High temperature superconducting (HTS) flux pumps can generate ultra-high currents (>1 kA) without the requirement for thermally inefficient room temperature current leads. Ultra-high currents enable physically smaller magnets with significantly less inductance unlocking new design opportunities. However, limited by intrinsically low electrical power efficiencies, existing HTS flux pumps cannot output high voltage or high power. In this work, we design, devise, and experimentally verify a transformer–rectifier type HTS flux pump using Jc(B) switching. We show that the rectification can be achieved by exploiting the HTS E-J relation with the application of DC magnetic fields. A quasi-persistent current of 54.5 A has been achieved at 77 K only limited by the load coil critical current. In addition, the electrical power efficiencies of both half-wave and full-wave flux pump are derived. We illustrate that the fundamental Jc(B) mechanism provides significantly higher efficiency than existing HTS flux pumps. This advancement will overthrow the common knowledge that HTS flux pumps could only be used for maintaining rather than fast ramping magnetic fields.

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We investigated electron cyclotron (EC) wave assisted low voltage Ohmic start-up in the conventional field null configuration (FNC) and the trapped-particle configuration (TPC) in the TST-2 spherical tokamak device. The upper pressure limit for successful burn-through increased when EC power was applied for both the FNC and TPC. On the other hand, at low prefill pressure, breakdown was delayed in the FNC start-up. The achievable plasma current also decreased especially at high EC power. By applying the TPC, fast breakdown was recovered even at high EC power. The plasma current ramp-up rate was also greater with TPC compared with FNC at the same loop voltage waveform. The lower prefill pressure limit for successful breakdown expanded in the TPC compared to that in the FNC. The higher vertical field decay index resulted in faster EC breakdown. The reduction of the upper pressure limit due to impurities was the same in the FNC and TPC indicating that the poloidal field configuration did not significantly affect the upper pressure limit for successful burn-through.

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