placed between them, determines the RF power transfer from RF generator to the plasma load. The impedance of plasma load depends on the plasma parameters through skin depth and plasma conductivity or resistivity. Therefore, for long pulse operation of inductively coupled plasmas, particularly for high power (∼100 kW or more) where plasma load condition may vary due to different reasons (e.g., pressure, power, and thermal), online tuning of impedance matching circuit is necessary through feedback. In fusion grade ion source operation, such online methodology through feedback is not present but offline remote tuning by adjusting the matching circuit capacitors and tuning the driving frequency of the RF generator between the ion source operation pulses is envisaged. The present model is an approach for remote impedance tuning methodology for long pulse operation and corresponding online impedance matching algorithm based on RF coil antenna current measurement or coil antenna calorimetric measurement may be useful in this regard.

Experimental Results of Large-Current Capacity HTS Conductors

Large-current capacity high-temperature superconducting (HTS) conductors suitable for high-field large-scale DC magnets, such as of helical-type fusion reactor FFHR, are being designed by employing YBCO tapes. As the first step of R&D of such an HTS conductor, we have fabricated short conductor samples using Bi-2223/Ag tapes. The HTS conductor was made by simply stacking and soldering Bi-2223/Ag tapes inside a copper sheath. The HTS conductor sample was fabricated in a hair-pin configuration and was tested under a bias magnetic field of 8 T. The conductor was thermally insulated by epoxy and GFRP and was conduction-cooled by liquid helium from the ends. Resistive heaters attached to the surface of the GFRP-insulated conductor were used to increase the temperature of the conductor for testing at elevated temperatures up to 30 K. The critical currents of the HTS conductor at 4.2 K, 10 K, 20 K, and 30 K were measured in a bias magnetic field of 8 T. The stability margin of the HTS conductor was examined in conduction-cooled conditions at different temperatures. The conductor was found to be very stable even at high currents closer to the critical currents.

Cryogenic Stability of LTS/HTS Hybrid Conductors

Hybrid-type superconductors are proposed by utilizing a bundle of high-temperature superconducting (HTS) tapes as a stabilizer of low-temperature superconducting (LTS) cables in order to extend the basic research on the cryogenic stability of solid composite-type superconductors and to explore its potential. Since the effective resistivity of HTS is significantly lower than that of pure metals of equivalent cross-sectional area, a bundle of HTS tapes may work as a good stabilizer to achieve high current density. Short sample experiments have been carried out by modifying the aluminum-stabilized superconductor used for the LHD helical coils and the cryogenic stability was examined.

Experimental Investigation of the Minimum Propagation Currents and Quench Characteristics of LTS/HTS Hybrid Conductors

An increase in minimum propagation currents of low temperature superconducting (LTS) NbTi/Cu conductor has been observed when high temperature superconducting (HTS) tapes are used as stabilizers. This kind of superconductor is being called an LTS/HTS hybrid conductor. The objective of the LTS/HTS conductor concept is to extend the basic research of cryogenic stability of LTS conductors such as stabilized with pure aluminum. Short samples of a NbTi/Cu conductor, a NbTi/Cu/Al conductor, and the NbTi/Cu/HTS conductors have been prepared and tested successfully, and the differences in the cryogenic stability were investigated.

Experiments of Bending Strain on Reduced-Scale HTS Conductors for Fusion Energy Reactors

High-temperature superconductors (HTS) are considered as a competitive candidate to be used for the heliotron-type fusion energy reactor FFHR. In order to confirm the bending strain behavior of large-scale HTS conductors, degradation of critical currents by bending strain has been examined in liquid nitrogen (at 77 K) for several samples. The experiment started with two types of reduced-scale conductors with four YBCO tapes, one with copper and YBCO tapes distributed uniformly (Type-A) and the other with YBCO concentrated at the center (Type-B). In order to reconstruct the obtained results, another series of experiment were done by bending a single YBCO tape in two forms: soldered on to a single copper tape and embedded in a copper jacket. We found that even the bending strain of 0.5% defined for the whole conductor thickness should not be a problem from the viewpoint of critical current degradation if we adopt Type-B design with HTS tapes located at the center of the conductor.

