Azusa Fukano

Study of ion-ion plasma formation in negative ion sources by a three-dimensional in real space and three-dimensional in velocity space particle in cell model

Recently, in large-scale hydrogen negative ion sources, the experimental results have shown that ion-ion plasma is formed in the vicinity of the extraction hole under the surface negative ion production case. The purpose of this paper is to clarify the mechanism of the ion-ion plasma formation by our three dimensional particle-in-cell simulation. In the present model, the electron loss along the magnetic filter field is taken into account by the “ τ///τ⊥ model.” The simulation results show that the ion-ion plasma formation is due to the electron loss along the magnetic filter field. Moreover, the potential profile for the ion-ion plasma case has been looked into carefully in order to discuss the ion-ion plasma formation. Our present results show that the potential drop of the virtual cathode in front of the plasma grid is large when the ion-ion plasma is formed. This tendency has been explained by a relationship between the virtual cathode depth and the net particle flux density at the virtual cathode.

Study of plasma meniscus formation and beam halo in negative hydrogen ion sources

A meniscus of plasma-beam boundary in H− ion sources largely affects the extracted H− ion beam optics. Recently it is shown that the beam halo is mainly caused by the meniscus, i.e. ion emissive surface, close to the plasma grid (PG) where its curvature is large. The purpose of this study is to clarify the effect of H− surface production rate on plasma meniscus and beam halo formation with PIC (particle-in-cell) modeling. It is shown that the plasma meniscus and beam halo formation is strongly dependent on the amount of surface produced H− ions.

Modeling of the rf discharge initiation in a negative ion source.

The maintenance free rf ion source is expected to be one of the most promising candidates for the negative ion sources of plasma heating for future fusion reactors. As an alternative to the arc-discharge sources, the rf negative ion sources have been developed for H(-) production. In order to make clear the condition for the discharge initiation of the rf source, we are developing a numerical model using the finite difference time domain Monte Carlo method to analyze the electron energy distribution function in rf field. The numerical result shows that the discharge is not successfully initiated due to the wall loss unless the wall potential is considered. More self-consistent model including ion dynamics to evaluate the wall potential and the electron loss at the wall will be needed in the future.

Analysis of Discharge Initiation in a RF Hydrogen Negative Ion Source

The maintenance free RF ion source is expected to be one of the most promising candidates for the negative ion sources of plasma heating for fusion reactors. In order to make clear the condition for the discharge initiation of the RF source, we are developing an electromagnetic PIC model. The numerical result shows that a positive potential built‐up with respect to the wall. As a result, the electron wall loss decreases and the electron density increases. The positive potential plays a key role for the suppression of wall loss and the electron confinement. The electromagnetic‐PIC model developed is useful for the analysis of discharge initiation of the RF source.

Analysis of the double-ion plasma in the extraction region in hydrogen negative ion sources

To understand the plasma characteristics in the extraction region of hydrogen negative ion sources is very important for the optimization of H− extraction from the sources. The profile of plasma density in the extraction region is analyzed with a 2D PIC modeling of the NIFS-R&D H− sources. The simulation results make clear the physics process forming a double-ion plasma layer (which consists of positive H+ and negative H− ions) recently observed in the Cs-seeded experiments of the NIFS-R&D source in the vicinity of the extraction hole and the PG (plasma grid).

A study of effective condition for MAR in detached divertor plasma

The effective conditions of MAR and the physical mechanism have been investigated by solving a set of time-dependent rate equations numerically. Preliminary results show that the effect of MAR becomes small in the high density of background plasma. This is due to lack of vibrational molecules available for MAR. The vibrational molecules are destroyed through IC and DA before reaching the sufficient density for MAR when a flow rate of hydrogen molecular source is not sufficient. In fact, the effect of MAR is appeared remarkably when the flow rate increases by an order of magnitude. These results indicate that not only the plasma density but also the flow rate of hydrogen molecules are important parameters for the onset of MAR.

Estimation of the cusp loss width in negative-ion sources

To estimate loss width in the cusp magnetic field in multicusp negative-ion sources, diffusion of plasma across magnetic field is investigated analytically. The transport process of plasma depends on various plasma conditions. Diffusion coefficients are classified by degrees of plasma ionization. In weakly ionized plasma, where a case of electron-neutral particle collision is dominant as compared with electron-ion collision, there is a possibility that ambipolar diffusion or electron short circuit occurs in plasma. This strongly depends on the conditions of the plasma, the system length, and the wall material. On the other hand, in fully ionized plasma, only ambipolar diffusion occurs automatically due to momentum conservation of electron and ion. In not fully but strongly ionized plasma, plasma diffusion depends on electron-neutral particle collision, ion-neutral particle collision, and electron-ion collision. Being based on the classification of the diffusion coefficients, we derive expressions of loss ...

Study of plasma meniscus formation and beam halo in negative ion source using the 3D3VPIC model

In this paper, the effect of the electron confinement time on the plasma meniscus and the fraction of the beam halo is investigated by 3D3V-PIC (three dimension in real space and three dimension in velocity space) (Particle in Cell) simulation in the extraction region of negative ion source. The electron confinement time depends on the characteristic time of electron escape along the magnetic field as well as the characteristic time of diffusion across the magnetic field. Our 3D3V-PIC results support the previous result by 2D3V-PIC results i.e., it is confirmed that the penetration of the plasma meniscus becomes deep into the source plasma region when the effective confinement time is short.

Kinetic modeling of particle dynamics in H(-) negative ion sources (invited).

Progress in the kinetic modeling of particle dynamics in H(-) negative ion source plasmas and their comparisons with experiments are reviewed, and discussed with some new results. Main focus is placed on the following two topics, which are important for the research and development of large negative ion sources and high power H(-) ion beams: (i) Effects of non-equilibrium features of EEDF (electron energy distribution function) on H(-) production, and (ii) extraction physics of H(-) ions and beam optics.

Electron Transport across Magnetic Filter in Negative Hydrogen Ion Source

Profiles of electron temperature and number density in a negative-ion source are investigated theoretically. Spatial dependence over the magnetic filter region is obtained using the equations of electron flux and electron heat flux that include the effect of interference of forces by the density gradient and temperature gradient. Due to the effect of the magnetic filter, temperature and density of the electron decrease from the source chamber to the extraction chamber, and the decrease depends on the magnitude of the magnetic flux. The effect of the magnetic filter on the production and destruction rates of the negative hydrogen ion is examined. The reaction rate for the dissociative attachment reaction which produces the negative hydrogen ion increases with the decrease of the electron temperature. However, the production rate per one vibrationally excited hydrogen molecule decreases with the decrease of electron density. On the other hand, the destruction probability of the negative ion by the electron detachment reaction decreases significantly by the decrease of the electron density and temperature. The magnetic filter does not enhance the production of the negative hydrogen ion, but it reduces the destruction of the negative ion because of the decrease of the electron density.