S. S. Medley

Alpha Particle Physics Experiments in the Tokamak Fusion Test Reactor

Alpha particle physics experiments were done on TFTR during its DT run from 1993 to 1997. These experiments utilized several new alpha particle diagnostics and hundreds of DT discharges to characterize the alpha particle confinement and wave-particle interactions. In general, the results from the alpha particle diagnostics agreed with the classical single particle confinement model in MHD quiescent discharges. The alpha loss due to toroidal field ripple was identified in some cases, and the low radial diffusivity inferred for high energy alphas was consistent with orbit averaging over small scale turbulence. Finally, the observed alpha particle interactions with sawteeth, toroidal Alfven eigenmodes and ICRF waves were approximately consistent with theoretical modelling. What was learned is reviewed and what remains to be understood is identified.

Studies of energetic confined alphas using the pellet charge exchange diagnostic on TFTR

Results from recent DT experiments on TFTR to measure the energy distribution and radial density profile of fast confined alphas with the use of Li pellets and neutral particle analysis are presented. When a pellet is injected into the plasma, a toroidally extended ablation cloud is formed. A small fraction of the fusion alphas incident on the cloud is converted to helium neutrals as a result of electron capture processes. The escaping energetic helium neutrals are analysed and detected by the neutral particle analyser. Radially resolved energy spectra of trapped confined alphas in 0.5-2 MeV range and radial alpha density profiles are presented in this paper. The experimental data are compared with modelling results obtained with the TRANSP Monte Carlo code and with a specially developed Fokker-Planck post-processor (FPP) that uses the alpha source distribution produced by TRANSP. Comparison of the experimental data with TRANSP and FPP shows that the alphas in the plasma core of sawtooth free discharges in TFTR are well confined and slow down classically. The energy and radial profiles distributions outside the plasma core show the influence of stochastic ripple losses on alphas. Measurements for sawtoothing plasmas show a significant outward radial transport of trapped alphas

Modelling TF ripple loss of alpha particles in TFTR DT experiments

Modelling of TF ripple loss of alphas in DT experiments on TFTR now includes neoclassical calculations of first orbit loss, stochastic ripple diffusion, ripple trapping and collisional effects. A rapid way to simulate experiment has been developed which uses a simple stochastic domain model for TF ripple loss within the TRANSP analysis code, with the ripple diffusion threshold evaluated by comparison with more accurate but computationally expensive Hamiltonian co-ordinate guiding centre code simulations. Typical TF collisional ripple loss predictions are 6-10% loss of alphas for TFTR DT experiments at Ip=1.0-2.0 MA and R=2.52 m

Experimental results from detached plasmas in TFTR

Abstract Detached plasmas are formed in TFTR which have the principal property that the boundary to the high temperature plasma core is defined by a radiating layer. This paper documents the properties of TFTR ohmic detached plasmas with a range of plasma densities at two different plasma currents.

Modelling of alpha particle slowing down, confinement and redistribution by sawteeth in TFTR using the FPPT code

Results from recent deuterium-tritium (DT) experiments on TFTR to measure the energy spectra and radial profiles of well trapped confined alpha particle distributions using the pellet charge exchange (PCX) diagnostic in quiescent plasmas are compared with a numerical modelling using the Fokker-Planck post-TRANSP (FPPT) processor code and show a classical slowing down behaviour of the alphas (Fisher, R.K., et al., Phys. Rev. Lett. 75 (1995) 846). However, in the presence of sawtooth oscillations PCX experimental data indicate a significant broadening of the trapped alpha radial distributions (Petrov, M.P., et al., Nucl. Fusion 35 (1995) 1437). Conventional models consistent with measured sawtooth effects on passing particles do not provide satisfactory simulations of the trapped particle mixing measured by the PCX diagnostic. A mechanism is proposed for fast particle mixing during the sawtooth crash to explain the trapped alpha particle radial profile broadening after the crash. The model is based on the fast particle orbit averaged toroidal drift in a perturbed helical electric field with an adjustable absolute value (similar to that in Kolesnichenko, Ya.I., et al., Nucl. Fusion 36 (1996) 159). Such a drift of the fast particles results in a change of their energy and a redistribution id phase space. To show the sensitivity of trapped particles to sawteeth, the redistribution in toroidal momentum Pphi (or in minor radius) was calculated in two ways. The first is based on the assumption that Pphi redistribution is stochastic with a large diffusion coefficient and was taken to be flat. The second way is to apply Kolesnichenkos inversion formula (Kolesnichenko, Ya.I., et al., Nucl. Fusion 32 (1992) 449). Both methods are compared with PCX data. The distribution function in a pre-sawtooth plasma and its evolution in a post-sawtooth crash plasma are simulated using the FPPT code. It is shown that FPPT calculated alpha particle distributions are consistent with TRANSP Monte Carlo calculations. Compariso

