Charge exchange neutrals

The most essential plasma device characteristics that are needed in order to obtain impurity release rates are the fluxes of photons and of charged and neutral particles to the wall. It will be necessary to have detailed information on the energy spectra and fluxes to walls, limiters and beam dumps of thermal electrons and ions, photons, a-particles, runaway electrons, charge exchange neutrals, neutral beam and impurity neutrals and ions. The effects of sheath potentials, secondary electron emission and unipolar arcing need to be included in these calculations. 

Although much has been done to improve the data base in these areas in the last decade, little is known about low energy (E < 400 eV) charge exchange neutrals, a-particle, and thermal and runaway electron fluxes. 

Fig. 9. Energy spectrum of the calculated flux of charge exchange neutrals which escape from TFTR immediately after neutral beam heating23 ...

Since the plasma is in part pumped away by the divertor, efficient refueling is necessary in order to keep a constant plasma density. An adequate refueling method has to maintain a sufficiently low rate of production of charge exchange neutrals. The possible effects of the divertor on the MHD-stability of Tokamak plasmas and a detailed understanding of the processes in the scrape-off layer require further investigations. 

The peak heat load to the divertor plate is supposed to be limited with the help of impurity radiation and charge exchange processes. Both, photons and charge exchange neutrals, are not affected by the magnetic field, thus allow to distribute the power on the whole vessel surface. 

Fig. 5.6. Simulated result for birth positions of C+ ions originated from CH4 molecules released from the private flux region by charge exchange neutrals (a) without and (b) with a dome in the private flux region [15]...

Charge-exchange (charge transfer) ionization. Occurs when an ion/atom or ion/molecule reaction takes place in which the chaise on the ion is transferred to the neutral species without any dissociation of either. 

Oil Contamination of Helium Gas. For more than 20 years, helium gas has been used in a variety of nuclear experiments to collect, carry, and concentrate fission-recoil fragments and other nuclear reaction products. Reaction products, often isotropically distributed, come to rest in helium at atmospheric concentration by coUisional energy exchange. The helium is then allowed to flow through a capillary and then through a pinhole into a much higher vacuum. The helium thus collects, carries, and concentrates products that are much heavier than itself, electrically charged or neutral, onto a detector... 

In charge exchange collisions the cross-section depends upon the energetics of the reaction. To compute the energy defect, the initial and final states of the colliding particles must be specified. This can be done easily for the bombarded neutral molecule, which usually can be assumed to be in the ground state before the collision, but not for the incident ion which is often in one of its metastable states. 

If charge exchange occurs when the incident positive ion passes the neutral gas molecule with a certain velocity, transfer of translational energy will usually take [place. This transfer of translational energy... 

Positive ions start to capture electrons from the medium when their velocity is comparable to that of an electron in an ls-orbital around itself. On further slowing, at first the captured electron is soon lost, and then another electron is captured. Thus cycles of capture and loss continue until it is energetically impossible to lose the captured electron. If the incident particle is multiply charged, another charge exchange cycle will soon be set up, and so on until the particle is reduced to a neutral atom. 

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