Soumen Kar

Electrothermal Behavior of the Joint of Binary Current Lead of Conduction-Cooled Magnet

A 6-T cryogen-free NbTi magnet system with a warm bore has been developed using a two-stage cryocooler (CCR). The binary current lead is one of the main components of any conduction-cooled magnet system. The heat flow scheme through the binary current leads greatly determines the operating points and the thermal stability of the magnet system. A pair of binary current leads has been developed for a 102-A current to achieve a 6-T field. The thermal interception of a binary current lead along with their electrical isolation to the corresponding cooling stages of the CCR greatly influences the electrothermal behavior of the lead, which eventually influences the stability of the magnet. Low-electrical-resistance and low-thermal-resistance joints have been developed for the binary current leads. The equilibrium temperatures are 37.75 and 4.55 K, respectively, for the first- and second-stage lead joints at zero current. In addition, the corresponding equilibrium temperatures of lead joints are 42.04 and 6.55 K at 102-A current. The average thermal impedance of the corresponding joints is measured to be 0.6 and 13.7 K/W.

Experimental studies on thermal behavior of 6 Tesla cryogen-free superconducting magnet system

A 6 T cryogen-free superconducting magnet system (CFMS) with warm bore has been designed, built and tested for carrying out physical property measurements at high magnetic field. The CFMS has a vertical Ψ 50 mm clear room temperature working bore. A two-stage GM-cryocooler of 35 W at 50 K and 1.5 W at 4 K cooling capacity has been used to cool the thermal radiation shield, hybrid current leads and the NbTi magnet. The magnet was charged to 102 A, which produced a field of 6 T, in less than 20 minutes. The outer surface temperature of magnet rose from 3.3 K at zero current to 3.9 K at 102 A current. The thermal link between cold head and magnet greatly influences the excitation rate and the stability of the magnet. Based upon our thermal analysis on the magnet system, an experimental study has been carried out on the thermal behavior of the magnet during its operation. A novel technique has been used for connecting the high temperature superconducting current leads to the magnet so as to reduce both electrical and thermal contact resistance at the 2nd stage of the cryocooler. This paper briefly describes the design, development and results of experimental studies of thermal behavior of the system.

Current distribution mapping in insulated (Gd,Y)BCO based stabilizer-free coated conductor after AC over-current test for R-SFCL application

Uniformity of critical current (Ic ) over long lengths of (GdY)-Ba-Cu-O ((Gd,Y)BCO)-based high temperature superconducting (HTS) tapes after long periods of AC current excitation is an important criterion in their selection for resistive type superconducting fault current limiter (R-SFCL). The present work describes such critical current (Ic ) uniformity measurements performed over 1m long, stabilizer-free (SF), 12 mm wide, 2nd generation (2G) (Gd,Y)BCO based HTS tape. A non-destructive method using a static hall probe (Tapestar®) with moving HTS tape configuration was employed for estimation of Ic uniformity. Scanning Hall probe microscopy (SHPM) was then used to examine the weak superconducting regions (i.e. less Ic ) with a static HTS tape. Remanent field distribution on the HTS tape was measured to yield the critical current density distribution. Except for small degradation of Ic at some locations, these studies confirmed near-uniform critical current distribution over meter-long (Gd,Y)BCO tapes, both in virgin state and after exposure to AC over current.

Conceptual Design of a 440 V/800 A Resistive-Type Superconducting Fault Current Limiter Based on High

A conceptual design of a high-Tc-based resistive-type superconducting fault current limiter (SFCL) using second-generation coated conductors (CCs) has been carried out in this paper for possible power applications in India. In the design aspect, noninductive SFCL coils made with stabilizer-free CCs have been used for analytical calculations. In electrothermal analysis, a short-circuit characterization has been simulated to get the maximum resistance and temperature achieved during fault limitation in a 440 V/800 A single-phase SFCL. Furthermore, the recovery under no load is studied, and recovery time is calculated.

T_{c}

Current leads are the key components which make an electrical connection between room temperature power supply and superconducting magnets at 4.2 K. In any cryogenfree magnet system, major heat input comes from the copper current leads of the magnet. The dimensional optimization of the copper part of the hybrid current leads is done for a particular operating current between room temperature to the operating temperature of the 1stst stage of cryocooler (CCR). The conduction-cooled magnet may not necessarily be operated at the optimized current all the time. Hence for non-optimum current, the heat input per unit current will be increased both at under-current and over-current operation. The stabilized temperature, of the 1st stage of CCR corresponding to each operating current, would mainly be governed by the dynamic heat input by the current lead. A pair of copper current leads working between the temperatures 300 K to 33 K has been optimized for 85 A optimum current. A simple analytical technique has bee...

