Influence of Magnetic Fields and Biasing on the Plasma of a RF Driven Negative Ion Source

Abstract

Abstract. The development of a RF driven negative-ion source currently in progress at IPP Garching has made remarkable progresses during the last years. Optimization of the source strongly depends on the understanding of the physics of generation, destruction and extraction of negative ions. Numerical model calculations can provide important information needed for the improvement of this understanding. Since negative ions are mainly produced on Cesium covered surfaces of the source, and have a survival length in the range of a few cm, models for the region close to the plasma grid are of particular interest. The plasma in this region is affected by magnetic filter fields. Another important feature is biasing of the plasma grid which strongly influences the co-extracted electron current. The different effects are treated in two separate models. Their current status is presented: a particle-in-cell-code (PIC) is used to study the influence of filter fields and bias voltage on the plasma flow onto the plasma grid. The motion of the charged particles due to their own and externally applied electric field is simulated in a selfconsistent way. In the second code the survival probability of negative ions produced on the surface of the plasma grid is estimated using test-particles. The Monte-Carlo technique is applied for the treatment of inelastic and elastic collisions.

Negative Ion Extraction With Finite Element Solvers and Ray Maps

“…Analytical 1-D studies of the presheath region confirm the decrease of N e , physically due to the fact that region 3 is a sink of electrons [12]. In the present code, we restrict the simulation to regions 3 and 4, making generic assumption on region 2 based on previous studies [11], [12] and on extracted total current. This strategy has the disadvantage of not being a complete simulation of the whole source and has the advantage of not depending on collisional effects (strongly dependent on ion temperature) and ion production mechanism (depending on conditions which are still debated like cesium distribution), which strongly affect the presheath [1], [8], [10] but hardly modify the beam acceleration in the sheath.…”

“…Electric fields are correspondingly different. The still standing need of separating numerical 2-D studies of the sheath and presheath regions for the different length scales involved is well documented in the recent literature [11]. Thus, we should add another region, i.e., region 3, (corresponding to the sheath) in negative source models, with the identification of presheath to region 2 for simplicity.…”

“…Curve "a" must match with the strong extraction field −u ,z 1/λ D at the region 4 border (u ,z is the derivative of u along z) and should approximately have a plateau (called here as "crest" with position z cr ) in the presheath region; the filter field makes the small potential differences with the bulk plasma region 1 possible. Off axis, we can still have a gentle fall of u toward the PG surface (curve b), considering that the magnetic field reduces the electron mobility, or a gentle rise (curve c, similar to a traditional sheath), depending on the bias voltage and the size of the bias plate [11].…”

“…A simultaneous simulation of regions 2, 3, and 4 is still impossible due to the different length scales involved: the Debye length λ D , the absorption lengths, and also the gyroradii and the mean free paths. From 2-D particle-in-cell simulations of region 2, limited to low plasma density for numerical reason [11], we expected the electron density N e to decrease (more or less linearly) toward the wall WF, i.e., the wall facing the plasma of the first electrode called plasma electrode or plasma grid (PG) (Fig. 1).…”

Two-Dimensional Simulations of H-Ions Extraction

“…Computer simulations are useful tools that support that task. A lot of effort has been made in order to model negative ion extraction in both 1D [5], 2D [6] and 3D [7][8][9][10][11][12][13][14]. The results obtained using these codes show e.g.…”