Improved energy confinement in spheromaks with reduced field errors

Abstract

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

Suppression of MHD Fluctuations Leading to Improved Confinement in a Gun-Driven Spheromak

“…The T i measurement is chord averaged, but we assume T i T e and decreases with minor radius even though the majority of anomalous ion heating is expected in the edge. This results in t E ഠ 150 6 70 ms, the same as the highest value previously reported (CTX [19]) for a decaying spheromak and is the highest value reported for a driven spheromak.…”

Energy confinement and magnetic field generation in the SSPX spheromak

“…Previous work at the CTX spheromak at Los Alamos [6] resulted in temperatures of >150 eV for driven spheromaks and toroidal currents of >250 kA in discharges up to 2 ms in duration. Decaying spheromaks on CTX heated to 400eV [7].…”

Spheromak Experiment Using Separate Guns for Formation and Sustainment

“…This success was due in part to the use of close-fitting, solid flux conservers and proper wall conditioning. [6][7][8][9] The key to proper conditioning was the incorporation of titanium gettering of the flux conserver walls. 10,11 In addition, the CTX group had perfected the slow-formation technique 12 so that spheromaks could be sustained for times that were long compared with their resistive decay times.…”

“…where 8 is the interelectrode gap and Vfi ow is the flow velocity of toroidal flux and plasma out of the gun. To compute the flow velocity, we note that in steady state, Vfl OW is fixed, and the J X B force acting on the plasma is (9) gun are straightforward. A direct measurement of the mass flow rate m = dm/dt would help corroborate this model.…”