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It now appears feasible to deposit positrons (e+) in a tokamak plasma by injecting bursts of neutral positronium atoms (e+e−), which are then ionized by the plasma. The annihilation time of these positrons in the plasma is long compared with typical particle containment times. Thus the subsequent transport of the positrons can be studied by monitoring the time dependence of the annihilation, gamma radiation produced when the positrons strike a limiter. This paper discusses the design of such an experiment, the kinds of data which can be obtained, and the physics questions which this experiment might address. This diagnostic technique could also be useful in studying transport in other magnetic confinement devices such as reversed-field pinches and magnetic mirrors.

Large improvements in spheromak parameters and new understanding have been obtained from the CTX experiment at Los Alamos [Phys. Rev. Lett. 51, 39 (1983); 61, 2457 (1988)]. In one experiment the global energy confinement time has been increased an order of magnitude over previous experiments to 0.2 msec and the magnetic-energy decay time increased to 2 msec. These results were achieved in a decaying spheromak by reducing the helicity dissipation in the edge. In another smaller spheromak, record electron temperatures (∼400 eV) and record magnetic field strengths (∼30 kG) have been obtained.

An increase in the global energy confinement time (TE) was obtained in the CTX spheromak by replacing the high-field-error mesh-wall flux conserver with a low-field-error solid-wall flux conserver. The maximum TE is now 0.18 ms, an order of magnitude greater than previously reported values of ;S0.017 ms. Both TE and the magnetic energy decay time ixw) now increase with central electron temperature, which was not previously observed. These new results are consistent with a previously proposed energyloss mechanism associated with high edge helicity dissipation.PACS numbers: 52.55.Hc, 52.70.Kz A spheromak 1 is a toroidal magnetic configuration with large internal plasma currents and self-generated internal magnetic fields. This configuration has been studied for many years with the hope that it would make an attractive fusion reactor. However, an apparent condemning feature of the spheromak for reactor use was the short global energy confinement times (T^) previously reported, 2,3 in the 5-20-jis range. It has been proposed that the dominant energy loss has been a consequence of enhanced helicity dissipation in the edge region of the spheromak induced by magnetic-field errors. 2 " 4 (Helicity 5 K is the quantitative measure of the "knottedness" of magnetic-field lines, i.e., flux linkage.) In the case of CTX with a mesh-wall flux conserver, 2,6 the field errors were due to the bridges crossing the midplane gap, the coarseness of the mesh, and the nonzero resistivity of the copper rods. Edge field lines either contacted the rods themselves, or the vacuum tank surrounding the flux conserver. It was estimated that approximately 25% of the poloidal flux intersected metal, and it appears that the resistivity of the open field lines was dominated by electron-neutral collisions 2 (rj e . n is much higher than r/spitzer in this region). These field lines are influenced by the minimum-energy principle, 7 which states that ^=//oJ' B/| B | 2 (where J is the current density and B is the magnetic field) should be a spatial constant. Therefore, current is driven on the highresistance open field lines, primarily by instabilities in the bulk plasma induced by Vk. The exact nature of these instabilities is not yet understood, but it is generally accepted that they cause direct ion heating at the expense of magnetic energy (W), giving 2,3,6 ion temperatures (Tj) higher than the electron temperature (T e ). The enhanced decay rate of W (due to instabilities and direct ion heating) must be accompanied by an approximately equal helicity decay rate (T/T'= -K/K), because W/Koc{X) (volume-averaged A.), which changes only slightly with VA. Enhanced AT is a direct result of large edge 77J, since 8 Koc J V0 \T]J' Bd 3 x. (One can think of this as enhanced "untying" of the "magnetic knot" in the resistive edge.) The severe impact on x E came from high charge-exchange rates involving the hot, directly

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