Divertor Region Of Tokamak Research Articles

Space-resolved vacuum-ultraviolet spectroscopy for measuring impurity emission from divertor region of EAST tokamak

The measurement of impurity distribution in the divertor region of tokamaks is key to studying edge impurity transport. Therefore, a space-resolved vacuum-ultraviolet (VUV) spectrometer is designed to measure impurity emission in the divertor region on EAST. For good spectral resolution, an eagle-type VUV spectrometer with 1 m long focal length with spherical holograph grating is used in the system. For light collection, a collimating mirror is installed between the EAST plasma and the VUV spectrometer to extend the observing range to cover the upper divertor region. Two types of detectors, i.e. a back-illuminated charge-coupled device detector and a photomultiplier-tube detector, are adopted for the spectral measurement and high-frequency intensity measurement for feedback control, respectively. The angle between the entrance and exit optical axis is fixed at 15°. The detector can be moved along the exit axis to maintain a good focusing position when the wavelength is scanned by rotating the grating. The profile of impurity emissions is projected through the space-resolved slit, which is set horizontally. The spectrometer is equipped with two gratings with 2400 grooves/mm and 2160 grooves/mm, respectively. The overall aberration of the system is reduced by accurate detector positioning. As a result, the total spectral broadening can be reduced to about 0.013 nm. The simulated performance of the system is found to satisfy the requirement of measurement of impurity emissions from the divertor area of the EAST tokamak.

Direct measurements of electronic ground state ro-vibrationally excited D2 molecules produced on ECR plasma-facing materials by means of VUV-FT absorption spectroscopy

Ro-vibrationally excited molecules of deuterium are involved in non-equilibrium chemical reactions in divertor region of tokamak (see molecular assisted recombination) or in neutral beam injector to produce negative ion (see dissociative electron attachment) for fusion plasmas. These molecules produced both in a plasma volume and on surfaces are no longer in their electronic ground state X1∑g+(v”⩾0,J”) and populate non-uniformly different vibrational (v″) and rotational levels (J″). The high resolution VUV Fourier Transform spectrometer of the DESIRS beam line (SOLEIL synchrotron) is applied to directly scrutinize the ro-vibrationally excited levels of the D2 ground state from v″ = 0 to 10 and J″ up to 8 in an electron cyclotron resonance (ECR) cold plasma. This is performed for different plasma-facing materials, i.e. quartz, tantalum, tungsten, and stainless steel, in order to compare their relative impact in molecular excitation through recombinative desorption from surfaces. A significant effect of these materials on the absolute distribution of the vibrationally states has been found above v″ > 3. The experimental detection limit for Quartz is v″ = 7 whereas tungsten and stainlees steel excite the molecules up to v″ = 9 and tantalum up to v″ = 10. The use of a bare, temperature-controlled surface of Quartz as a reference, compared to metallic surfaces, allows us to determine the relative production of these excited levels. The recombinative desorption rate on tungsten and stainless steel compared to quartz is ~ 2.6 higher and reaches ~ 5 for tantalum.

Measurements of Flow Velocity in the Plasma Generator PSI-2

The plasma flow velocity in the Plasma Generator PSI-2 has been investigated by using of Mach probe. PSI-2 is a stationary high-current arc discharge in which the quasi-neutral plasma expands along the magnetic field lines. The low-temperature (Te < 20 eV), medium density (ne ∼ 1018— 1019 m—3 ) plasma in the discharge is similar to the plasma in the divertor region of tokamaks. From the ratio of ion saturation currents collected from opposite sides of the probe the flow velocities (Mach numbers) in argon and hydrogen discharges are obtained.

Deuterium atomic cross section uncertainties and molecular recycling effects in the divertor region of tokamaks

How predictions of deuterium atomic reaction rates in tokamak divertors are altered by changes in cross section data which are comparable with the uncertainty in published data is investigated. An extended neutral diffusion theory calculation, which includes D atoms and ground and excited state D2 molecules, is performed on a fixed background plasma. For the atoms, the principal relevant mechanisms are elastic scattering, charge exchange, ionization, and recombination. It is evaluated that the charge exchange and electron processes are known to an accuracy better than +/−25%, and it is concluded that this level of accuracy is sufficient for the present state of divertor calculations. Of significant concern is the fact that classical elastic cross sections which are widely used in divertor calculations differ substantially from a more accurate set of cross sections based on quantum mechanics, and that the latter produce predictions of neutral density and ionization rate which differ by orders of magnitude from predictions based on the classical cross sections. Opacity of the plasma to Lyman alpha radiation influences transport for high density plasma situations. The explicit representation of incident particles recycling as molecules is shown to significantly affect the neutral atom density and reaction rates near the divertor plate. In particular, the dissociation of even a small fraction of excited molecules significantly reduces the plasma ion population and increases the neutral atom population just in front of the plate in nearly detached plasmas.

Chemical erosion of pyrolytic graphite by low-energy H + and D + impact

Recent developments have lead to prospects of lower plasma edge temperatures in the divertor region of tokamaks and, consequently, a growing interest in the interaction of graphite with low-energy hydrogen ions (10s of eV). Here we present experimental measurements of methane and heavier hydrocarbon yields of pyrolytic graphite (300 K–1000 K temperature) for H + impact energies of 10–200 eV. Heavier hydrocarbons contribute significantly to the total chemical erosion yield at low energies. Comparison of CH 4 yields due to H + irradiation of graphite with similar results for CD 4 yields due to D + shows a smaller isotopic effect than anticipated based on previous H + and D + measurements. Comparisons are also made between the experimental results and a recently proposed model.

Three-dimensional neutral atom imaging of tokamak plasmas

Recent technological advances in neutral atom imaging of space plasmas can be applied to the next generation of long pulse, high density fusion experiments. In particular, the small size of these neutral atom imagers makes them ideal for measurements of charge exchange neutrals in the edge and divertor region of tokamaks. In the edge of tokamak plasmas, the lower energy neutrals are created locally and the higher energy neutrals are generated within a few mean free paths of the edge. This charge exchange neutral flux can be imaged and its energy spectrum analyzed. The energy spectrum measurements provide the radial information necessary for a three-dimensional (3D) image of the convolved ion and neutral populations.