Neutral Recycling Research Articles

Neutral Pumping Rates for a Next Step Tokamak Ignition Device

Neutral pumping rates are calculated for pump-limiter and divertor options of a next step tokamak ignition device using a method that accounts for the coupled effects of neutral transport and plasma transport. For both pump limiters and divertors the plasma flow into the channel surrounding the neutralizer plate is greatly reduced by the neutral recycling. The fraction of this flow that is pumped can be large (>50%) but in general is dependent on the particular geometry and plasma conditions. It is estimated that pumping speeds greater than or approximately 10/sup 5/ L/s are adequate for the exhaust requirements in the pump-limiter and the divertor cases.

Density limits in tokamaks

The energy loss from a tokamak plasma due to neutral hydrogen radiation and recycling is of great importance for the energy balance at the periphery. It is shown that the requirement for thermal equilibrium implies a constraint on the maximum attainable edge density. The relation to other density limits is discussed. The average plasma density is shown to be a strong function of the refuelling deposition profile.

Two-point transport model for cold divertor plasmas

A two-point model for plasma transport along divertor field lines is presented here. Solutions for the steady-state plasma temperature, density, and flow speed at the divertor throat and plate are found for a given throat heat flux, divertor field line length, neutral recycling coefficient, and throat density. Because of the simplicity of the model, it is useful for parameter surveys and indications of general trends. Results from the model show that more than one steady-state solution can exist for a particular throat density and that the range of throat densities for which solutions exist is limited. Also, for a constant plasma flux at the divertor throat, as neutral recycling increases, the throat density must also increase to compensate for a large decrease in plasma flow speed at the throat.

An analytic one-dimensional divertor model with neutral sources

Divertor operation with high-density, low-temperature plasmas near the neutralizer plate offers the possibility of impurity control for high-power, long-pulse fusion experiments. Such plasmas can be produced by intense neutral recycling near the neutralizer plate. To complement large scale computational modeling of such divertors, we have developed a simple analytic model to describe this divertor regime. Continuity equations for neutral atom and plasma transport are analytically solved in a one-dimensional model, and the role of recycling is examined. The results are parametrized in a dimensionless form and compared with both theoretical and experimental results.

Results from the bundle divertor experiment on DITE with neutral beam heating

Characteristics of the operation of the DITE tokamak with the MkII bundle divertor and neutral beam heating are described. In these experiments the divertor was pumped by a liquid helium cooled cryopump, and fuelling was by gas input into the tokamak vacuum vessel. Generally a fuelling rate of 1–2 × 10 21 atoms/s is required to sustain a constant density with the divertor operating, and about 50% of the gas input flows to the divertor. Global power balances have been carried out using instrumentation distributed around the tokamak and in the divertor. It is found that the wall loading exhibits substantial toroidal and poloidal asymmetries, with a high loading near the gas feed when the divertor is operating. The percentage power exhaust to the divertor is highest, up to 70%, in ohmic discharges with the gas feed turned off and the density falling. With 1.4 MW of neutral beam heating, about 24% of the power is seen at the divertor plates. Typically, between 70% and 80% of the input power can be counted for. Measurements in the scrape-off plasma show that the density is about 1× 10 19 m −3 at 30 to 40 mm outside the separatrix, when the line average density in the tokamak is 2−2.5 × 10 19 m −3. However there is less than 10% reionisation of recycled neutrals in the divertor, as determined from H α measurements. In addition, the strong coupling observed between the particle and power flow to the divertor suggests that the exhaust is convective, and this may be a consequence of the strong mirror field characteristic of the bundle divertor magnetic geometry.

Role of particle recycling in beam heated expanded boundary divertor discharges in D-III

Discharges with improved energy confinement are produced in Doublet III when the major recycling location is shifted away from the main plasma by forming an expanded boundary (XB) divertor configuration with a separatrix to limiter separation Δ > 1.5 cm. Monte Carlo calculations indicate that over a wide range in density the divertor plasma effectively limits the penetration of recycled neutrals to the main plasma. This results in large particle recycling rates at the divertor plates and a reduction of particle recycling near the main plasma. This divertor function apparently plays an important role in obtaining improved energy confinement. Particle containment, as inferred from variations in D α emission near the main plasma, is found to decrease with n e and B T, and increases rapidly (faster than quadratic) with I P whereas energy confinement is relatively insensitive to variations in n e and B T and increases linearly with I P. This improved energy confinement with the XB divertor configuration is suggestive of particle confinement playing an important role. However, in view of their rather different parametric dependences, the correlation between τ P and τ E, if any, remains unclear.

A computational study of operating regimes for poloidal divertors

We have identified three theoretical operating regimes for poloidal divertors. These regimes are determined by the geometry of the divertor and the input energy and particle fluxes, and are characterized by the divertor plasma density and temperature. A fully self-consistent two-dimensional model for the plasma and neutral atom and molecule transport [1,2] was used to study poloidal divertor operation. Extensions of our previous calculations [2] important to this study were the inclusion of parallel electron and ion thermal conduction. We find that the key physics in divertor operation is the neutral recycling near the neutralizer plate. This can be parametrized by R = Γ p Γ 0 , the ratio of particle flux striking the neutralizer plate to the particle flux entering the divertor. Values of R ~ 1 can be produced by large pumping rates near the neutralizer plates resulting in low neutral recycling and a high temperature, low density divertor plasma. By decreasing the pumping near the neutralizer plate, R can be raised to an intermediate value of 5–10, the plasma temperature lowered by the same factor, and the density raised by a factor of 10–30. In this regime, escape of the neutrals back to the main plasma is virtually blocked. By further restricting the pumping, R can be raised to twenty or more, thereby lowering the temperature by a factor of twenty or more and raising the density by a factor of ninety or more. Such high density regimes have been observed on D-III [3] and appear to offer the most promise for impurity control and particle control on large reactor experiments such as INTOR or FED. In this paper, we explore the range 3 < R < 16.

Neutral particle modelling assumptions: Theory and experiment

The neutral particle recycling models that have been used to date in numerical plasma confinement simulations are by no means equivalent but may lead, instead, to divergent results. As a consequence of this confinement studies based on individual neutral assumptions must be received with a healthy degree of scepticism.

Steady-state diffusion in a tokamak

Steady-state neoclassical diffusion calculations are described for conditions appropriate to tokamaks. Particle loss by diffusion is balanced by neutral recycling. It is found that with the boundary condition that the particles convect out their own energy, the electron temperature and current density are large at the boundary. The means by which the behaviour of the temperature at the boundary may be brought closer to that observed in experiments are discussed.

Tokamak boundary in contact with a limiter

A theoretical and computational study of a tokamak boundary in contact with a limiter is given. Parallel field flows of plasma and heat induced by the sheath conditions at the limiter are treated with model sources and sinks in the cross-field transport equations. A charge-exchange neutral code is used for the re-cycled neutrals. It is found that the boundary conditions at the limiter radius have little effect on the energy flow to the boundary unless the ionization zone is well attached to the limiter. Exemplary heuristic boundary conditions useful for transport codes are given. The partitions of particle and energy flow to the limiter and wall are discussed.