Tandem Mirror Experiment Upgrade Research Articles

ECRH in Tandem Mirror Machines

Electron cyclotron resonance heating (ECRH) is important in tandem mirror devices which incorporate thermal barriers to enhance plasma confinement. ECRH in the end cells generates hot mirror-confined electrons to form the thermal barrier and creates the plugging potential to reduce ion end loss from the central cell. Important considerations for heating include a) the proper choice of wave polarization and harmonic number for wave penetration and efficient absorption, b) the possibility for control of hot electron energy and anisotropy by spatially limited heating, and c) the particle feed for hot electrons due to RF trapping processes. Fokker-Planck and Monte Carlo computer codes are useful for understanding these effects. The Tandem Mirror Experiment Upgrade (TMX-U) was constructed to investigate plasma confinement for the thermal barrier mode of operation. We carried out experiments to generate the hot electrons that are required for the thermal barrier. Heating efficiencies as large as 30 percent were observed. By combining the hot electron population with neutral-beam-injected hot ions and ECRH at the location of the desired potential peak, we strongly reduced the end loss of ions and measured plugging potentials up to 700 V. These results are consistent with the thermal barrier model.

Characteristics of the neutral beam injectors for the Tandem Mirror Experiment Upgrade

Neutral beams are used to fuel and heat the Tandem Mirror Experiment Upgrade (TMX–U) at LLNL. We have studied these beams spectroscopically to determine their species mix (H, H2, H+3) and impurity content (H2O). From the shapes of the spectral lines, we measured the divergences of the various components and compared them with the numerically determined divergence. By measuring the area under the various Doppler-shifted lines, we determined the species mix and found that the atomic species fraction is lower than expected (30% to 45% rather than 60%). Although there is good agreement between the measured and calculated divergence for the atomic component of the beam, the molecular components have larger divergences, part of which can be accounted for by the dissociation of the molecules.

Measurement of sloshing-ion spatial profiles in end cell of tandem mirror experiment-upgrade (TMX-U)

Neutral-beam injection is used to establish an energetic sloshing-ion distribution in the end cells of the Tandem Mirror Experiment-Upgrade (TMX-U). The angular and radial distributions of these ions are measured with arrays of secondary-emission detectors. Unfolding the angular distribution gives the axial extent of the sloshing ions. Sloshing-ion density measurements from the charge-exchange flux indicate a high-density (nis≂3 to 6×1012 cm−3) but narrow (FWHM ∼16 cm) sloshing-ion plasma.

Dynamic gas flow during plasma operation in TMX-U

Control of the neutral density outside of the plasma radius is essential for proper operation of the various plasma configurations in Lawrence Livermore National Laboratory’s (LLNL) Tandem Mirror Experiment-Upgrade (TMX-U). TMX-U excess-beam, stream-gun, gas-box, and beam-reflux gases are pumped internally in regions defined by 73° Ti-gettered liners and warm Ti-gettered plasma liners. The array of fast and slow ion gauges—a large TMX-U diagnostic—has been used to measure the dynamic pressure in many of the liner-defined regions on three time scales. The natural divertor action, or plasma pump effect, of mirror plasmas has been measured using the ion gauge diagnostics on a fast time scale during operation of TMX-U with ECRH start up. Routine operation of TMX-U is enhanced by the ability to verify the effectiveness of gettering and to locate leaks using pressure data collected on the two slow time scales. A computer code, dynavac 6, which treats TMX-U as a set of conductance-coupled regions with pumping and sources in each region, has been used to successfully model the overall gas dynamics during all phases of TMX-U operation.

Magnetic Field Errors in Quadrupole Tandem Mirrors

Coil misalignments in quadrupole tandem mirrors generate dipole and quadrupole field errors. The effects of these errors on the magnetic field are calculated. Misalignments in the thin, quadrupole fans have particularly large effects on field lines. The results are applied to the Tandem Mirror Experiment (TMX) and to the TMX Upgrade.

Design of first walls and beam dumps for Tandem Mirror Experiment Upgrade

Neutral reflux from the first walls and beam dumps of the Tandem Mirror Experiment (TMX) Upgrade could erode the neutral-beam-fueled end-cell plasma or cool the neutral-beam-heated central-cell plasma. To allow the TMX Upgrade to meet its design parameters, the neutral reflux to the plasma must be much less than the fluxes of both charge-exchange neutral products to the first walls and transmitted neutral-beam atoms to the beam dumps. To achieve this, we intend to evaporate titanium as a getter on the first wall and install vanadium-foil beam dumps. The vanadium beam dumps will be heated by the neutral beam during the shot, releasing most of the implanted hydrogen as they cool radiatively after the shot. The deposited titanium will be thick enough to retain the implanted charge-exchange neutral products. We describe our design of these components, report our estimates of their performance, and compare their performance to the needs of TMX Upgrade.

The LLNL tandem mirror experiment (TMX) upgrade vacuum system

The tandem mirror experiment (TMX) upgrade is a large, tandem, magnetic‐mirror fusion experiment with stringent requirements on base pressure (10−8 Torr), low H reflux from the first walls, and peak gas pressure (5×10−7 Torr) due to neutral beam gas during plasma operation. The 225 m3 vacuum vessel is initially evacuated by turbopumps. Cryopumps provide a continuous sink for gases other than helium, deuterium, and hydrogen. The neutral beam system introduces up to 480 l/s of H or D. The hydrogen isotopes are pumped at very high speed by titanium sublimed onto two cylindrical radially separated stainless steel quilted liners with a total surface area of 540 m2. These surfaces (when cooled to about 80 K) provide a pumping speed of 6×107 l/s for hydrogen. The titanium getter system is programmable and is used for heating as well as gettering. The inner plasma liner can be operated at elevated temperatures to enhance migration of gases away from the surfaces close to the plasma. Glow discharge cleaning is part of the pumpdown procedure. The design features are discussed in conjunction with the operating procedures developed to manage the dynamic vacuum conditions.