K. McGuire

Attainment of high confinement in neutral beam heated divertor discharges in the PDX tokamak

Abstract The PDX divertor configuration has recently been converted from an open to a closed geometry to inhibit the return of neutral gas from the divertor region to the main chamber. Since then, operation in a regime with high energy confinement in neutral beam heated discharges (ASDEX H-mode) has been routine over a wide range of operating conditions. These H-mode discharges are characterized by a sudden drop in divertor density and H α emission and a spontaneous rise in main chamber plasma density during neutral beam injection. The confinement time is found to scale nearly linearly with plasma current, but can be degraded due either to the presence of edge instabilities or heavy gas puffing. Detailed Thomson scattering temperature profiles show high values of T c near the plasma edge (∼ 450 eV) with sharp radial gradients (∼ 400 eV/cm) near the separatrix. Density profiles are broad and also exhibit steep gradients close to the separatrix.

Plasma rotation in the PDX tokamak

Toroidal and poloidal rotation has been measured in the Poloidal Divertor Experiment (PDX) tokamak in Ohmic and neutral-beam heated plasmas in a variety of discharge conditions and in both circular and diverted configurations. Rotation velocities were deduced from Doppler shifts of magnetic dipole (M1) lines and lines of optically allowed transitions in the visible and UV regions, from Kα emission, and also from an array of magnetic pickup loops. Poloidal and toroidal rotation velocities in Ohmically heated discharges were usually less than 3 × 105 cms−1. Near the plasma edge, the toroidal rotation velocity varies with poloidal angle both before and during neutral-beam injection. No systematic poloidal rotation was observed during neutral-beam injection centred about or displaced 10 cm from the horizontal midplane, which implies that the poloidal damping time τθ < 0.5 τii consistent with theoretical estimates. The central toroidal rotation velocity during neutral-beam injection scales linearly with the quantity and is independent of plasma current and toroidal magnetic field. The toroidal rotation velocity is higher in deuterium than in hydrogen plasmas, and also in diverted discharges as compared with circular ones. Toroidal rotation decay times after injection range from 80–100 ms at the centre to 160–180 ms at half the minor radius. Modelling of the radial profile of toroidal rotation indicates a central momentum diffusivity of the order of 8 × 103 cm2s−1. This is approximately a factor of three higher than the momentum diffusivity obtained from the decay time. All present theories are inadequate in accounting for the observed damping rate of v.

Parallel electric resistivity in the TFTR tokamak

The average parallel resistivity and the location of the q=1 surface are found to be consistent with the predictions of neoclassical transport theory and inconsistent with classical resistivity (uncorrected for toroidal effects) for Ohmic plasmas in the TFTR tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 51], both in near‐equilibrium and during ramping of the plasma current. These observations are incompatible with theories predicting anomalous parallel resistivity in concert with anomalous perpendicular transport.

High poloidal beta equilibria in the Tokamak Fusion Test Reactor limited by a natural inboard poloidal field null

Recent operation of the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Research 1, 51 (1986)] has produced plasma equilibria with values of Λ≡βp eq+li/2 as large as 7, eβp dia≡2μ0e〈p⊥〉/〈〈Bp〉〉2 as large as 1.6, and Troyon normalized diamagnetic beta [Plasma Phys. Controlled Fusion 26, 209 (1984); Phys. Lett. 110A, 29 (1985)], βNdia≡108〈βt⊥〉aB0/Ip as large as 4.7. When eβp dia≳1.25, a separatrix entered the vacuum chamber, producing a naturally diverted discharge that was sustained for many energy confinement times, τE. The largest values of eβp and plasma stored energy were obtained when the plasma current was ramped down prior to neutral beam injection. The measured peak ion and electron temperatures were as large as 24 and 8.5 keV, respectively. Plasma stored energy in excess of 2.5 MJ and τE greater than 130 msec were obtained. Confinement times of greater than 3 times that expected from L‐mode predictions have been achieved. The fusion power gain QDD reached a value of 1.3×10−...

Tomography of full sawtooth crashes on the Tokamak Fusion Test Reactor

Full sawtooth crashes in high temperature plasmas have been investigated on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)]. A strong asymmetry in the direction of major radius, a feature of the ballooning mode, and a remaining m=1 region after the crash have been observed with electron cyclotron emission image reconstructions. The TFTR data is not consistent with two‐dimensional (2‐D) models; it rather suggests a three‐dimensional (3‐D) localized reconnection arising on the bad curvature side. This process explains the phenomenon of fast heat transfer which keeps the condition q0<1.

Overview of TFTR transport studies

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

Correlations of heat and momentum transport in the TFTR tokamak

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low‐recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad‐density L‐mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test‐particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked‐density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In hi...

Impurity levels and power loading in the pdx tokamak with high power neutral beam injection

Abstract The PDX tokamak provides an experimental facility for the direct comparison of various impurity control techniques under reactor-like conditions. Four neutral beam lines inject > 6 MW for 300 ms. Carbon rail limiter discharges have been used to test the effectiveness of perpendicular injection, but non-disruptive full power operation for > 100 ms is difficult without extensive conditioning. Initial tests of a toroidal bumper limiter indicate reduced power loading and roughly similar impurity levels compared to the carbon rail limiter discharges. Poloidal divertor discharges with up to 5 MW of injected power are cleaner than similar circular discharges, and the power is deposited in a remote divertor chamber. High density divertor operation indicates a reduction of impurity flow velocity in the divertor and enhanced recycling in the divertor region during neutral injection.

β limit disruptions in the Tokamak Fusion Test Reactor

A disruptive β limit (β=plasma pressure/magnetic pressure) is observed in high‐performance plasmas in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Plasma Phys. Controlled Nuclear Fusion 1, 421 (1987)]. The magnetohydrodynamic character of these disruptions differs substantially from the disruptions in high‐density plasmas (density limit disruptions) on TFTR. The high β disruptions can occur with less than a millisecond warning in the form of a fast growing precursor. The precursor appears to be an n=1 kink strongly coupled through finite β effects and toroidal terms to higher m components. It does not have the ‘‘cold bubble’’ structure found in density limit disruptions. The n=1 kink, in turn, appears to excite a ballooning‐type mode that may contribute to the thermal quench.

Studies of energetic ion confinement during fishbone events in PDX

The 2.5 MeV neutron emission from the beam-target d(d, n)3He fusion reaction has been examined for all PDX deuterium plasmas which were heated by deuterium neutral beams. The magnitude of the emission was found to scale classically and to increase with as expected when electron drag is the primary energy degradation mechanism. The time evolution of the neutron emission through fishbone events was measured and used to determine the confinement properties of the energetic beam ions. Many of the experimental results are predicted by the Mode Particle Pumping Theory.