Route to High Temperatures by Current Amplification in the Sustained Spheromak Physics Experiment (SSPX)

For the spheromak to be attractive as a reactor concept it would be necessary to sustain the configuration with a low recycling power, reflected in the current amplification factor: A1 = Itor/Igun, where Itor is the toroidal current and Igun is the gun current. It is understood that A1 needs to be around 60 for a reactor [1], although the highest obtained so far in the spheromak has been ∼3 [2]. The spheromak is a simply connected toroidal confinement device related to the reversed field pinch in that the q-profile falls at the edge and the first wall is conducting, although the central solenoid is absent. In the spheromak, the paradigm for field generation (and hence current amplification) is the injection of helicity, K = ∫A.BdV = 2ΦΨ where φ and Ψ are linked fluxes. Helicity is additive in the process of electrostatic injection by a coaxial gun [3]: K = 2VgunΨgun, where Vgun is the voltage applied between two coaxial electrodes (giving the rate of toroidal flux injection) and Ψgun is the poloidal vacuum flux connecting them. SSPX [4] is a 1m wide coaxial-gun-driven spheromak with W-coated copper electrodes (FIGURE 1) and a uniquely programmable vacuum field configuration. SSPX was built to assess if confinement can be reasonably preserved during injection, and to address the specific physics of the processes governing helicity injection