Ion heating and current drive from relaxation in decaying spheromaks in mesh flux conservers

Fernández, Juan; Jarboe, T. R.; Knox, S.O.; Hennis, I.; Marklin, G.J.

doi:10.1088/0029-5515/30/1/007

Cited by 25 publications

(14 citation statements)

References 19 publications

(65 reference statements)

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“…2,3,6 Another observation consistent with edge-dominated helicity decay was that both x E and the global magnetic energy decay time (r^) did not depend on the central electron temperature. 2 Observations and conditions in other spheromak devices are also consistent with this model.…”

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confidence: 63%

Improved energy confinement in spheromaks with reduced field errors

et al. 1990

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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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confidence: 63%

Improved energy confinement in spheromaks with reduced field errors

et al. 1990

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“…However, we now present results from SPHEX which for the first time show that (vxb) is nonzero, and of the right magnitude to drive the current. We shall show that both single-mode and turbulent dynamos are present, although the former forms an ''antidynamo" opposing the applied electric field [12].…”

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confidence: 84%

Observations of the magnetohydrodynamic dynamo effect in a spheromak plasma

et al. 1993

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We present measurements of the magnetohydrodynamic ''dynamo" due to correlated fluctuations of velocity and magnetic field in the SPHEX spheromak. We show that there are both single-mode and turbulent dynamo processes present, although the single-mode process is in this case an "antidynamo" opposing the externally applied electric field. The size of the turbulent dynamo at the magnetic axis is close to that required to drive the toroidal current there.

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“…It has been demonstrated that particle transport [8] and energy transport [3,4] are enhanced during periods of the discharge in which there is strong relaxation activity. Together with strong plasma relaxation, a significant decrease in the magnetic field decay time (due to relaxation current drive in the resistive edge) and high ion temperatures (T| <> 5T e ) [3,9,10] have been observed. Charge exchange (CX) of these anomalously hot ions and subsequent escape of hot neutrals appear to dominate the energy balance of these plasmas.…”

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confidence: 99%

Time of flight measurement of ion temperatures in spheromaks

et al. 1991

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The energy distribution of neutral particles is detected from spheromak plasmas by a time of flight neutral atom spectrometer. These particles originate in charge exchange interactions within the bulk plasma and carry the temperature of the majority ion component. This is the first direct measurement of temperatures of the main ion species in spheromaks. The data are found to be in the range of 500 eV during the sustainment and late decay phases and are in agreement with impurity Doppler temperatures for species expected to be localized to the plasma centre.

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Ion heating and current drive from relaxation in decaying spheromaks in mesh flux conservers

Cited by 25 publications

References 19 publications

Improved energy confinement in spheromaks with reduced field errors

Improved energy confinement in spheromaks with reduced field errors

Observations of the magnetohydrodynamic dynamo effect in a spheromak plasma

Time of flight measurement of ion temperatures in spheromaks

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