Two RF Driver-Based Negative Ion Source Experiment

Multidriver-based radio-frequency (RF) ion sources have emerged as the most promising option for producing the plasma in large ion sources required for fusion applications. In India, under domestic long-term fusion program, a negative ion source development program has been initiated. A 100-kW inductively coupled single RF driver-based negative ion source (ROBIN) has been installed and it is operational at the Institute for Plasma Research. The next step is to move to a multidriver source and address to several issues related to configuration and operation. A two-driver-based twin source has been configured for this purpose. The experimental plan is divided into two phases: 1) plasma operation phase and beam extraction and 2) acceleration phase. A vacuum chamber has been specially designed to accommodate these phases of operations. The data acquisition and control system (DACS) of the experimental system is designed based on International Thermonuclear Experimental Reactor-COntrol Data Access and Communication (ITER-CODAC) guidelines. Probe and spectroscopy diagnostics constitute the principal diagnostics. This paper presents the experimental system with emphasis on the aspects of mechanical, electrical, and DACS integration.

Power supply system for negative ion source at IPR

The first step in the Indian program on negative ion beams is the setting up of Negative ion Experimental Assembly – RF based, where 100 kW of RF power shall be coupled to a plasma source producing plasma of density ~5 × 1012 cm-3, from which ~ 10 A of negative ion beam shall be produced and accelerated to 35 kV, through an electrostatic ion accelerator. The experimental system is modelled similar to the RF based negative ion source, BATMAN presently operating at IPP, Garching, Germany. The mechanical system for Negative Ion Source Assembly is close to the IPP source, remaining systems are designed and procured principally from indigenous sources, keeping the IPP configuration as a base line. High voltage (HV) and low voltage (LV) power supplies are two key constituents of the experimental setup. The HV power supplies for extraction and acceleration are rated for high voltage (~15 to 35kV), and high current (~ 15 to 35A). Other attributes are, fast rate of voltage rise (< 5ms), good regulation (< ±1%), low ripple (< ±2%), isolation (~50kV), low energy content (< 10J) and fast cut-off (< 100μs). The low voltage (LV) supplies required for biasing and providing heating power to the Cesium oven and the plasma grids; have attributes of low ripple, high stability, fast and precise regulation, programmability and remote operation. These power supplies are also equipped with over-voltage, over-current and current limit (CC Mode) protections. Fault diagnostics, to distinguish abnormal rise in currents (breakdown faults) with over-currents is enabled using fast response breakdown and over-current protection scheme. To restrict the fault energy deposited on the ion source, specially designed snubbers are implemented in each (extraction and acceleration) high voltage path to swap the surge energy. Moreover, the monitoring status and control signals from these power supplies are required to be electrically (~ 50kV) isolated from the system. The paper shall present the design basis, topology selection, manufacturing, testing, commissioning, integration and control strategy of these HVPS. A complete power interconnection scheme, which includes all protective devices and measuring devices, low & high voltage power supplies, monitoring and control signals etc. shall also be discussed. The paper also discusses the protocols involved in grounding and shielding, particularly in operating the system in RF environment.

Improvements of Current Decay Behavior of HTS Coils in Persistent Current Operations

In order to improve the current decay behavior of high-temperature superconducting (HTS) coils using silver-sheathed Bi-2223 tapes in persistent current operations, the effect of the shielding current on the current decay behavior has been studied for magnetically levitated superconducting coils in internal ring devices. Generally, the current decay behavior of HTS coils is evaluated by the inductance of the HTS coils, the joint resistance and electric field versus current density relationship of the HTS tapes. In this paper, the effect of shielding currents in HTS tapes is discussed. It is shown that improvements of their properties of HTS tapes, which are the n-value and the critical current, are not so effective to decrease the shielding current in HTS tapes. Numerical electromagnetic analysis using finite element method and experiments have been carried out. Furthermore, improvements of the current decay behavior using excitation techniques are discussed.

Indian Test Facility (INTF) - a status update

The Indian Test Facility (INTF) for Neutral Beams is designed with the objective to not only test the performance of an ion source but also to characterize the transmission of the Neutral Beam to a length of 20.66 m and thereby generate adequate database on the quality of the beam that is required for the CXRS diagnostics for measurement of Helium ash content in the fusion plasma.