Yu. A. Tsidulko

EXPERIMENTAL STUDY OF CURVATURE-DRIVEN FLUTE INSTABILITY IN THE GAS-DYNAMIC TRAP

A curvature‐driven flute instability will be excited in the magnetized plasmas if the magnetic field lines curve toward the entire plasma boundary. Conditions under which it can be effectively stabilized in axisymmetric geometry have been experimentally studied in a gas‐dynamic trap (GDT) at Novosibirsk. Flexible design of the experimental device and the availability of neutral beams and ion cyclotron heating enabled the pressure‐weighted curvature to be varied over a wide range. The stability limits were thus measured and compared with those predicted by the modified Rosenbluth–Longmire criterion. Characteristics of unstable curvature‐driven flute modes were also measured and found to conform to a theory including finite ion Larmor radius (FLR) effects. Stable operation during neutral beam injection was achieved with a cusp end cell, resulting in an increase in Te to 45 eV, limited by end losses rather than anomalous power losses.

Observation of magnetohydrodynamic stability limit in a cusp-anchored gas-dynamic trap

In this paper, the observation of magnetohydrodynamic (MHD) stability limit in a fully axisymmetric gas-dynamic trap is reported. Transition through the stability boundary was studied by varying the plasma pressure in the stabilizing cusp end cell and simultaneously measuring the particle and energy lifetimes in the central cell. Energy and particle balance of the neutral beam heated plasma was measured and compared in unstable and stable regimes of operation. It was observed that if the calculations based on the energy principle predicted the plasma to be stable, experimentally measured transverse losses appeared to be smaller than the longitudinal ones. In the opposite case, the transverse losses were dominant thus indicating transition through the MHD stability boundary in the cusp-anchored gas-dynamic trap.

Novosibirsk Project of Gas-Dynamic Multiple-Mirror Trap

Development of a new linear device for confinement of fusion plasmas is under way in the Budker Institute of Nuclear Physics, Novosibirsk. The new device combines features of existing GOL-3 and GDT traps, namely, the central GDT-like cell with sloshing ions produced by intense neutral beam injection, and the multiple-mirror end sections for suppression of axial plasma losses. It is designed as a prototype of an energy-efficient neutron source and a testbed for development of mirror-based fusion reactors.

Fast ion confinement and stability in a neutral beam injected reversed field pinch

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3–5% D2) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfvenic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial...

Experimental investigation of coherent structures in a low-energy electron beam

A sharp transition to a space-charge dominated regime is induced in a low-energy electron beam produced in a Malmberg–Penning trap by increasing the emission current of the source. The transition is characterized by the appearance of a region, around the axis of the beam, not accessible to beam electrons, and by the fast development of coherent structures in the remaining electron plasma, due to the sharp increase of local vorticity. The results are interpreted in the framework of a cold fluid drift–Poisson model, and using a three-dimensional particle-in-cell simulation code.

A first step in the development of a powerful 14 MeV neutron source

This paper reviews the latest results of the numerical optimization of the powerful 14 MeV neutron source based on gas dynamic trap (GDT). Further experiments on the existing GDT device in Novosibirsk, which are planned to prove the key physical issues of the plasma confinement in the neutron source, are also discussed here.

Neoclassical transport and plasma mode damping caused by collisionless scattering across an asymmetric separatrix

Plasma loss due to apparatus asymmetries is a ubiquitous phenomenon in magnetic plasma confinement. When the plasma equilibrium has locally-trapped particle populations partitioned by a separatrix from one another and from passing particles, the asymmetry transport is enhanced. The trapped and passing particle populations react differently to the asymmetries, leading to the standard 1/ν and ν transport regimes of superbanana orbit theory as particles collisionally scatter from one orbit type to another. However, when the separatrix is itself asymmetric, particles can collisionlessly transit from trapped to passing and back, leading to the enhanced diffusion and mobility that is calculated here. The effect of this collisionless scattering across an asymmetric separatrix on the damping rate of trapped particle diocotron modes is also considered.

Full particle orbit tracing with the RIO code in the presence of broad-spectrum MHD activity in a reversed-field pinch

In order to better understand the behaviour of both neutral beam injected and spontaneously generated fast ions in the Madison Symmetric Torus reversed-field pinch, we have developed the full orbit-following code random ion orbits (RIO). The low magnetic field and relatively large level of MHD activity present in MST require a full orbit code as the guiding centre assumptions are violated even for ions with modest energy. Furthermore, quasi-periodic bursts of MHD activity (sawteeth) generate large transient electric fields and significant modifications to the equilibrium magnetic fields. Understanding the full effect of these sawteeth on the spatial and velocity distribution of the fast ions is of great interest. To this end, RIO now has the ability to take the full 3D, time evolving, magnetic and electric fields produced by the visco-resistive MHD code DEBS as input. In static cases, where broad-spectrum magnetic perturbations from DEBS are input, but fixed in time, beam injected ions are found to be generally well confined with the core fast ion density profile largely unaffected by the magnetic modes while the fast ion density in the mid-radius is substantially reduced. In the dynamic case, the large amplitude magnetic fluctuations that occur at the sawtooth crash produce substantial fast ion loss. Those fast ions that are not lost are accelerated by a large, transient, parallel electric field in the co-current direction. This causes the average energy of the beam ions to increase by ∼20%, consistent with recent experimental measurements.

Formation of a Narrow Radial Density Profile of Fast Ions in the GDT Device

The radial density profile of fast ions with a mean energy of 10 keV is measured in experiments with a two-component high-β plasma in the GDT device. Fast ions are produced by injecting neutral beams into a warm plasma. The measured fast-ion density profile is found to be narrower than that calculated with allowance for the neutral beam trapping and Coulomb scattering. Special experiments with a movable limiter have indicated that the formation of a narrow fast-ion density profile in GDT cannot be attributed to the loss of fast ions. Possible mechanisms responsible for this effect are discussed.

Alfvén Ion-Cyclotron Instability in a Mirror Trap with Skew Injection of Neutral Beams

The Alfvén ion cyclotron (AIC) instability margin in a mirror trap with skew injection of fast neutral beams into a target plasma is investigated in the present work. The instability is driven by inverse population of trajectories of resonant ions having velocity near the injection velocity. So, the stability margin depends strongly on injection details, in particular an injection angle and angular width. The absolute instability margin analysis as well as WKB-analysis on longitudinal and transversal coordinates are used for the stability threshold studying. Simple estimations relating the wave parameters to injection parameters are presented. The stabilizing effect of strong transversal non-uniformity is shown.