THE DESIGN AND OPERATION OF THE SPHEX SPHEROMAK

We describe the design and operation of the SPHEX spheromak device and present an overview of its behaviour. The plasma is formed by ejectio n from a magnetized Marshall gun, and can be sustained as long as the gun is energized; The plasma is divided into the annulus comprising th e closed toroidal Bur, linked with the open Bur forming the central co lumn. The column current is driven directly by the central gun electro de, and the toroidal current in the annulus is driven indirectly by a mechanism associated with a coherent n = 1 oscillation of the column. The configuration exemplifies the operation of the process of relaxati on to a state of minimum magnetic energy, which leads to magnetic conf igurations similar to those observed; to sustain these configurations requires some mechanism of toroidal current drive. Associated with thi s is the amplification of the poloidal flux, which is typically a fact or of about five larger than the flux generated by the gun solenoid; t he constancy (to a first approximation) of this factor plays a control ling role in spheromak behaviour. In standard operating conditions the re is a 'hard' limit, set by the solenoid flux, on the current carried by the column; any current driven by the external circuit above this apparently does not emerge from the gun. Evidence is presented that th e column current is carried largely (>50%) by accelerated ions with en ergy up to the gun voltage (approximate to 500 V for a typical gun cur rent of 60 kA). These ions are poorly magnetized and can escape across the magnetic field to the wall, a likely mechanism for the observed ' loss' of current. Hydrogen is the normal operating gas: other gases (D -2 and He) have been used, but the current drive is found to be less e ffective than in H-2, With lower toroidal current maintained in the an nulus.