V. V. Maximov

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.

RESULTS OF RECENT EXPERIMENTS ON GDT DEVICE AFTER UPGRADE OF HEATING NEUTRAL BEAMS

Abstract The status of the experiments on the axially symmetric magnetic mirror device gas dynamic trap (GDT) is discussed. The plasma has been heated by skewed injection of 20-keV, 3.5-MW, 5-ms deuterium/hydrogen neutral beams at the center of the device, which produces anisotropic fast ions. Neither enhanced transverse losses of the plasma nor anomalies in the fast ion scattering and slowing down were observed. Extension of neutral beam injection pulse duration from 1 to 5 ms resulted in an increase in the on-axis transverse beta (ratio of the transverse plasma pressure to magnetic field pressure) from 0.4 at the fast ion turning points near the end mirrors to about 0.6. The measured beta value is rather close to or even higher than that expected in different versions of the GDT-based 14-MeV neutron source for fusion materials testing. The density of fast ions with the mean energy of 10 to 12 keV reached 5 × 1019 m−3 near the turning points. The electron temperature at the same time reached ≈200 eV. The radial plasma losses were suppressed by sheared plasma rotation in the periphery driven by biasing of end wall segments and the radial limiter in the central solenoid.

Spatial profiles of fusion product flux in the gas dynamic trap with deuterium neutral beam injection

Recently, a plasma with energetic deuterons has been produced in the gas dynamic trap (GDT) experiment under skew injection of 4 MW, 15–17 keV deuterium neutral beams. The GDT is a long, axially symmetric magnetic mirror device with a high mirror ratio. The deuterium neutral beams have been injected at the mid-plane of the device under 45° to the axis. High anisotropy of the fast ion angular distribution results in a strong peaking of the fast ion density at the turning points near the end mirrors. The axial profile of DD fusion product fluxes has been measured and found to be strongly peaked in the same regions. The characteristics of the profiles are consistent with the classical mechanism of fast ion relaxation caused by two-body Coulomb collisions with plasma particles. This observation validates an approach used in a GDT based neutron source, in which the regions of high neutron flux would be surrounded by the testing zones for fusion material irradiations.

Confinement of Hot Ion Plasma with β = 0.6 in the Gas Dynamic Trap

Abstract A so called vortex confinement of plasma in axially symmetric mirror device was studied. This recently developed approach enables to significantly reduce transverse particle and heat losses typically caused by MHD instabilities which can be excited in this case. Vortex confinement regime was established by application of different potentials to the radial plasma limiters and end-plates. As a result, the sheared plasma flow at periphery appears which wraps the plasma core. Experiments were carried out on the gas dynamic trap device, where hot ions with a mean energy of Eh ≈ 9 keV and the maximum density of energetic ions nh ≈ 5·1019m-3 were produced by oblique injection of deuterium or hydrogen neutral beams into a collisional warm plasma with the electron temperature up to 250 eV and density nw ≈ 2·1019m-3. Local plasma β approaching 0.6 was measured. The measured transverse heat losses were considerably smaller than the axial ones. The measured axial losses were found to be in a good agreement with the results of numerical simulations. Recent experimental results support the concept of the neutron source based on the gas dynamic trap.

Progress in Mirror-Based Fusion Neutron Source Development

The Budker Institute of Nuclear Physics in worldwide collaboration has developed a project of a 14 MeV neutron source for fusion material studies and other applications. The projected neutron source of the plasma type is based on the gas dynamic trap (GDT), which is a special magnetic mirror system for plasma confinement. Essential progress in plasma parameters has been achieved in recent experiments at the GDT facility in the Budker Institute, which is a hydrogen (deuterium) prototype of the source. Stable confinement of hot-ion plasmas with the relative pressure exceeding 0.5 was demonstrated. The electron temperature was increased up to 0.9 keV in the regime with additional electron cyclotron resonance heating (ECRH) of a moderate power. These parameters are the record for axisymmetric open mirror traps. These achievements elevate the projects of a GDT-based neutron source on a higher level of competitive ability and make it possible to construct a source with parameters suitable for materials testing today. The paper presents the progress in experimental studies and numerical simulations of the mirror-based fusion neutron source and its possible applications including a fusion material test facility and a fusion-fission hybrid system.

Electron Cyclotron Resonance Heating Experiment in the GDT Magnetic Mirror: Recent Experiments and Future Plans

Abstract A system for electron cyclotron resonance plasma heating (ECRH) has been recently installed at the GDT (Gas Dynamic Trap) facility at Budker Institute. The system is based on two 5.5-mm gyrotrons and is designed to deliver two microwave beams with total power of 700 kW and X-mode polarization that are absorbed at the fundamental cyclotron harmonic. A significant increase of basic plasma parameters (energy content, electron temperature, neutron flux) during the injection of microwave radiation has been registered. In particular, the on-axis electron temperature was increased from 200 eV to 600 eV in several shots with ECRH, which establishes a new record for this class of magnetic installation.

The GDT Experiment: Status and Recent Progress in Plasma Parameters

Abstract This paper presents a brief review of experimental results obtained on the Gas Dynamic Trap (GDT) device during the last few years. Special attention is paid to the problems of longitudinal plasma confinement and suppression of transverse transport caused by magnetohydrodynamic instabilities in mirror traps with an axisymmetric magnetic field configuration. We also consider problems of auxiliary electron cyclotron resonance heating in the GDT plasma. Electromagnetic fluctuations driven by anisotropic high pressure plasma in GDT will be discussed as well as influence of these fluctuations on plasma confinement.

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.

Recent progress of plasma confinement and heating studies in the gas dynamic trap

The paper includes a brief overview of previous researches on the stabilization of MHD instabilities, study of micro-instabilities, and demonstration a tangible increase of the electron temperature with application of auxiliary ECR heating. A review of the results of recent researches related to application of microwave radiation for plasma generation, and plasma heating in the GDT device is presented. The paper summarizes also recent results of researches that oriented on study of expander physics.