The Spheromak path to fusion energy

Abstract

The spheromak is a simple and robust magnetofluid configuration with several attractive reactor attributes including compact geometry, no material center post, high engineering p, and sustained steady state operation through helicity injection. Spheromak physics was extensively studied in the US program and abroad (especially Japan) in the 1980's with work continuing into the 1990s in Japan and the UK. Scientific results included demonstration of self-organization at constant helicity, control of the tilt and shift modes by shaped flux conservers, elucidation of the role of magnetic reconnection in the magnetic dynamo, and sustainment of a spheromak by helicity injection.Several groups attained electron temperatures above 100 eV in decaying plasmas, with CTX reaching 400 eV. This experiment had high magnetic field (>l T on the edge and -3 T near the symmetry axis) and good confinement. More recently, analysis of CTX found the energy confinement in the plasma core to be consistent with Rechester-Rosenbluth transport in a fluctuating magnetic field, potentially scaling to good confinement at higher electron temperatures. The SPHEX group developed an understanding of the dynamo in sustained spheromaks but in a relatively cold device. These and other physics results provide a foundation for a new "concept exploration" experiment to study the physics of a hot, sustained spheromak.If successful, this work leads to a next generation, proof-of-principle program.The new SSPX experiment will address the physics of a large-scale sustained spheromak in a national laboratory (LLNL) setting. The key issue in near term spheromak research will be to explore the possibly deleterious effects of sustainment on confinement.Other important issues include exploring the P scaling of confinement, scaling with Lundquist number S, and determining the need for active current-profile control. Collaborators from universities and other national laboratories are contributing experience from previous work, diagnostics, and physics support. Experiments at PPPL and Swarthmore are being conducted on the physics of magnetic reconnection, yielding physics results which should help advance the confinement work.A spheromak reactor will require steady state operation with the equilibrium fully supported by external coils. Although the present generation of experiments can provide data on the initial stages of the transition from short-pulsed operation, sustainment longer than the wall resistance time will be addressed in the proof-of-principle experiments.-lContents

Imposed-dynamo current drive

“…Integrating equation (7) gives the time behaviour of the spheromak current predicted by the IDCD model. Using [13] showing data from each of the four local toroidal current measurements, the average toroidal current, and the separatrix current.…”

“…However, this result shows that the dependence of the current drive is on the amplitude of the imposed fluctuations and the density is accurately predicted by the model. Equation (7) with dI/dt = 0 indicates current amplification is increased with lower n, higher L/R time, and/or higher injector current. In addition, figure 9 shows that the model explains, through equation ( 11), the strange j/n-of-the-spheromak dependence of the injector impedance.…”

“…The solenoid free startup method developed on HIT-II using CHI has been successful on NSTX [2] and the first DEMO will probably not have the transformer coil set. Of the remaining two coils only the equilibrium coils are fundamental for toroidal confinement and stable equilibria have been produced transiently that have good confinement at temperatures in the kilovolt range using very little [3] or no [4][5][6][7][8][9] externally produced toroidal field. However, the benefits of the large external toroidal field are considerable.…”

Recent results from the HIT-SI experiment

“…The solenoid-free startup method developed on HIT-II using CHI has been very successful on NSTX [11] and the first DEMO will probably not have the transformer coil. Of the remaining two coils only the equilibrium coils are fundamental for toroidal confinement and stable equilibria have been produced transiently that have good confinement at temperatures in the kilovolt range using very little [12] or no [13][14][15][16][17][18] externally produced toroidal field. The benefits of a strong external toroidal field for allowing a broad range of MHD-stable current profiles are considerable, but if an efficient steady-state current drive with sufficient current profile control is developed then this coil may not be needed.…”

Final results of the first phase of the PROTO-SPHERA experiment: obtainment of the full current stable screw pinch and first evidences of the jet + torus combined plasma configuration

“…Unpleasant effects for the plasma torus are associated with any kind of filamentation of the plasma current: the plasma filaments act as a relevant source of magnetic disturbances to the overall axial symmetry of the magnetic configuration. The two main disturbances from current carrying filaments are: (a) to add a counterflowing current on the outboard edge of the torus and (b) to introduce an ergodic component to the magnetic field inside the torus [32]. The results would be that: (a) the filamentation could therefore contrast the helicity injection and (b) the integrity of the magnetic toroidal flux surfaces could be spoiled.…”