Phenomenology of MARFEs in TFTR

Abstract MARFEs and related detached plasmas are observed in TFTR. The MARFE is a toroidally symmetric, poloidally asymmetric luminous band that, in TFTR, occurs within the last closed flux surface at the inside plasma edge near the density limit. The edge cooling due to high radiation from the MARFE can result in further evolution into a poloidally symmetric state of reduced minor radius that is detached from the limiter. The time evolution of the 2-D spatial distribution of radiated power, providing an unambiguous picture of the location and evolution of the MARFE, is presented. At the time the bolometer arrays indicate that the MARFE passes through the horizontal midplane, rapid rises are observed in signals from midplane viewing diagnostics for line-integral density and for low-ionization states of both C and O, while the signals from the midplane viewing diagnostic monitoring H-like O and C radiation remain nearly constant. Also shown is the first evidence of density transfer from the rest of the plasma edge to the MARFE.

Nuclear reaction diagnostics of fast confined and escaping alpha particles

The resonant nuclear reactions D(α,γ)u20096Li, 6Li(α,γ)u200910B, and 7Li(α,γ)u200911B are examined as diagnostics of the energy distribution of confined fast alpha particles in tokamak plasmas. Reaction rates for Q=1 D‐T plasmas are estimated. The design of and preliminary results from the prototype fusion gamma ray detector on the tokamak fusion test reactor (TFTR) will be presented. The activation reactions 10B(α,n)u200913N, 14N(α,γ)u200918F, 25Mg(α,p)u200928Al, and 27Al(α,p)u200930P are similarly examined as diagnostics of fast escaping alpha particles. Count rate estimates for Q=1 D‐T plasmas will be presented.

Survey of Features in Radiative Power Loss Profiles in the Tokamak Fusion Test Reactor

Although the total radiated power in the Tokamak Fusion Test Reactor is often as high as 70% of the heating power, most of the radiation is concentrated near the surface of the plasma, and the interior loss is almost negligible. Fractional radiation loss declines during neutral beam heating. Under most interesting plasma conditions, the radiation profiles are dominated by asymmetrical peaks, which indicate locally intense edge radiation. As the high-density limit is approached, under most conditions, a bright band of radiation (a marfe) appears on the inner side of the plasma column. Marfe location is affected by toroidal field direction, neutral beam direction, and nearness to the high-density limit. Marfes have been observed to drift under the plasma column to the lower outside plasma edge. Marfes naturally develop into detached plasmas. In enhanced confinement discharges (supershots), an unexplained peculiar bright band, distinct from a marfe, appears in the lower outside part of the vacuum vessel, outside of the limiter radius. In high-density pellet-fueled discharges, there is a central peak that shows evidence for inward impurity convection.

Effect of the boundary plasma on TFTR ohmic discharges

The role of the boundary plasma in determining the power and particle balance of tokamak discharges is discussed. Detailed boundary plasma measurements of edge density, edge temperature, deuterium influx and carbon influx are reported from ohmically heated TFTR discharges over a range of plasma densities. The experimental results are compared with predictions from a simple zero-dimensional model based on power and particle balance. Reasonable agreement is obtained. More detailed impurity modelling is performed with the LIM impurity production and transport code. The complementary modelling approaches reveal, amongst other results, the important role of the sputtering yield at the limiter in determining the central effective charge of the discharge, the ability of a densified boundary plasma to screen impurities from the central plasma and the importance of cross-field particle transport to the TFTR limiter. The authors demonstrate that, under the conditions of this experiment, the behaviour of the boundary plasma and its effect on the central plasma appear to be explicable using rather simple considerations of power and particle balance

Control of disruption-generated runaway plasmas in TFTR

Many disruptions in the Tokamak Fusion Test Reactor (TFTR) (Meade and the TFTR Group 1991 Proc. Int. Conf. on Plasma Physics and Controlled Nuclear Fusion (Washington, DC, 1990) vol 1 (Vienna: IAEA) pp 9–24) produced populations of runaway electrons which carried a significant fraction of the original plasma current. In this paper, we describe experiments where, following a disruption of a low-beta, reversed-shear plasma, currents of up to 1 MA carried mainly by runaway electrons were controlled and then ramped down to near zero using the ohmic transformer. In the longer lasting runaway plasmas, events resembling Parail–Pogutse instabilities were observed.