Onion-peeling inversion of stellarator images

Hammond, K. C.; Diaz-Pacheco, R.; Kornbluth, Yosef; Volpe, F.; Wei, Y. L.

doi:10.1063/1.4960309

Cited by 3 publications

(4 citation statements)

References 12 publications

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“…1) is uniquely well-suited for this research. The device, originally constructed to study non-neutral and pure-electron plasmas [12][13][14][15] , has since been repurposed to investigate quasi-neutral plasmas, and has addressed issues relevant to magnetic fusion energy such as error-field diagnosis 16 and processing of stellarator images 17 . From the point of view of high β stability, CNT is attractive for two main reasons: (1) it could reach high values of β by deploying relatively small amounts of heating power a) kch2124@columbia.edu b) fvolpe@columbia.edu and (2) its stability limit is expected to be lower and thus more easily accessible than in other devices.…”

Section: Introductionmentioning

confidence: 99%

High-β equilibrium and ballooning stability of the low aspect ratio CNT stellarator

2017

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The existence and ballooning-stability of low aspect ratio stellarator equilibria is predicted for the Columbia Neutral Torus (CNT) with the aid of 3D numerical tools. In addition to having a low aspect ratio, CNT is characterized by a low magnetic field and small plasma volume. Also, highly overdense plasmas were recently heated in CNT by means of microwaves. These characteristics suggest that CNT might attain relatively high values of plasma beta and thus be of use in the experimental study of stellarator stability to high-beta instabilities such as ballooning modes. As a first step in that direction, here the ballooning stability limit is found numerically. Depending on the particular magnetic configuration we expect volume-averaged β limits in the range 0.9-3.0%, and possibly higher, and observe indications of a second region of ballooning stability. As the aspect ratio is reduced, stability is found to increase in some configurations and decrease in others. Energy-balance estimates using stellarator scaling laws indicate that the lower β limit may be attainable with overdense heating at powers of of 40 to 100 kW. The present study serves the additional purpose of testing VMEC and other stellarator codes at high values of β and at low aspect ratios. For this reason, the study was carried out both for free boundary, for maximum fidelity to experiment, as well as with a fixed boundary, as a numerical test.

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Section: Introductionmentioning

confidence: 99%

High-β equilibrium and ballooning stability of the low aspect ratio CNT stellarator

2017

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“…CNT was the first device to toroidally confine non-neutral plasmas [32][33][34] and plasmas with various degrees of quasi-neutrality [35]. Its focus has recently shifted to 3D diagnostic image inversion [36], error fields [37], high beta [38] and overdense microwave heating [39] in neutral stellarator plasmas.…”

Section: Tilted Interlinked Planar Coil Torsatronsmentioning

confidence: 99%

Large vacuum flux surfaces generated by tilted planar coils

Austin

Hammond

et al. 2019

Plasma Phys. Control. Fusion

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Helical equilibria can be generated by arrangements of planar coils similar to tokamaks, but without a central solenoid and with the toroidal field (TF) coils tilted with respect to the vertical. This is known from earlier numerical works, e.g. P.E. Moroz, Phys. Plasmas 2, 4269 (1995). However, such concept tends to need large coils (of low aspect ratio) but form small plasmas (of large aspect ratio). Here it is numerically shown that larger, more attractive vacuum flux surfaces -relative to the size of the device-can be generated by carefully optimizing the inclination of the TF coils and currents in the various coil-sets. Vacuum configurations of aspect ratios as low as 4 are found for 6 tilted TF circular coils. Higher numbers of TF coils have advantages (smaller effective ripple) and disadvantages (lower rotational transform, smaller plasma). Finally, the aspect-ratio A of the vacuum flux surfaces is quantified as a function of the ratio A c of the coil-radius to the radial location of the coil-center. It is found that, in order to minimize A, it is beneficial to interlink or marginally interlink the TF coils (A c 1).

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“…CNT was originally dedicated to nonneutral and quasi-neutral plasma research, [15,16] but has recently been re-purposed to investigate fusion-relevant problems such as error field analysis, [17] high-β stability, [18] and inversion of stellarator images. [19] As part of this, it was equipped with microwave heating at 2.45 GHz. The corresponding vacuum wavelength, λ 0 = 12.2 cm, is comparable with the plasma minor radius a ≃ 13 cm and about one-third of the major radius R ≃ 30 cm.…”

Section: Introductionmentioning

confidence: 99%

“…The low field, low aspect ratio CNT stellarator [19] can be viewed as a further extension of the TJ-K long-wavelength regime. CNT was originally dedicated to non-neutral and quasi-neutral plasma research [20,21], but has recently been re-purposed to investigate error fields [22], high-β stability [23] and image inversion [24] in neutral stellarator plasmas. As part of this, it was equipped with microwave heating at 2.45 GHz.…”

Section: Introductionmentioning

confidence: 99%

Overdense microwave plasma heating in the CNT stellarator

Hammond

Diaz-Pacheco

Köhn

et al. 2018

Plasma Phys. Control. Fusion

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Abstract. Overdense plasmas have been attained with 2.45 GHz microwave heating in the low-field, low-aspect-ratio CNT stellarator. Densities higher than four times the ordinary (O) mode cutoff density were measured with 8 kW of power injected in the O-mode and, alternatively, with 6.5 kW in the extraordinary (X) mode. The temperature profiles peak at the plasma edge. This was ascribed to collisional damping of the X-mode at the upper hybrid resonant layer. The X-mode reaches that location by tunneling, mode-conversions or after polarization-scrambling reflections off the wall and in-vessel coils, regardless of the initial launch being in O-or X-mode. This interpretation was confirmed by full-wave numerical simulations. Also, as the CNT plasma is not completely ionized at these low microwave power levels, electron density was shown to increase with power. A dependence on magnetic field strength was also observed (for O-mode launch) and discussed.

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Onion-peeling inversion of stellarator images

Cited by 3 publications

References 12 publications

High-β equilibrium and ballooning stability of the low aspect ratio CNT stellarator

High-β equilibrium and ballooning stability of the low aspect ratio CNT stellarator

Large vacuum flux surfaces generated by tilted planar coils

Overdense microwave plasma heating in the CNT stellarator

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