Vertical asymmetry of the helical magnetic-field ripple in torsatrons and heliotrons

Smirnova, M. S.

doi:10.1063/1.873119

Cited by 5 publications

(7 citation statements)

References 27 publications

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“…Probably more complicated manifestations of the asymmetry in Heliotron E, for example the polarity reversal of the non-ambipolar divertor flows at the top and bottom with a rise of heating power [8], are related to the radial electric field being much stronger in Heliotron E (∼100 V/cm or even more, in comparison with ∼10 V/cm or less in the U-3M case). Moreover, as has been shown in Refs [20,21], a certain contribution to the vertical asymmetry of divertor flows in Heliotron E can also be made by the vertical asymmetry of helical ripple well co-ordinates, which is specific for the magnetic configuration of this heliotron/torsatron. The presence of such asymmetry facilitates a considerable increase in the vertical component of the drift velocity of fast particles with energy near the boundary of particle trapping in the helical ripple wells.…”

Section: Discussionmentioning

confidence: 53%

Plasma flow asymmetries in the natural helical divertor of anl= 3 torsatron and their relation to particle losses

et al. 2002

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In the l = 3/m = 9 Uragan-3M (U-3M) torsatron (R 0 = 1 m, ā ≈ 0.12 m, B φ = 0.72 T, ι( ā)/2π ≈ 0.4), an open helical divertor is realized. A hydrogen plasma with ne ≈ 2 × 10 18 m −3 , T e ≈ 0.3 keV, T i ≈ 0.1 keV is produced and heated by RF fields (ω ≈ ω ci ). The flows of diverted plasma are detected by 78 plane Langmuir probes aligned poloidally in the spacings between the helical coils in two geometrically symmetric poloidal cross-sections of the torus. In measurements of the distributions of ambipolar (e.g. the ion saturation current I s ) and non-ambipolar (e.g. the current to a grounded probe I p ) plasma flows, a strong vertical asymmetry of these distributions is observed, its main characteristics being a many-fold difference in the values of I s in the outgoing flows in the upper and lower parts of the torus and the opposite signs of I p in these flows, with the positive current corresponding to the larger ambipolar flow of the diverted plasma. Reversal of the direction of the toroidal magnetic field results in the reversal of the asymmetry, with the larger flux (and I p > 0) always flowing in the ion B × ∇B drift direction. On this basis, it is concluded that the asymmetry is related to direct (non-diffusive) losses of charged particles from the confinement volume. This conclusion is validated by numerical modelling of thermal and fast particle orbits in U-3M, where qualitative agreement has been revealed between the calculated distribution of the angular co-ordinates of lost particles and the measured poloidal distributions of the flows of diverted plasma.

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Section: Discussionmentioning

confidence: 53%

Plasma flow asymmetries in the natural helical divertor of anl= 3 torsatron and their relation to particle losses

et al. 2002

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“…The helical ripple asymmetry favours an increase in vertical asymmetry of convective particle flows, provided banana or superbanana ions, being sensitive to the helical ripple asymmetry, dominate in the loss cone losses. It is the difference in the structure of ion loss cone in U-3M [5] and H-E [17] that might result in the divertor flow asymmetry in H-E to be several times higher than in the l = 3 torsatron.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…A characteristic feature of the H-E magnetic configuration is the natural vertical asymmetry of the helical magnetic field ripple [17]. Such an asymmetry is inherent to all l = 2 helical devices, to a larger or lesser extent.…”

Section: Summary and Discussionmentioning

confidence: 99%

Plasma heating effects on divertor flow vertical asymmetries in the Uragan-3M torsatron