Coated Conductors

The present study investigates the effects of high voltage impulses and low frequency high current pulses on Ni powder compacts and on alloys like Ni₈₀Si₂₀ or Ni₉₅Si₅. An increase in density has been observed due to pulse application on samples of ball milled Ni powder and Ni₈₀Si₂₀. It is suggested that pulse application along with simultaneous pressure application be employed in field assisted sintering techniques like SPS and EDC. These alternative novel methods are proving better than conventional sintering methods like hot isostatic pressing and cold isostatic methods.

Peformance test and thermal analysis of conduction-cooled optimized current leads at non-optimum operation

Two stage GM cryocooler is one of the major components for development of any conduction cooled superconducting magnet system. Recently, IUAC New Delhi has developed a 6T conduction cooled NbTi magnet system using a two stage GM cryocooler (Sumitomo SRDK-415D) of refrigeration capacity of 1.5W at 4.2K and 40W at 50K. The commercially available typical load map of the SRDK-415D cryocooler shows few ‘discrete’ points. The commercial load map most of the time is not sufficient to analyse the thermal characteristics of the NbTi magnet system. This paper briefly discusses the full scaled practical load map for the SRDK-415D cryocooler generated using a test rig. This paper also gives the detail of the continuous load curves for both the stages of the cryocooler. Details of the test setup, measurement methodology to generate the practical load curves, parameters responsible for temperature fluctuation, have been described.

Investigations on pulsed power application for nano-material consolidation

IUAC has recently commissioned a superconducting quadrupole doublet magnet cryostat with zero boil off technology using 2 number of GM cryocooler each having refrigeration capacity 1.5 watt @ 4.2 K. Both the magnets are designed to be operated up to 84 Ampere for the required magnetic field gradient. To reduce the heat load to the 2nd stage of the cryocooler 4 number of high temperature superconductor (HTS) leads have been used to power the magnet in the cryostat. Each HTS lead requires two thermal stations cum electrical isolator for its efficient functioning. In the first design Aluminum Nitride (AlN) was used as electrical isolator cum thermal conductor. In the recently conducted cold test of the magnet, we have observed a temperature rise in the 2nd stage joint during current charging (From 4.4 K to 6.7 K). The quantum of temperature rise is not acceptable for the stable operation. To minimize the temperature rise in the 2nd stage an experimental study was conducted to select a design having better thermal conductivity. In the new design the thermal conductivity ratio improved by four times compared to the 1st design and when implemented the base temperature reduced from 4.4 K to 3.4 K. This paper will highlight the effort made to realize a better electrical isolator cum thermal conductor at low temperature & its implementation in the existing cryostat.

Experimental load map of two stage 1.5 W at 4.2 K (SRDK-415D) GM cryocooler

Hybrid current lead is one of the important parts for the development of cryogen free superconducting magnet system. The thermal stability of the NbTi magnet is greatly dependent on the performance of the hybrid current lead. Hybrid current lead, which is a combination of the metallic (copper) lead and HTS lead, has some inter-lead joints which need to be electrically insulating and thermally conducting with the different stages of the cryocooler. A thermal anchoring block of oxygen-free high-conductivity (OFHC) copper is developed for inter-lead thermal joint. A detailed experimental study is carried out using Kapton and Aluminium nitride (AlN) as interlayer insulation material in the thermal anchoring block. The thermal resistance of the interface for 19 mm2 contact surface area, is found to be ˜7 K/W and ˜15 K/W for Kapton and Aluminium nitride (AlN) respectively at 4.2 K temperature range and ˜0.6 K/W (nearly similar) for both Kapton and Aluminium nitride (AlN) at 40 K temperature range. Dependency of the thermal impedance on the contact surface area is also discussed in details.

Studies on an electrical isolator cum thermal conductor for a superconducting quadrapole magnet at IUAC

Design and development of a rectangular current pulse generator is presented. The generator produces 15 ms wide, 100 A amplitude unipolar pulses into an 8 Omega load. Pulses with duration of 10-20 ms are essential for the development of compacts of the nanomaterial samples while avoiding grain growth on the samples dominant at long pulses. The pulses are produced by discharging the charged artificial transmission line into the load impedance equal to the surge impedance of the transmission line. These pulses obtained are applied for sintering of samples like aluminum and graphite.