The physics of divertors in tokamaks is reviewed, primarily from an experimental point of view, although where possible simple analytic modelling is included. The paper covers the four main subject areas at issue in divertor research: (1) the wide dispersal of plasma power exhausted from the main plasma, (2) the production of sufficiently high gas pressures in the vicinity of pump ducts to enable the removal of fuel and helium (`ash') gas from the system, (3) the elimination or reduction of impurity production and (4) the screening of impurities produced, or intentionally added, at the plasma boundary from the plasma core. A simple analytic model, the `two-point' model, is introduced early in the paper and provides a framework for comparison of the experimental results, drawn from many machines, with simply derived expectations. Conclusions regarding the direction of future research priorities are made.

https://iopscience.iop.org/article/10.1088/0741-3335/39/6/001/pdf

Experimental divertor physics

In the last fifteen years, great strides have been made in controlled fusion research. The plasma temperatures, beta, and confinement needed for a reactor have been approached in recent experiments (Table 1). In the next few years, experiments on JET (EC), TFTR (USA), JT-60 (Japan), and T-15 (USSR) are expected to answer many of the remaining physics questions as to whether it is possible to obtain reactor grade plasmas in large tokamaks. The major physics questions are: 

(1)

confinement




(2)

beta




(3)

heating and current drive




(4)

impurity and particle control.

Introduction to the Physics of Plasma Wall Interactions in Controlled Fusion

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

The impurity flow parallel to the magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated. The rate equations are solved treating each ionization state as a test fluid which interacts with the given hydrogen background plasma via collisions and the ambipolar electric field. Results for typical impurities (O, Fe, He, etc.) show that collisional friction usually forces the impurities to flow nearly at hydrogen speed. Thermal forces, however, can become dominant locally for small Mach number and large temperature gradient (e.g. strong target recycling), causing impurity flow reversal and subsequent accumulation outside the recycling region. The criterion for flow reversal is roughly M <λi/λT where λi is the hydrogen ion mean free path and λT is the temperature gradient length. Self-sputtering at the target plates is calculated, showing the importance of fractional impurity acceleration in addition to the charge-statedependent electrostatic energy gain.

/pdf/modelling-of-impurity-flow-in-the-tokamak-scrape-off-layer-kg2rv2gjag.pdf

Modelling of Impurity Flow in the Tokamak Scrape-off Layer

Experimental results and theoretical models for the boundary layer of toroidal confinement devices are described. Emphasis is put on the scrape-off layer of poloidal divertor tokamaks which have demonstrated a significant reduction in impurity content as compared to limiter operation. The competition of transport processes parallel and perpendicular to the field lines, determining the profiles of plasma density, temperatures and impurity concentrations, is described. The role of hydrogen recycling in the divertor chamber is underlined, which decides the flow Mach number in the scrape off, the plasma parameters in the target plate vicinity and the regime of energy transport along the field lines. Limiter and divertor operation in ASDEX are compared and the favourable results of the latter explained by the reduced impurity production of the target plates and their distance from closed field lines, which impurity ions would have to overcome by diffusion against the plasma flow into the divertor chamber.

https://iopscience.iop.org/article/10.1088/0031-8949/1982/T2B/022/pdf

Plasma Boundary Layer in Limiter and Divertor Tokamaks

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

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