Electron divertors

The consideration of higher hydrocarbons was introduced. The former version of the ERO code considered only the methane family CHy whereas higher hydrocarbons CxHy were not included. Especially at low electron temperatures as they can develop in the divertor higher hydrocarbons become more and more important [54]. Thus, the higher hydrocarbons C2Hy and C3Hy are now included in the code. 

Under typical high recycling divertor conditions, when molecules travel in a bath of 7-8 eV (or hotter) electrons, the molecules are destroyed before they reach such vibrational levels. Hence this chain of reactions is irrelevant there. For detached divertor plasma conditions this is not necessarily so. 

From inspecting the atomic database of the EIRENE code [31], which is used in many applications to a large number of different tokamaks, including for the ITER design, in particular its collisional-radiative models for molecules, it was clear that matters can be more complicated. The relaxation time for establishing a vibrational distribution of the ground state molecule is comparable to the transport time of the molecule, hence the applicability of local collisional-radiative approximations is questionable. Furthermore, one of the two atoms created in dissociative recombination is electronically excited, and, hence, can be ionized very effectively even at low divertor plasma temperatures (instead of radiative decay). In this case, the whole chain of reactions would be just an enhanced ( molecular activated ) dissociation (MAD, i.e., dissociative excitation of those H]]", which have been produced... 

Fig. 2.8. Divertor electron temperature profiles, left case 1, right case 2. Temperatures above 20 eV are not resolved and fall into the bright color region. Near target temperatures in case 1 are below 5eV (left target, blue colors) and below 10eV (right target, yellow colors). Both temperatures are increased significantly in case 2, to values above 20 eV at the outer target...

Fig. 3.3. Time traces for a JET discharge evolving from L-mode to Type I ELMy H-mode [15]. (NBI) is the input power into the plasma by fast neutral beams, Wtot is the total plasma energy (thermal + non-thermal component), Da (divertor) is the deuterium emission in the Balmer a line from neutrals coming into the plasma in the divertor area, average n is the average density of the discharge, Ti>core and Te,core are the ion and electron temperatures in the centre of the discharge and TelPed is the electron temperature at the top of the pedestal...

Fig. 3.12. Measurements with high time resolution ( 4(is) of the MHD activity (measured with Mirnov coils), pedestal temperature (Te,pea) and soft X-ray emission collapse, and outer divertor Da emission and inner divertor X-ray bremsstrahlung (from hot electron impact) during a Type I ELM in JET. The collapse of Te,ped, pedestal soft X-ray emission and the increase of the inner divertor bremsstrahlung emission occur over a time interval of 200-300 ps, similar to the period of large MHD activity [27]...

In this chapter, some topics of divertor spectroscopy with molecular transport are presented, mainly based on recent studies in JT-60U, which is a large tokamak (the major radius is around 3.4 m, and the minor radius is around 1.0 m) with a W-shaped poloidal divertor in the bottom [4]. (General molecular diagnostics without transport analysis are described in [5].) The plasma parameters in the divertor plasma change as two-dimensional spatial functions, and analysis with consideration of the divertor structure is necessary for understanding the particle behavior. On the other hand, molecular reactions are very complex. Thus, transport codes using Monte Carlo techniques become useful for analysis of the molecular behavior. Applications of molecular data and the data requirements for the analysis are also discussed. In the attached divertor plasma, where the electron temperature is high (> 5eV)... 

Fig. 5,2. Fulcher lines observed near the divertor plates in an attached divertor plasma in JT-60U [7], Calculated Q-branch line intensities for the v = 0 — 0,1 — 1,2 — 2 transitions are also shown as vertical lines. The electron temperature and density measured with a Langmuir probe near the separatrix strike point were 20 eV and 0.7 x 1019 m-3, respectively...

Fig. 5.4. Spectral profiles of Da line [12]. (a) 8cm and (b) 0.5 cm away from the outer divertor plates. The viewing chords are shown in the upper figure. The thick and thin lines indicate the observed and simulated profiles, respectively. Compositions for the simulated 7r-component line are also shown. The kinetic energy of the D atoms corresponding to the wavelength shift is indicated above figure (a). The electron temperature and density were, respectively, 90 eV and 0.5 x 1019m 3 at the outer separatrix strike point. The toroidal magnetic field at the outer separatrix strike point was 4.15 T...

Fig. 5.8. Profiles of (a) electron temperature, (b) density, and (c) ion flux measured with Langmuir probes at the divertor plates in an attached and a detached divertor plasma. The distance from the strike point (dl) is measured along the outer divertor plates as shown in the right-hand figure...

The investigation of molecular behavior in the divertor plasmas is important for establishment of heat and particle control in the tokamak. For attached divertor plasmas, where the electron temperature is high and the molecules... 

In divertor regions with temperatures below a few eV, II ions may be formed by radiative electron attachment... 

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