Neutral-Beam Injection
Neutral beams inject fast neutral atoms that ionize inside the plasma and transfer their energy to the bulk, heating and helping fuel the machine.
Neutral-beam injection (NBI) accelerates ions to high energy, neutralizes them so they can cross the confining field, and injects them into the plasma. Once inside, the fast neutrals are re-ionized and become trapped fast ions that slow down by collisions, giving their energy to the plasma. NBI is a mainstay of high-temperature plasma heating and is well suited to reaching the burner's near-90 keV target.
In the tandem mirror, beams do double duty. In the central cell they raise ion energy toward burn conditions and can help fuel the plasma. In the plugs they build and sustain the dense fast-ion population whose charge separation creates the ambipolar potential. Beam aiming, energy, and current are tuned differently for these two roles.
What NBI provides
- Bulk ion heating toward the ~90 keV burn point
- Fast-ion population that sustains the plug potential
- A fueling contribution alongside gas injection
- Steady-state operation matched to the continuous burn
- Directed momentum and profile control
- Independent aiming for central-cell and plug roles
Beam energy is set by penetration: a beam that stops at the plasma edge heats the wrong region and can drive impurities, so the energy is chosen against the local density and machine size. In the plugs the beams must also sustain a fast-ion population against continuous slowing-down, which sets their required current.
Design constraints
Beams must penetrate to the plasma core without being stopped at the edge, which sets a minimum beam energy for a given plasma density and size. They deposit heat on beam dumps and vessel surfaces that must be cooled, and their duct is a line-of-sight opening that neutron and radiation shielding must account for. Beam availability feeds directly into machine availability.
All figures are design-and-simulation values for an unbuilt machine.