Ziad Melhem

The Development of High Field Magnets Utilizing Bi-2212 Wind & React Insert Coils

Wind & react Bi-2212 inserts have been manufactured and tested inside a wide-bore NbTi - Nb3Sn magnet providing a background field up to 20 T at 4.2 K. A pair of six-layer concentric coils both achieved critical currents of 350 A (JE = 200 A/mm2) in a 20 T background field. A thicker 14-layer insert made from 119 m of round wire had a critical quench current IQ of 287 A (JE = 162 A/mm2) at the same field and contributed to a combined central field of 22.5 T. This is a record for a fully superconducting magnet at 4.2 K. The 14-layer coil, equipped with an external protective shunt, was used for an extensive series of quench measurements and endured > 150 quenches without damage. Minimum quench energies were found to be in the range of 200-500 mJ in background fields of 15-20 T when the coil carried 70-95% of its critical quench current.

Present and Future Applications for Advanced Superconducting Materials in High Field Magnets

Advances in high field magnets are driven primarily by the availability of high current density conductors. The restack rod process (RRP), internal Sn superconductors have achieved engineering current densities nearly five times that of bronze route conductors at high fields. Careful utilization of this low temperature superconductor (LTS) enables the production of magnets beyond the previous benchmark of 21 Tesla without an associated increase in magnet and cryostat volume. Steps to realize extremely compact high field magnets for a variety of applications are described. The next significant challenge is to produce magnetic fields beyond 25 Tesla solely using superconducting solenoids. High temperature superconductors (HTS) will be required and, to this end, Bi-2212/Ag matrix wires are at an advanced stage of development. The tangible objective is a new generation of compact, ultra-high field magnets.

Development Trends in High Field Magnet Technology

The production of high magnetic fields using low temperature superconductors (LTS) has become common place. However, large magnet sizes and associated high cooling costs have often precluded the full utilization of these research capabilities. Recent advances in internal Sn superconductors and cryogen free technology have opened up a new era in superconducting magnet development. Ultra-compact, laboratory sized magnets producing fields up to 22 Tesla are available. This new class of high field magnet weighs under 200 kg and is suitable for general laboratory installation. In addition, extremely compact, high field, split pair magnets with open access are now operating at the elevated temperature of T = 4.2 K. Beyond conventional wet magnet technology, there is a growing trend to utilize cryogen free technology. Cryogen free magnets do not require liquid Helium baths and, with the addition of active shielding, both the experimental sample access and siting availability is improved. The influence of enabling technologies required to realize the above practical applications for high field, superconducting magnet systems is described.

Recent Advances on the Spherical Tokamak Route to Fusion Power

Stambaugh developed the Peng-Hicks concept of a fusion reactor based on a solid copper center-post spherical tokamak (ST). Using the promising results from the START experiment, they produced a vision for a path to fusion power. This path had two elements such as the ability to produce high fusion gain from an ST and of equal importance, the ability to demonstrate this in a small (and therefore relatively low cost) pilot plant device. In this paper, we review various attempts to pursue this vision, and try to elucidate the reason why success has not yet been achieved. However, we show that the advent of high temperature superconductors may overcome some of the problems, and we suggest a revised version of the small, low entry cost route to fusion power.

High Temperature Superconducting (HTS) Coils for a Compact Spherical Tokamak

High temperature superconductors (HTSs) have the potential to impact the future development of superconducting applications for research magnets in physical sciences and industrial products. A compact spherical tokamak of major radius 25 cm has been designed and engineered using the second generation HTS material, REBCO tapes. It was developed with the primary objective to test out the feasibility of a fully superconducting device made entirely from HTS materials and hence be the first tokamak to demonstrate the practicality of this new medium operating at temperatures above 4.2 K. The tokamak has 6-D shaped toroidal field coils, and two poloidal field coils, each of which is wound in a pancake geometry. Each coil is made in one continuous winding and the coils are assembled together. Results of initial coils and magnet cooling and energisation will be presented.

The spherical Tokamak path to fusion power — Revisited

In their pioneering paper [1] Stambaugh et al developed the Peng-Hicks concept [2] of a fusion reactor based on a solid copper centre-post Spherical Tokamak (ST). Using the promising results from the START experiment [3], they produced a vision for a path to Fusion Power. This path had two elements: the ability to produce high fusion gain from an ST, and equally importantly, the ability to demonstrate this in a small (and therefore low cost) pilot plant device. In this paper we review various attempts to pursue this vision, and try to elucidate why success has not yet been achieved. However we show that the advent of high temperature superconductors (HTS) may overcome some of the problems, and we suggest a revised version of the small, low entry cost route to Fusion Power.

Electromagnetic Characteristics of Bi-2212 Test Coils at High Field During Quench

We present further progress on the development of insert coils using round wires of the high-temperature superconductor Bi-2212 material. Several different-sized configurations of coils have been manufactured with variation of coil length as well as number of layers. The coils have been tested at 4.2 K in various background field values up to 20 T using a wide bore low-temperature superconducting magnet. The electromagnetic stress characteristics of the insert coils and performance under quench will be discussed.

Quench Analysis of LTS and HTS Coils Used in Wide Bore Compact High Field Superconducting Magnets

Realizing a high field compact system for research applications requires a magnet constructed from low temperature superconductors (LTS) coils and insert coils developed from high temperature superconductor materials (HTS) like Bi-2212 round wires. The understanding of the quench behavior of such an integrated system is essential for the safe energisation and operation of compact high field magnets where quench management of LTS differ for those of HTS coils. In the present study, the quench behavior of the HTS coils and the LTS coils used to provide the background field were measured and analysed. The analysis is highly relevant to the design of high field wide bore magnets as well as ultra high field magnets using integrated LTS and HTS coils.

High magnetic fields for fundamental physics

Abstract Various fundamental-physics experiments such as measurement of the magnetic birefringence of the vacuum, searches for ultralight dark-matter particles (e.g., axions), and precision spectroscopy of complex systems (including exotic atoms containing antimatter constituents) are enabled by high-field magnets. We give an overview of current and future experiments and discuss the state-of-the-art DC- and pulsed-magnet technologies and prospects for future developments.