Molecular Assisted Recombination Research Articles

Effect of Initial Vibrational Excitation of Molecular Hydrogen on Molecular Assisted Recombination in Divertor Plasmas

We have calculated the effective rate coefficient for molecular assisted recombination (MAR) by using our collisional-radiative model for atomic and molecular hydrogen with recent atomic and molecular data incorporated. The result for the vibrational ground state molecule agrees very well with the original rate coefficient proposed by Krashennikov et al. The rate coefficient depends strongly on the initial vibrational excitation of the molecule released from the wall.

Characteristics of detached plasmas associated with electron-ion and molecular assisted recombinations in NAGDIS-II

Recombination in a helium and helium-hydrogen plasma wasexperimentally investigated in the linear divertor simulatorNAGDIS-II. To identify the physical mechanisms, electron-ionrecombination (EIR) and molecular assisted recombination (MAR), theplasma parameters in the recombining region have been measured. Weused spectroscopic techniques on the basis of continuum emission andline emission from highly excited levels due to EIR and Langmuir probes. The dependence of the plasmaparameters on the neutral pressure in a pure helium plasma and in ahelium-hydrogen mixture shows clearly that recombination is caused bydifferent processes in the two cases. In a pure helium plasma aroll-over and a following decrease of the electron density as well asa monotonic decrease of the electron temperature down to 0.1 eV wereobserved when increasing the helium pressure. Here EIR is therecombination mechanism. Conversely, when hydrogen is added to thehelium plasma, the electron density decreases monotonically while thecentral electron temperature remains above 1 eV. This particularbehaviour in the helium-hydrogen plasma can only be explained by thedominant action of MAR.

Collisional Radiative Kinetics of Molecular Assisted Recombination in Edge Plasmas

Recombination of hydrogen plasma in the divertor volume is a simple explanation for divertor detachment phenomena experimentally observed on many tokamaks and divertor plasma simulators. The presence of vibrationally excited molecules in detached plasma provides an additional mechanism for plasma recombination, the so-called molecular assisted recombination (MAR). The cross-section data on atomic and molecular processes relevant for divertor plasma conditions are reviewed. Collisional radiative kinetics for cascade excitation of vibrational and electronic states as well as for the dissociation, recombination, and ionization of hydrogen molecule and molecular ions is studied. The results of calculations on the effective rate coefficient for MAR in detached divertor plasmas are presented. A comparison of the MAR rate coefficient with the rate coefficients for molecular dissociation, ionization, and electron-ion recombination is given. The sensitivity of MAR rate to the accuracy and completeness of atomic physics data is discussed

Molecule Assisted Recombination (MAR): Mechanisms and Plasma Conditions for Effective Operation

The main features of plasma and neutral gas recycling at the edge of magnetic confinement devices are discussed and the crucial impact of plasma volumetric recombination on power load on material surfaces is demonstrated. The physics of molecular assisted recombination (MAR) involving ro-vibrationally excited molecular hydrogen is discussed. It is shown that although conventional electron-ion recombination (EIR) alone can strongly alter edge plasma parameters, MAR may even exceed the impact of EIR for magnetic confinement devices. However, unlike EIR, the MAR crucially depends on recycling of the molecules on the wall surface and, therefore, the MAR efficiency can be sensitive to geometrical effects.