Role Of Drifts Research Articles

The role of drifts on the isotope effect on divertor plasma detachment in JET Ohmic discharges

• The magnitude of the isotope effect is determined by the pressure in the sub-divertor. • Pumping of gas within the JET divertor is effective near the outer target only. • The re-distribution of the neutrals due to drifts decreases the throughput. • The total decrease of the throughput reduces the isotope effect. Experiments in JET-ILW Ohmic confinement mode plasmas show that the line-averaged detachment onset density in deuterium discharges is approx. 10% lower than in hydrogen discharges. The magnitude of the isotope effect on the detachment onset density depends on the divertor geometry, the magnetic configuration and the throughput of the sub-divertor/divertor cryopump system. Simulations with the edge fluid code EDGE2D-EIRENE revealed that the pumping of neutral gas within the JET divertor is effective near the outer divertor target only. The studies show that the magnitude of the isotope effect is determined by the molecular pressure in the sub-divertor pumping plenum. According to the simulations, operating in vertical configurations or closer proximity of the strike point increases the molecular pressure (and thus throughput) in front of the outer pumping plenum by up to 15% compared to the horizontal configuration, thus producing a stronger isotope effect on the detachment onset density. Similarly, EDGE2D-EIRENE predicts that plasma, hence neutral, redistribution due to E × B drifts in favourable B T configurations (ion B × grad(B) towards the divertor) decreases the throughput. The total decrease of the throughput reduces the isotope effect on the detachment onset density, and decreases the detachment onset density for both isotopes.

The role of drifts in the plasma transport at the tokamak core–SOL interface

The interface between the core (inside the magnetic separatrix in X-point configurations) and the scrape-off layer (SOL) of tokamaks is a delicate region of the magnetic topology transition from closed to open field lines where neither the standard neoclassical theory nor the SOL physics fully apply. Sharp gradients of plasma parameters in the outer core, caused by the proximity of divertor sinks in the near SOL, invalidate some ordering assumptions of the neoclassical theory. At the same time, the existence of closed flux surfaces in the core enforces ambipolarity of radial plasma flows, in difference to the situation in the SOL where the current loop may close through the divertor. Detailed analysis of the plasma transport and flows with the emphasis on the outer core region, just inside the separatrix, is carried out in the paper, based on EDGE2D modelling and analytical formulas.

Hysteresis between solar activity and cosmic rays during cycle 22: the role of drifts, and the modulation region

Our studies of the neutron monitor observatories data with different cut-off rigidities during solar cycle 22 have made it possible to investigate the hysteresis character of the relationship between the variations in solar activity and in galactic cosmic ray intensity. This effect arises from the delay of interplanetary processes (responsible for cosmic ray modulation) with respect to the initiating solar processes, which correspond to some effective solar wind velocity and shock waves propagation. It gives the possibility to determine the effective dimension of modulation region in depending on the effective energy of galactic cosmic rays. In addition to our previous investigations of cosmic ray—solar activity hysteresis phenomenon made in the frame of convection-diffusion model, we now try to take into account drifts that change sign in periods of solar magnetic field reversal. From comparisons with experimental data on long-term cosmic ray variation in cycle 22, we determine the role of convection-diffusion and drifts in global modulation, and the effective dimension of the modulation region.

Time-dependent cosmic-ray modulation: Role of drifts and interaction regions

The combination of gradient, curvature and heliospheric neutral sheet (HNS) drifts with interaction regions (IRs) occurring in the inner heliosphere and merged interaction regions (MIRs) in the outer heliosphere would enable one to successfully simulate complete 11 and 22-yr cycles with comprehensive, global modulation models. This is possible because both drifts and MIRs happen on a large scale. With this scenario, drifts are primarily responsible for long-term modulation during low to moderate solar activity for both magnetic cycles. In terms of the “tilt angle”, α, of the wavy neutral sheet this happens when α ≤ (35 ± 5)°. With moderate to high solar activity MIRs become progressively more important, but still in combination with drifts, to eventually dominate during maximum solar activity when global drift effects cannot occur any longer. The rate at which solar activity decreases (increases) before (after) minimum modulation will then become crucial in determining when and what process begins to dominate.

Role of drifts and global merged interaction regions in the long‐term modulation of cosmic rays

It is shown that the long‐term modulation of 0.2‐ to 3‐GV galactic and anomalous cosmic rays over the 22‐year heliomagnetic cycle is principally a combination of two solar related processes, the cumulative effect of long‐lived global merged interaction regions (GMIRs) and large‐scale particle gradient and curvature drifts in the interplanetary magnetic field. This paradigm for cosmic ray modulation is based on the observed changes in cosmic ray intensity from solar minimum to solar maximum over successive solar cycles (21 and 22) using data from 1 AU and from the outer heliosphere. For cycle 21 (when positive ions flow in over the solar pole and out along the heliospheric neutral current sheet; qA>0), the 1977–1980 modulation is dominated by GMIRs. While drifts may play a role in particle transport in the heliosphere at this time, the changing inclination of the heliospheric neutral sheet has a negligible effect on the intensity of cosmic ray nuclei. In cycle 22 when this flow pattern is reversed, it is shown that drifts are an important but not dominant factor for galactic cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidity R and heliocentric distance r. Anomalous cosmic rays are much more sensitive to changes in the current sheet inclination. The observed changes in the galactic cosmic ray intensity over the 1987–1988 period due to the increasing inclination of the current sheet, α, are a factor of 3–5 smaller than predicted by the time‐dependent model of Potgieter and Le Roux (1992). The latitude variation at Voyager 1 (heliolatitude 32°) of the dependence of galactic cosmic ray intensity on α is consistent with theoretical expectations. The strong role of GMIRs and their episodic nature requires a long lifetime (1.5–1.8 years) and a magnetic structure that effectively extends over the solar poles. Such a long GMIR lifetime implies a modulation boundary of the order of ∼175 AU.

The role of drifts in diffusive shock acceleration

The role played by shock-associated drifts during the diffusive acceleration of charged particles at collisionless MHD shocks is evaluated. In the rest frame of the shock, the total energy gained by a particle is shown to result from two coupled acceleration mechanisms, the usual first-order Fermi mechanism and the drift mechanism. When averaged over a distribution of particles, the ratio of the drift-associated energy gain to the total energy is found to be independent of the total energy at a given theta1 (the angle between the shock normal and the unperturbed upstream magnetic field) in agreement with theoretical predictions. No evidence is found for drift-associated deceleration, suggesting that drifts always augment acceleration.