Effects of the magnetic field topology on the co-extracted electron current in a negative ion source for fusion

Abstract

The negative hydrogen ion current that can be extracted from ion sources for neutral beam heating in fusion experiments can be strongly restricted by the amount of co-extracted electrons and their increase over time, particularly during long pulses (up to 1 h). Models describing the underlying physics of particle extraction from a low-temperature plasma with a high amount of negative ions are essential for identifying measures for reducing and stabilizing the co-extracted electrons. In this work, the 3D PIC-MCC code ONIX (Orsay Negative Ion eXtraction) for the plasma volume around one extraction aperture in the first grid of the extraction system is used for analyzing the effect of the magnetic field configuration on the co-extracted electrons and the extracted negative ions. The magnetic field topology is the result of superimposing two different fields that are perpendicular to each other, the filter field (dominant in the ion source volume) and the electron deflection field (dominant in the extraction system). A parametric study changing the relative intensity of these two fields is performed. It is demonstrated that on the local scale of the simulation, the strength of the filter field does not affect the amount of co-extracted electrons, while a significant reduction of the co-extracted electron current is observed when strengthening the electron deflection field.

Tutorial: Modeling of the extraction and acceleration of negative ions from plasma sources using particle-based methods

“…An attempt to use the ONAC code was made on a GPU machine 74 . Recent simulations of the BAT-MAN extraction region have been achieved using the ONIX code with 4096 cores (Intel Sandy Bridge) of the HYDRA supercomputer at the Max Planck Computing and Data Facility (MPCDF), Garching, Germany 75,76 . The second option is called particle decomposition and based on the assignment on each of the cores of a subset of the initial population of particles.…”

“…48, 77, and 80. Different strategies regarding the communication between cores have also been implemented, either exclusively using MPI (message passing interface) libraries to exchange information between cores [73][74][75][76] , or adopting a hybrid approach by limiting the use of MPI to transfer data between sockets or nodes, while shared memory capabilities are implemented between cores via OpenMP (Open Multi-Processing) 48,[77][78][79] . The latter technique has the advantage of reducing the cost of communication between threads saving computational time.…”

“…Numerically, any particle at a given location y = y old and crossing the y = 0 (y = y max ) boundary, is re-injected at a new position y new = y old + y max (y new = y old − y max ), respectively, while its velocity components remain unchanged. For 3D simulations 39,76,120 , periodic boundary conditions are also considered along the z direction. Negative ions H − are injected from the PG surface which is assumed at ground potential, whereas the EG is at a fixed positive electric potential (to extract negative charges).…”

“…11(b), while it is asymmetric close to the aperture in the xy plane, which is a telltale sign of a cross B drift induced by the magnetic field of the suppression magnets. This mechanism drives the electron current outward toward the accelerator 76,147 . The difference in the plasma density on the PG surface leaves an imprint on the depth of the virtual cathode, which as a consequence, is asymmetric as well as shown in Fig.…”

Simulation of Beam Formation in the CERN Negative Ion Source for the Linac4 Accelerator

“…Negative ion beam formation and plasma in the vicinity of the PE extraction aperture have been simulated with the ONIX code [8][9][10][11], used to the modelling of the extraction of negative hydrogen ions and co-extracted electrons in negative ion multi-aperture sources for ITER's Neutral Beam Injector (NBI). ONIX has been modified and adapted to match extraction region of the Linac4 H − source [12].…”