Description
Neutral Beam Injection systems for ITER will heat and drive current in the
plasma. For such a large tokamak, high beam energies (1 MeV and 0.87 MeV in
hydrogen (H) and deuterium (D), respectively) are required. The negative ion
source will have to satisfy the stringent ITER requirements: high extracted negative ion current density, low co-extracted electron current density, high beam
uniformity, and low single-beamlet divergence for long pulses in H and D. Achieving these parameters is challenging, particularly in D operation, where a higher
co-extracted electron current density is measured and observed to increase with
time.
The properties of the extracted negative ions and co-extracted electrons are
strongly influenced by the physics in the vicinity of the extraction apertures.
Isotopic differences are observed in the plasma properties, and are expected
to influence virtual cathode (local build-ups of negative space charge) formation at the plasma grid and the meniscus (plasma-beam boundary). This region
is, however, inaccessible to plasma diagnostics. Therefore, a 3D3V Particle-in-Cell Monte-Carlo Collision (PIC-MCC) code, the Orsay Negative Ion eXtraction
(ONIX) code, is used to shed light on the physics at play (see Ref. [1] and references therein).
In this work, a quasi-1D domain (significantly fewer cells in the y and z directions than in x) is used, with a plasma composed of electrons and monoatomic
positive and negative ions. The differences between hydrogen and deuterium are
investigated through a series of parametric studies, in which deuterium is found
to consistently form a deeper and broader virtual cathode than hydrogen across
all scans. This arises from the higher mass of D+ ions, leading to lower thermal
velocities and enhanced space-charge confinement of emitted negative ions.
These results reveal a fundamental isotope dependency in sheath physics, providing physical insight and expectations for the more complex 3D simulations.
Future work will extend similar parametric scans to full 3D with realistic species
and plasma densities to further understand the plasma’s influence on the topology of the meniscus and resulting source performance, bringing us closer to
ITER’s requirements.
[1] Lindqvist, M., et al. ”Particle injection methods in 3D-PIC MCC simulations
applied to plasma grid biasing.” Physics of Plasmas 31.3 (2024).Lindqvist, M.,
et al. ”Particle injection methods in 3D-PIC MCC simulations applied to plasma
grid biasing.” Physics of Plasmas 31.3 (2024).