et al. 2003

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In the l = 3/m = 9 Uragan-3M (U-3M) torsatron (R0 = 1 m, abar; ≈ 12 m, Bϕ = 0.7 T, ι(abar)/2π ≈ 0.4), an open helical divertor has been realized. Recently, under RF plasma production and heating conditions, a strong up–down asymmetry of diverted plasma flow has been observed as a result of measurements of distributions of this flow in two symmetric poloidal cross-sections of the U-3M torus. In many aspects, this asymmetry is similar to that observed in the l = 2 Heliotron E (H-E) heliotron/torsatron under neutral beam injection and electron cyclotron heating conditions. The main feature of the asymmetry is a predominant outflow of the diverted plasma in the ion toroidal drift direction. On this basis, the asymmetry can be related to non-uniformity of the distribution of direct charged particle losses in the minor azimuth. In the work reported, the magnitude of diverted plasma flow in U-3M and the degree of its vertical asymmetry are studied as functions of the heating parameter , P being the RF power absorbed in the plasma, and are juxtaposed with corresponding P-related changes in the density, , and suprathermal ion content in the plasma. As the heating power increases, both the temperature of the main ion group and the relative content of suprathermal ions increase. At the same time, a decrease in plasma density is observed, evidencing a rise of particle loss. The rise of particle loss with heating could result from both a shift of diffusion regime towards a lower collisionality and a rise of direct (non-diffusive) loss of high-energy particles. Outside the confinement volume, the total flow of diverted plasma increases together with an increase of vertical flow asymmetry towards the ion toroidal drift side. Such a mutual accordance between the processes in the confinement volume and in the divertor region validates the hypothesis on a dominating role of fast particle loss in the formation of vertical asymmetry of divertor flow in U-3M. In conclusion, the results obtained on U-3M are compared with those from similar research on H-E.

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“…In the Heliotron E device main particle losses, similar to those in U-3M [15], should be associated with fast toroidal banana-ions, which were trapped in the helical ripple wells at the plasma periphery, and the non-compensated ion B Â HB drift should be a determinative mechanism of particle losses and the vertical asymmetry of poloidal distributions of these losses. However, in the H-E case, the degree of vertical asymmetry of ion loss distribution underwent a significant increase due to the natural vertical asymmetry of the helical magnetic field ripple, according to results of calculations in [16,17]. This kind of asymmetry differs from that caused by a single-null divertor in a tokamak or a vertical displacement of the plasma column [18] and is inherent, to a larger or lesser extent, to all l ¼ 2 heliotrons=torsatrons.…”

Section: On the Role Of Trapped Particles In Up-down Asymmetrymentioning

confidence: 77%

“…12) which crossed the CP surface. Therefore, it is impossible to make a quantitative comparison of calculated locations on OMSs of lost particles [16,17] with observed behavior of divertor flows.…”

Section: Difference Of the Up-down Asymmetry For Different Regimes Ofmentioning

confidence: 98%

Influence of Experimental Conditions on Distribution of Divertor Plasma Flow in the Heliotron-E Fusion Device

Voitsenya¹,

Chechkin²,

Mizuuchi³

et al. 2002

Plasma Devices and Operations

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A comprehensive study of the divertor plasma flow (DPF) distribution in the Heliotron-E helical device has been carried out by means of collector arrays under various experimental conditions. A strong inhomogeneity of DPF distribution is observed in all the investigated regimes. This inhomogeneity is characterized by a significant asymmetry of plasma flows recorded by symmetrically displaced collectors. The degree of asymmetry in horizontal (in-out) and in vertical (up-down) directions depends on characteristics of magnetic configuration as well as on experimental conditions: method of plasma heating, injected power and power absorbed by the plasma, plasma density, gas puffing time, pellet injection, etc. A certain degree of asymmetry also remains after termination of the active phase of discharge, i.e., at the stage of decaying plasma. By comparison of asymmetry indices at the active stages of discharge and at the stage of afterglow plasma, the contribution of trapped particles into the asymmetry of DPF distribution has been estimated.

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Vertical asymmetry of the helical magnetic-field ripple in torsatrons and heliotrons

Cited by 5 publications

References 27 publications

Plasma flow asymmetries in the natural helical divertor of anl= 3 torsatron and their relation to particle losses

Plasma flow asymmetries in the natural helical divertor of anl= 3 torsatron and their relation to particle losses

Plasma heating effects on divertor flow vertical asymmetries in the Uragan-3M torsatron

Influence of Experimental Conditions on Distribution of Divertor Plasma Flow in the Heliotron-E Fusion Device

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