Anomalous plasma diffusion in ion cyclotron resonance

Abstract

We have carried out experimental investigations of the increased diffusion of a low-temperature plasma across the lines of force of a constant homogeneous magnetic field under conditions of high-frequency heating for a frequency approaching ion cyclotron frequency. The plasma was created in a glass tube by a pulse discharge with oscillating electrons in hydrogen at a pressure range of 5 × 10−4–5 × 10−2 torr. Initial plasma density exceeded 1.7 × 1013 cm−3. A high-frequency electric field with a frequency of 7.45 × 106 Hz and an axial period of 23 cm was excited in the plasma by an alternating current flowing along a spiral surrounding the discharge tube. The spiral and the capacitors connected to it formed an artificial line that was fed from the pulse generator with self-excitation. In the region of magnetic fields where there occurs a strong absorption of highfrequency energy by the plasma caused by ion cyclotron resonance time of plasma decay decreases sharply if the amplitude of the high-frequency current in the spiral exceeds some critical value depending on neutral gas pressure. Here ion diffusion across the magnetic field lines increases and chaotic oscillations arise in the plasma. It has been ascertained in experiments that for neutral gas pressures exceeding 10−2 torr the ratio between the critical current value and the pressure remains constant. On these grounds the assumption was made that anomalous diffusion arises when the ion drift velocity under the effect of an alternating azimuthal electric field exceeds a critical value. This is possibly associated with the appearance of “flute” instability. The possibility for flute instability to arise is apparently confirmed also by this fact: as is shown by evaluation, the velocity gained by the ion between two collisions in an azimuthal electric field for a current approaching a critical current equals in order of magnitude thermal ion velocity. We note that the investigated phenomenon of anomalous plasma diffusion in ion cyclotron resonance can be the reason for increased velocity of plasma decay in all devices in which the phenomenon of strong damping of an ion cyclotron wave in the region of ion cyclotron resonance is used for plasma heating and where considerable high-frequency powers are introduced into the plasma.

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

“…In some basic features, such as a many-fold difference in the values of symmetrically positioned flows of the diverted plasma at the top and bottom, the opposite polarities of non-ambipolar divertor flows at the top and bottom, and the consistency between the direction of the dominating divertor flow and that of the ion B × ∇B drift, the vertical asymmetry of diverted plasma distributions observed in the l = 3 U-3M torsatron with an RF heated plasma is similar to that observed earlier in the l = 2 Heliotron E heliotron/torsatron under NBI and ECH conditions [6][7][8]. In this sense, we may speak about the universality of this effect for helical devices.…”

“…Proceeding from these data, it has been hypothesized that in the limit of zero power the observed asymmetry was caused by an additional field line distortion in the divertor layer due to small error fields, while the enhancement of the asymmetry and its modification with heating were related to convection-like losses of fast ions and electrons. The validity of the explanation based on the role of fast particles has been confirmed qualitatively by studies of non-ambipolar flows (electric currents) in the diverted plasma of Heliotron E [8].…”

“…In this sense, we may speak about the universality of this effect for helical devices. 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.…”

“…Since a potential difference is always present between the wall and the adjacent plasma, the initial plasma flow is not ambipolar as is supposed when estimating the flow by I s . The value and direction of the corresponding electric current (plasma current) I p ≡ I (0) = I s [1 − exp(−eV f /T e )] [13] and its spatial distribution are closely related to physical processes in the SOL and confined plasma, and hence a study of this current should also be of interest [8]. With all this taken into account, not only the current I s but also the current I p to a grounded probe is measured and its behaviour is analysed in the present work.…”

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

“…This conclusion has been confirmed by measurements of diverted plasma flow distributions in the helical divertor of the l = 3 Uragan-3M (U-3M) torsatron under conditions of RF plasma production and heating [5]. In some characteristics, such as the predominant outflow of the diverted plasma in the ion toroidal drift direction [1][2][3][4][5] and the agreement between the larger ambipolar divertor flow and an excess of ions in the corresponding non-ambipolar flow [5][6][7] and the rise of asymmetry with heating power [1,2] (see, also, present communication, section 3), the asymmetries in H-E and U-3M appear to be similar despite a substantial difference between these devices in magnetic configuration, plasma heating methods and confined plasma parameters. This is an indication of the universality of the effect considered.…”