UC San Diego

Detached plasma has been observed on different fusion devices, such as DIII-D, JET. Traditionally, Electron-Ion Recombination (EIR) is considered the dominant volume recombination process to cause detachment in detaching divertors. However, evidence for a new process, Molecular- Activated Recombination (MAR), was found both in PISCES-A and NAGDIS-II, which could be an important path to make a detaching plasma, and was also suggested by theoretical investigation and simulations. In this dissertation, we describes measurements that quantitatively show the existence and rate of MAR in a low temperature recombining plasma. The experiment is performed on the PISCES-A divertor simulator in University of California, San Diego. In partially detached pure helium plasma, EIR can be demonstrated directly by high excited state emissions. A small amount of H₂ gas is then injected into the target region of this plasma, resulting in the collapse of EIR emission with sufficient H₂ gas density. The plasma is maintained in the partially detached state, therefore indicating that in the presence of H₂ gas another loss process is operative. Since MAR produces H neutrals in a low excited or ground states in contrast to EIR associated with highly excited states, the existence of MAR is also implied by the anomalous large ratio of [H_\alpha]/[H_\ gamma]. Concurrent with this collapse, an increase in the [H_\alpha]/[H_\gamma] ratio in the cool (< 1 eV) outer halo plasma is observed. The results are quite different from the behavior of a standard EIR of H₂ in the cold region, and are consistent with the presence of MAR in the halo region. Using an integral form of the particle conservation equation, we show that this loss process is associated with a volumetric ion loss mechanism associated with the presence of H₂, in other words, it is due to a MAR process. The magnitude of the total volumetric particle MAR sink is derived with the measured parallel flux, ionization source, anomalous radial flux to the wall and EIR sink. The EIR sink rate is obtained in a magnetized He plasma column by absolutely calibrated high n (principle quantum number) He-I line emission associated with EIR. The difference between the total and EIR volumetric sink term is then attributed to the MAR process. The results indicate that the MAR is the dominant recombination mechanism when the H₂ concentration exceeds a few percent. In all cases, radial transport loss is the dominant ion loss term, which is consistent with previous experimental results in pure H₂ plasmas. With the measured radial profiles of MAR, it is seen that the MAR expands from the larger radii toward the center of the plasma column during the detachment. The role and evolution of MAR in detaching plasma reveals how MAR takes over from EIR during detachment in a mixed plasma. Keywords: Volume recombination, Molecular-Activated Recombination, Electron -Ion Recombination, Detachment, PISCES-A, Recombining plasma