Study on Kinetics of Vibrationally Excited Hydrogen Molecule in Hydrogen Negative Ion Source

Abstract

Hydrogen negative ion source is a very useful ion source which is utilized in a proton accelerator or a neutral beam injector for fusion plasma heating and current drive. The negative ion is favored in these applications by its high neutralization efficiency. The negative ion production mechanism can be categorized as two types. The first one is a volume production mechanism which produces the negative ion via a dissociative electron attachment (DA) reaction with a vibrationally excited hydrogen molecule and a low energy electron. The other mechanism is a surface production occurred when a hydrogen atom or an atomic ion collide with the surface deposited by a low work function material such as the cesium. The negative ion source in Seoul National University adopts the volume production mechanism. In order to enhance an efficiency of the volume production, there is a magnetic filter field perpendicular to the axis of the ion source. The magnetic filter field generates a large gradient of the electron temperature between a driver region and an extraction region. The high energy electron in the driver region maintains a discharge and efficiently excites vibrational states of the molecule. Contrarily, the low energy electron at the extraction region is favorable to the DA reaction and prevents the destruction of the negative ion by an electron impact detachment. In order to understand the physics and characterize the ion source designed according to the volume production mechanism and the magnetic filter concept, various diagnostics are carried out. The plasma parameters such as the electron temperature and density are measured by an axially moving Langmuir probe. The negative ion density at the extraction region is diagnosed by a photo-detachment technique with Nd:YAG laser. To determine a vibrational state distribution function (VDF), the Fulcher-α spectroscopy is introduced. However, the Fulcher-α spectroscopy could not give an information on the higher vibrational states (υ>4) which are key elements in the DA reaction. Therefore, the VDF is modeled by a particle balance equation and compared with the spectroscopic measurement. As a result, it is confirmed that the VDF of the present negative ion source is in a non-Boltzmann distribution which requires a special treatment of the Fulcher-α spectroscopy for the VDF determination. A zero-dimensional model is developed to get a correct VDF by the Fulcher-α spectroscopy. Though the zero-D VDF model successfully identifies the relationship between the VDF and the Fulcher-α spectrum, the negative ion density estimation from the zero-D model is still deviated from that of the laser photo-detachment diagnostics. Thus, one-dimensional model is developed to account for the effect by the collision of excited hydrogen molecules during the transport from the driver region to the extraction region. It is found by the 1-D model that higher vibrational states are relaxed by the collision with the hydrogen atom or molecule. Consequently, the molecules in the higher vibrational states transit into the lower states when they arrive at the extraction region. Thus, the 1-D model greatly improves the negative ion density estimation in the consideration of the vibrational relaxation. Especially, the vibrational-translational (V-T) energy transfer reaction becomes important at high pressure regime. Thus, the over-estimation tendencies of the 0-D and the Boltzmann VDF model are corrected in the 1-D model at the high pressure regime. Also, the 1-D model is applied to a short length ion source and its result is compared with the diagnostic result, in order to examine effect of the chamber length on the VDF. The result of 1-D model is well matched when the transport length is set as the shortened length, while the negative ion density is under-estimated when the transport length is assumed as the same as that of the long length ion source. Thus, it is confirmed that the transport of the hydrogen molecule depends on the chamber length. Based on the numerical and experimental study, it is identified that the vibrational relaxation of the hydrogen molecule can be represented by the two characteristics length, the effective mean free path and the transport length. The effective mean free path is defined as the diffusive speed divided by the effective collision frequency. The effective mean free path represents the rate of the vibrational relaxation during the vibrationally excited molecule travels from the driver region to the extraction region. The effective mean free path strongly depends on the operating pressure of the ion source. It is shortened at the high pressure by the V-T energy transfer collision with the hydrogen atom and molecule. The transport length represents the distance that the vibrationally excited molecule should be transported, in order to produce negative ion at the extraction region. The transport length is mainly determined by the source geometry and adjusted by the plasma parameter profile. Thus, the efficiency of the negative ion production is enhanced when the effective mean free path is longer than the transport length. In order to satisfy this condition in the negative ion source, the negative ion source should be operated with lower pressure. Also, the strong filter field is required to control the increased electron temperature at the extraction region by the low operating pressure. In addition, thickness of the magnetic filter region should be made short because the transport length should be sustained as short as possible. This concept can be examined by the experiment conducted at TRIUMF. TRIUMF tried several types of the magnetic filter configurations, where the strong and thin magnetic filter was found to be the most effective without knowing the reason why the thin and strong magnetic filter was favored in the negative ion source. Based on this study, the dependency of the negative ion density on the magnetic filter configuration can be easily understood. Other important parameter to characterize molecular behavior is the degree of dissociation since the dissociated atom is a main source for the V-T relaxation and the negative ion destruction. In addition, higher dissociation degree leads toward depletion of the hydrogen molecule which is a precursor of the negative ion. Thus, it is investigated that the relationship between the negative ion density and the degree of dissociation which is determined by the optical diagnostic measurements with numerical analyses. The degree of dissociation increases as the rise of the electron temperature and the density. Thus, if the negative ion source is operated at the condition mentioned in previous paragraph, there should be an optimum pressure to maximize the negative ion density since the dissociation of the hydrogen molecule by the high electron temperature at low pressure causes deleterious effect on the negative ion production. Also, the electron density enhancement with higher power cannot increase the negative ion density indefinitely. The negative ion density at high power is saturated and the power efficiency is declined. In this study an improved one-dimensional VDF model considering the plasma parameters in the negative ion source is developed by accounting for the limitation when the non-Boltzmann VDF is diagnosed by the conventional Fulcher-α spectroscopy. Also, the improved 1-D model can simulate the transport of the hydrogen molecule and the vibrational state relaxation by the collision during the transport from the driver region to the extraction region. In addition, it is examined that the effective mean free path of the vibrationally excited molecule should be longer than the transport length to improve the negative ion production. Also, to optimize the operating condition of the negative ion source, it should be considered that the kinetics of the hydrogen molecule such as the VDF and the degree of dissociation as well as the plasma parameters. This study is expected to be useful to comprehend the magnetically filtered negative ion source.