[en] This volume brings together papers on atomic processes that have been important in fusion research during the past 30 years. Topics include: Atomic radiation from low density plasma; Properties of magnetically confined plasmas in tokomaks; Diagnostics and; Heating by energetic particles. Each chapter includes references

[en] The main objective of the panel as presented in this paper was to examine envisaged scenarios for reactor start-up in the light of present experimental and theoretical knowledge and to identify long term guidelines for impurity control. Topics covered in the discussion are: ionisation and current initiation; ramp-up in inductive scenarios; non-inductive ramp-up; radio frequency heating; divertor power loading during burn; transformer recharging; and atomic processes

[en] Control of the release of impurities and their subsequent ingress and exhaust from tokamak plasmas has been the subject of intensive studies aimed at both the prediction of reactor burn condition and the interpretation of results from present experiments. Control concepts which are specific to current-initiation, current ramp-up and RF heating during start-up of a reactor such as NET have to date received little attention, according to the author. This paper presents an inductive start-up scenario which is typical of those presently being considered for NET. Shown are plasma configurations during start-up. Particularly significant issues are the advantages of forming the divertor configuration at the earliest practicable stage and the problems expected due to contamination of the limiter with divertor material

[en] The first successful colliding-beam study of inelastic atomic collisions was reported in 1969 by Harrison and co-workers at AERE Harwell. Since then the technique has provided a powerful and flexible tool for the measurement of many absolute cross sections, particularly those involving ions and atoms of relevance to nuclear fusion and astrophysics. Its particular merit lies in the ability to define precisely the velocity and atomic species of the colliding partners and to detect unambiguously all the products of the collision. This paper discusses the basic properties underlying the design of beam lines for such experiments. (author)

[en] The performance of the single-null divertor is modelled over a range of scrape-off plasma parameters in the INTOR Phase I and Phase IIA. The behaviour of beryllium and tungsten is compared. The models predict that the DT plasma within the divertor chamber is cooled by localised recycling and that most of the energy which flows into the chamber is eventually deposited upon the target by convective transport through the sheath. Catastrophic sputtering of a tungsten target is possible but unlikely. The temperature of the plasma at the sheath edge and also the effective sputtering of impurity atoms are sensitive to the plasma density in the scrape-off region. Increasing the scrape-off density to about 8 x 10₁₉ m^-₃ ensures that the effective sputter yield of tungsten does not exceed approximately 10^-₄. For a tungsten target the speed of the vacuum pumps needed to maintain 5% of helium within the scrape-off plasma is predicted to be about 150 m₃ s^-₁ at 300K. A beryllium target almost doubles the pumping requirements. It is concluded that tungsten should be the recommended target material for the divertor of INTOR. Target lifetime and plasma purity could be enhanced by operating the scrape-off plasma at a density in excess of the specified value, ie > 5 x 10₁₉ m^-₃. (author)

[en] A simple description is presented of the physics issues which influence the conceptual design of a pumped-limiter and single-null poloidal divertor in a next step, long burn tokamak of NET/INTOR scale. It is concluded that the performance of the divertor is superior and that it can be predicted with a reasonable degree of confidence. The viability of the limiter remains in doubt but the concept cannot be rejected at the present time. (author)

[en] Modelling of the pumped limiter envisaged for INTOR Phase IIA predicts than an effective exhaust of helium can be attained with a pumping speed of about 100 m₃ s^-₁ provided that the power flow to the under-surfaces of a tungsten limiter plate is not less than about 6 MW; compatible with a tip loading of 1 MW m^-₂. Plasma characteristics below the limiter plate are sensitive to the exhaust speed of the vacuum system. Tungsten and beryllium limiter plates are compared. The sputter rate of tungsten is low (typically < 1 cm/year); self-sputtering is not likely to be a problem even if the charged particle input power is as high as 40 MW. Sputter erosion of the beryllium plate is large (approximately 20 cm/year) and the helium exhaust pumping requirements for beryllium are about twice as demanding as those for tungsten. Localised recycling of DT ions in the immediate vicinity of the limiter is a significant transport mechanism when the scrape-off plasma density is approximately greater than 10₁₉ m^-₃. The charged particle power flow necessary to maintain this recycling is at least 4 MW and, there is a substantial temperature gradient between the sheath regions at the limiter plate and the bulk of the scrape-off plasma. The temperature at the separatrix distant from the plate may therefore be in excess of 50 eV. (author)

[en] This paper considers the relevance of atomic and molecular processes to research into controlled nuclear fusion and in particular their effects upon the magnetically confined plasma in Tokamak experiments and conceptual Tokamak reactors. The relative significance of collective phenomena and of single particle collisions to both plasma heating and loss processes are discussed and the pertinent principles of plasma refuelling and plasma diagnostics are outlined. The method by which atomic and molecular data are applied to these problems, the contributing effects of surface interactions and the consequent implications upon the accuracy and the type of data needed are described in a qualitative manner, Whilst particular atomic and molecular processes are not discussed in detail, sufficient information is given of the physical environments of Tokamak devices for significant processes to be self-evident. (Auth.)

[en] Some consequences of the release of electrons at a divertor target have been considered with particular reference to the collisionless exhaust of a D-T reactor. These electrons share the energy of the thermonuclear α particles and thereby cool the divertor plasma and lower the sheath potential at the target. A limit is set by space charge saturation in the sheath and some target materials have sufficiently large yields of secondary electrons for saturation to occur. Even so, the sheath potential will be in excess of the thresholds for unipolar arcing. (Auth.)

[en] The advent during the last five years of large tokamaks, exemplified by JET, has elevated fusion research from laboratory scale experiments to the status of reactor relevant studies. The results have provided further evidence of the impact of atomic and molecular processes upon the behavior of tokamak plasmas. Atomic radiation remains the main source of energy loss from limiter bounded plasmas and this has led to the adoption of low Z coatings or liners for the torus vessel. Divertor experiments have so far been restricted to smaller sized devices, e.g. ASDEX, PDX and DOUBLET III, nevertheless they have demonstrated the effectiveness of the divertor in reducing radiation losses from the main plasma and in providing a cool recycling plasma adjacent to the divertor target. The interest in atomic and molecular processes remains strong and ranges from processes involving highly charged impurity ions to thermal energy neutral molecules of H₂ and impurities such as O₂, CO, etc. Despite the substantial advances in experimental and theoretical atomic collision physics there are many unresolved problems. Precise data for dielectronic recombination in the plasma environment is one example, excitation and ionization data for medium charge state ions is another. Even apparently mundane processes pose problems which have not yet been adequately considered, one example is the assessment of the average energy dissipated by plasma electrons in producing H₂⁺, H⁰, H⁺, etc. from hydrogen molecules. In conclusion it should be noted that atomic interactions are not restricted to the hot plasma of a tokamak but extend to the detailed physics and chemistry at the surface of the vessel. 31 references, 24 figures