Princeton Large Torus Research Articles

Turbomolecular pump vacuum system for the Princeton Large Torus

A turbomolecular pump vacuum system has been designed and installed on the Princeton Large Torus (PLT). Four vertical shaft, oil-bearing, 1500-l/s turbomolecular pumps have been interfaced to the 6400-l PLT vacuum vessel to provide a net pumping speed of 3000 l/s for H2. The particular requirements and problems of tokamak vacuum systems are enumerated. A vacuum control system is described which protects the vacuum vessel from contamination and protects the turbomolecular pumps from damage under a variety of possible failure modes. The performance of the vacuum system is presented in terms of pumping speed mesurements and residual gas behavior.

Calculations of impurity radiation and its effects on tokamak experiments

The impurity radiation for typical tokamak parameters has been numerically calculated using an “average-ion model”. Coronal equilibrium values for the emission of oxygen, iron, molybdenum, tungsten and gold were determined from the steady-state solutions of a set of related rate equations which included the effects of electron collisional ionization and excitation, dielectronic and radiative recombination, Δn = 0 and Δn ≠ 0 line transitions, and bremsstrahlung. The results for oxygen, iron, and molybdenum compare very well with other calculations. Since impurities diffusing in a tokamak are not expected to be in coronal equilibrium, time-dependent radiation calculations were also performed. A comparison of these non-equilibrium loss rates with those calculated under the assumption of coronal equilibrium indicates that coronal radiation calculations do not significantly underestimate the moderate- and high-Z impurity radiation losses for neoclassical diffusion velocities in large tokamaks, such as Princeton Large Torus and Tokamak Fusion Test Reactor. Finally, the detailed steady-state emission rates were used to investigate the effects of various concentrations of impurities on the neτ requirements for breakeven, ignition, and Q = 5 beam-driven reactor experiments.

Are generalized or unified criteria possible?

The Tokamak Fusion Test Reactor (TFTR). the key toroidal confinement device of the US fusion energy research program, is directed toward the goal of demonstrating energy break-even under DT conditions by 1988. TFTR now is completing its second year of research operation. In this period it has extended the study of scaling in ohmic heated plasmas and found scaling to vary with the cube of length including observations relative to that of the Princeton Large Torus (PLT) tokamak; it has explored impurity effects and the influence of neutral beam heating on confinement. The ongoing experimental campaign is being conducted with plasma/machine parameters up to BT ~ 40 kG, Ip ~ 1.5 MA, PNB ~ 2 MW(H) and τNB ~ 0.3 s with τc up to 0.3 s. With completion of this two year start-up phase the TFTR will be operated for a two-year period of machine optimization concluding with Q ~ 1 for DD operation, followed by a two year phase of plasma optimization culminating with DT operation at Q ~ 1 in 1988. A subsequent phase of D-T research is under consideration.

Abstract: Surface analysis measurements in the Princeton Large Torus (PLT)

Abstract: Surface analysis measurements in the Princeton Large Torus (PLT)

Changes in surface conditions with first plasma in the princeton large torus (PLT)

We report in-situ measurements of the changes in surface composition of 305 stainless steel exposed to the first plasma discharges in PLT. Auger electron spectroscopy of pre-exposure surfaces shows a contamination by carbon and smaller amounts of oxygen. Surfaces subjected to increasing levels of plasma exposure show a substantial decrease of the carbon coverage, increased fractional-monolayer coverages of oxygen and iron and tenth-monolayer quantities of chlorine and sulfur.

Changes in surface conditions with first plasma in the princeton large torus (PLT)

Changes in surface conditions with first plasma in the princeton large torus (PLT)

