D. McCune

New techniques for calculating heat and particle source rates due to neutral beam injection in axisymmetric tokamaks

A set of numerical techniques for calculating heat and particle source rates due to neutral beam injection in axisymmetric tokamaks is described. While these techniques consume a substantial amount of computer time, they take into account a number of significant, and normally neglected, effects. Examples of these effects are reionization of escaping charge exchanged beam particles, finite fast ion orbit excursions, beam deposition through collisions of beam neutrals with circulating beam ions, and the transport of thermal neutrals in the plasma due to charge changing collisions with beam ions.

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.

Simulations of deuterium-tritium experiments in TFTR

A transport code (TRANSP) is used to simulate future deuterium-tritium (DT) experiments in TFTR. The simulations are derived from 14 TFTR DD discharges, and the modelling of one supershot is discussed in detail to indicate the degree of accuracy of the TRANSP modelling. Fusion energy yields and alpha particle parameters are calculated, including profiles of the alpha slowing down time, the alpha average energy, and the Alfven speed and frequency. Two types of simulation are discussed. The main emphasis is on the DT equivalent, where an equal mix of D and T is substituted for the D in the initial target plasma, and for the D0 in the neutral beam injection, but the other measured beam and plasma parameters are unchanged. This simulation does not assume that alpha heating will enhance the plasma parameters or that confinement will increase with the addition of tritium. The maximum relative fusion yield calculated for these simulations is QDT ~ 0.3, and the maximum alpha contribution to the central toroidal β is βα(0) ~ 0.5%. The stability of toroidicity induced Alfven eigenmodes (TAE) and kinetic ballooning modes (KBM) is discussed. The TAE mode is predicted to become unstable for some of the simulations, particularly after the termination of neutral beam injection. In the second type of simulation, empirical supershot scaling relations are used to project the performance at the maximum expected beam power. The MHD stability of the simulations is discussed

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.

Ion cyclotron emission measurements during JET deuterium-tritium experiments

In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) was measured in the frequency range v ≤ 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines, which correspond to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to tile last closed flux surface at major radius R approximately 4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs. In pure deuterium and mixed DT discharges ICE spectra are similar in form, but on changing from pure D to mixed D+T neutral beam injection at constant power, the intensity of the ICE rises in proportion to the increased neutron flux: this indicates that fusion alpha particles-and not beam ions-provide the free energy to generate ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time averaged ICE power increases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE signal, which is delayed by a time of the order of the fusion product slowing down time. This feature is well modelled by a TRANSP code simulation of the density of deeply trapped fusion products reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer midplane edge. There, the velocity distribution has a ring structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfven-ion Bernstein branch at all ion cyclotron harmonics. The paper shows how ICE provides a unique diagnostic for fusion alpha particles

Simulations of alpha parameters in a TFTR DT supershot with high fusion power

A TFTR supershot with a plasma current of 2.5 MA, a neutral beam heating power of 33.7 MW and a peak DT fusion power of 7.5 MW is studied using the TRANSP plasma analysis code. Simulations of alpha parameters such as the alpha heating, pressure and distributions in energy and v1/v are given. The effects of toroidal ripple and mixing of the fast alpha particles during the sawteeth observed after the neutral beam injection phase are modelled. The distributions of alpha particles on the outer midplane are peaked near forward and backward v1/v. Ripple losses deplete the distributions in the vicinity of v1/v=-0.2. Sawtooth mixing of fast alpha particles is computed to reduce their central density and broaden their width in energy

Predictions of H-mode performance in ITER

Time-dependent integrated predictive modelling is carried out using the PTRANSP code to predict fusion power and parameters such as alpha particle density and pressure in ITER H-mode plasmas. Auxiliary heating by negative ion neutralbeaminjectionandion-cyclotronheatingofHe 3 minorityionsaremodelled,andtheGLF23transportmodelis used in the prediction of the evolution of plasma temperature profiles. Effects of beam steering, beam torque, plasma rotation, beam current drive, pedestal temperatures, sawtooth oscillations, magnetic diffusion and accumulation of He ash are treated self-consistently. Variations in assumptions associated with physics uncertainties for standard base-line DT H-mode plasmas (with Ip = 15MA, BTF = 5.3T and Greenwald fraction = 0.86) lead to a range of predictions for DT fusion power PDT and quasi-steady state fusion QDT (≡PDT/Paux). Typical predictions assuming Paux = 50‐53MW yield PDT = 250‐720MW and QDT = 5‐14. In some cases where Paux is ramped down or shut off after initial flat-top conditions, quasi-steady QDT can be considerably higher, even infinite. Adverse physics assumptions such as the existence of an inward pinch of the helium ash and an ash recycling coefficient approaching unity lead to very low values for PDT. Alternative scenarios with different heating and reduced performance regimes are also considered including plasmas with only H or D isotopes, DT plasmas with toroidal field reduced 10% or 20% and discharges with reduced beam voltage. In full-performance D-only discharges, tritium burn up is predicted to generate central tritium densities up to 10 16 m −3 and DT neutron rates up to 5 ×10 16 s −1 , compared with the DD neutron rates of 6 × 10 17 s −1 . Predictions with the toroidal field reduced 10% or 20% below the planned 5.3T and keeping the same q98, Greenwald fraction and βn indicate that the fusion yield PDT and QDT will be lower by about a factor of two (scaling as B 3.5 ).

The diffusion of fast ions in Ohmic TFTR discharges

Short duration (20 msec) neutral deuterium beams are injected into the TFTR tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 51]. The subsequent confinement, thermalization, and diffusion of the beam ions are studied with multichannel neutron and charge exchange diagnostics. The central fast‐ion diffusion (<0.05 m2/sec ) is an order of magnitude smaller than typical thermal transport coefficients.

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...