George C. Vlases

Pellet Injection with Improved Confinement in ASDEX

Tokamak discharges with repetitive pellet fuelling were investigated in the ASDEX divertor device. The importance of sufficiently high divertor recycling for a high density at the separatrix and for successful density buildup in the bulk plasma was demonstrated. In contrast to low recycling discharges where no permanent improvement of the energy confinement time was achieved, in OH-heated discharges with high recycling an energy confinement time of 160 ms was reached, the normal value being 80 ms in the rollover region. The peaked density profiles in this case were accompanied by reduced or suppressed sawtooth activity and finally ended in a phase of strong central impurity accumulation. The particle transport was characterized by strong, non-classical inward drift, while the improved energy transport can be explained by the following alternatives: (la) a local model which assumes neo-Alcator χe for the electrons and χi= 3χneocl for the ions in the gas puff cases, reducing to χi= χneocl for the optimum pellet cases; (1b) the assumption χi= χneocl under all conditions and an electron energy confinement worse than neo-Alcator in the rollover region in gas-puff-discharges; (2) a profile consistency picture where Te(a) determines the energy confinement. Low power, NI heated discharges with pellet fuelling behave like Ohmic discharges, while for high power in the L-mode no successful density buildup was possible, and τE was not improved. The H-regime was extended from a density maximum e = 0.8 × 1020 m-3 without pellets to e = 1.2 × 1020 m-3 by the injection of pellets. In this case a density buildup takes place, but further density profile peaking could not be observed.

CO2 laser heating of plasma columns in a steady solenoid field

A 250−J CO2 laser has been used to produce plasma columns 12−20 cm in length, 2−4 mm in diameter, from initially uniform neutral hydrogen in a steady solenoidal field up to 100 kG. Interferometric and spectroscopic measurements indicate densities and temperatures on the order of 100 eV and 5×1017 cm−3, respectively. The backward−going laser−driven wave normally present in laser breakdown has been eliminated by pulsed filling of the tube.

Trapping and absorption of an axially directed CO2 laser beam by a θ‐pinch plasma

A deuterium‐filled θ pinch was axially irradiated in the first half‐cycle by a CO2 laser to demonstrate the effects of beam trapping and absorption by a plasma. Peak electron densities were 5×1016 cm−3 and the nonirradiated axial temperature was about 1.3 eV. Detection of the laser radiation transmitted through the plasma showed that beam trapping occurred when the density profile had a minimum on axis, and the maximum laser‐induced temperature increase of 2 eV occurred for irradiation at the time of maximum density, in agreement with inverse bremsstrahlung predictions. Furthermore, the plasma emission histories indicated that the column underwent a laser‐induced elongation of up to twice its original length.

Numerical Modelling of Pulsed Electric CO2 Lasers

A simple four‐level kinetics model is combined with a cavity intensity model to predict the output pulse characteristics of pulsed electric discharge CO2 lasers for pressures ranging from 0.1 to 10 atm. Both free‐running (gain‐switched) and Q‐switched cases are studied. Particular emphasis is placed on peak intensity and energy content of the initial spike as a function of pumping rate, gain at time of Q swithcing, mixture, and pressure.

Heating of Pinch Devices with Lasers

A simple thermodynamic model is used to determine the change in plasma density and temperature and in confining magnetic field strength when a θ or Z pinch is subjected to strong laser heating.

Improved confinement and current drive of high temperature field reversed configurations in the new translation, confinement, and sustainment upgrade devicea)

Previous work in the translation, confinement, and sustainment (TCS) device [Hoffman, Guo, Slough et al., Fusion Sci. Technol. 41, 92 (2002)] demonstrated formation and steady-state sustainment of field reversed configurations (FRC) by rotating magnetic fields (RMF). However, in TCS the plasma temperature was limited to several 10s of eV due to high impurity content. These impurities are greatly reduced in the new TCS upgrade device (TCSU), which was built with a bakable, ultrahigh vacuum chamber, and advanced wall conditioning capabilities. This led to dramatic improvements in TCSU with temperatures well over 200eV, using simple even-parity RMF drive. The higher temperatures, coupled with reduced recycling, allowed plasma to enter into a collisionless, high-ζ (ratio of average electron rotation frequency to RMF frequency) regime. These new FRC states exhibit the following key features: (1) Dramatic improvement in current drive efficiency with ζ approaching 100%, for the first time in TCSU; (2) up to thre...

Thomson scattering from a CO2−laser−heated magnetically confined plasma

Measurements of the interactions of CO2 laser radiation with a theta pinch plasma have been made using laser scattering as a diagnostic technique. The plasma was heated axially with a TEA CO2 pin laser. The scattering measurements have shown the time evolution of the laser−heated plasma during and after heating by the ’’spike’’ of the CO2 laser. The energies involved in the change of plasma conditions agree well with the theoretical absorption associated with the inverse bremsstrahlung mechanism.

Correlation of pellet penetration depths on ASDEX for Ohmic and auxiliary heated discharges

Penetration depths for frozen D2 pellets into ASDEX ohmically and auxiliary heated discharges are reported for a wide range of plasma and pellet parameters. It is found that the data are well correlated by a single parameter (pellet diameter, pellet velocity, central electron density and temperature), which emerges from the neutral shielding model. The pellet depths, however, are systematically smaller than is expected from the model, implying a higher ablation rate in the outer plasma layers.

Transverse diffusion of particles and energy in a laser-heated plasma column

In a laser‐heated magnetically confined column of plasma separated from material walls by a vacuum field, laser energy absorption near the center leads to a ’’light pipe’’ effect essential to the concept of heating long columns to fusion temperatures. Outward diffusion of particles at the vacuum‐plasma boundary tends to move the position of maximum density inward, and hence to narrow the light pipe, while over‐all expansion of the heated column against the field tends to widen it. A model is formulated to describe these effects and is treated analytically and numerically. The results indicate that for conditions of interest, the light pipe will persist long enough to permit heating of reactor‐scale columns.

Staging of theta pinches using laser‐heat addition and magnetic compression

The thermodynamics of using laser heating in conjunction with magnetic compression in θ‐pinch plasmas is discussed. It is shown that the minimum laser energy required to achieve a given plasma filling factor Λ (ratio of plasma to plasma tube area) is Elaser = Λ2/3Eplasma and that any filling factor up to Λ = 1 can be achieved. The proposed schemes have important implications for either increasing the performance of standard θ pinches or for replacing the single‐turn θ‐pinch coil by a multiturn solenoid.