Non-linear saturation of the dissipative trapped-ion mode by mode coupling

The non-linear saturation of the dissipative trapped-ion mode is analysed. The basic mechanism considered is the process whereby energy in long-wavelength unstable modes is non-linearly coupled via E⃗×B⃗ convection to shortwavelength modes stabilized by Landau damping due to both circulating and trapped ions. In the usual limit of the mode frequency being small relative to the effective electron collision frequency, a one-dimensional non-linear partial differential equation for the potential can be derived, as was first shown by LaQuey, Mahajan, Tang, and Rutherford. The stability and accessibility of the possible equilibria for this equation are examined in detail, both analytically and numerically. The equilibrium emphasized by LaQuey et al. is shown to be unstable. However, a class of non-linear saturated states which are stable to linear perturbations is found. Included in the analysis are the effects of both ion collisions and dispersion due to finite-ion-banana-width effects. Cross-field transport is estimated and the scaling of the results is considered for tokamak parameters (specifically those for the Princeton Large Torus). It is concluded that the anomalous cross-field transport can be much lower than the estimate of Kadomtsev and Pogutse, for relevant parameters near marginal stability for the linear modes.

Resistive instabilities in a tokamak

Application of resistive instability theory shows that toroidal effects can stabilize the tearing mode in devices like the Princeton Large Torus. Contraction of the current channel is destabilizing. Finite fluid compressibility is crucial to this phenomenon.

Ion-cyclotron waves in nonuniform plasmas and parametric instabilities

A general formalism for studying waves propagating in a plasma with density and magnetic field gradients in slab geometry is discussed. Specializing to electrostatic ion-cyclotron waves propagating perpendicular to the gradients, it is shown that a density profile can lead to local or nonlocal structure around points where N′ (x)/N (x) has a local maximum while inhomogeneous magnetic fields provide the wave with the profile of an Airy function. Application of the formalism to parametric instabilities driven by magnetosonic waves and with kyρi≫1 is discussed. In particular, it is concluded that density and magnetic field gradients should not be critical in the development of the cyclotron decay instability in the FM-1 (floating multipole-1), ATC (adiabatic toroidal compressor), and PLT (Princeton large torus) devices of Princeton.

Vacuum and wall problems in precursor reactor tokamaks

The Princeton Large Torus (PLT) will be completed in 1975 and two other CTR-oriented toroidal plasma devices, the Poloidal Divertor Experiment (PDX) and the Tokamak Fusion Test Reactor (TFTR), are planned to be completed in 1977 and 1981. The vacuum systems of these machines must satisfy stringent requirements because of unusual operating conditions, such as, magnetic-field-induced strains and eddy currents, energetic particle and photon bombardment, large transient gas loads, and the use of 100 Ci quantities of tritium. In addition, novel vacuum wall surfaces and fast moving mechanical or magnetic plasma limiters will be required to minimize the influx of impurities during discharges.

Effect of toroidal gradient drifts on the dissipative trapped-ion instability

Conditions for the onset of the dissipative trapped-ion instability in tokamaks with elliptic and circular cross sections are determined. Toroidal gradient drifts, ignored in earlier calculations, are included in the analysis and found to give rise to significant stabilizing effects. In particular, the results show that the ions trapped in regions of good gradient (dB/dρ > 0) contribute to the Landau damping of the unstable waves, and that ion collisional damping is also enhanced when the drifts are taken into account. Since vertical ellipticity increases the region of good gradient, these stabilizing effects are further enhanced in elliptical cross-section tokamaks. It is also shown that previously calculated restrictions on the allowable magnitude of temperature gradients for favorable damping can be relaxed. Results of the analysis are applied to expected parameters for the Princeton Large Torus experiment and growth rate estimates given for conditions violating the stability criteria.

Effect of ellipticity on the collisional trapped-particle instability

Stability criteria for the collisional trapped-particle instability in an axisymmetric, large-aspect-ratio tokamak with an elliptic cross-section are derived. Comparison with the circular configuration indicates that an increase in the vertical ellipticity can lead to a factor-of-two reduction in the critical density below which the mode is unstable. The improvement is a result of an enhancement of both ion Landau damping and ion collisional damping in the boundary layer of the trapped ions. Collisions are treated via the Fokker-Planck operator and the resulting problem is solved, as in earlier work, by a variational method. Unlike previous treatments of resonant effects, Landau damping by trapped rather than untrapped ions is considered, and the damping rate is found to be approximately the same in magnitude as the untrapped rate derived in earlier work. Results are applied to design parameters for Princeton's PLT (Princeton Large Torus) device and for a reactor, and stability conditions for several tokamaks presently in operation are also determined.