Page 305 - Fundamentals of Magnetic Thermonuclear Reactor Design

Plasma Heating Systems Chapter | 9 285

FIGURE 9.2 Coefficient of H ions recharge on a hydrogen target as function of ion energy.

Let us consider the GLOBUS-M neutral beam injector (NBI). It operates
on the positively charged ions, using beams of ∼1 MW with particle energy
of 30 keV and pulse duration of 30 ms. A gas-discharge–based plasma emitter
acts as an ion source. The injector’s energy effectiveness is close to 0.6–0.75 A/
kW. A non-independent discharge in a gas discharge chamber is driven by a
thermionic emission from tungsten cathode pins. Homogeneous plasma with an
2
extracted ion current density of up to 0.5 A/cm is yielded at the outlet end of the
gas discharge chamber. The ion beam is recovered and formed by an ion-optical
system with electrodes in the form of a molybdenum-alloy profiled mesh [6].
A pump-out system prevents the injector gas flow and accumulation inside
the tokamak chamber. To this end, a cryosorption vacuum pump in the form of
a hollow cylindrical tube, cooled with liquid nitrogen, is placed in the injector
case. The pump surface has a titanium getter coating, deposited with an electric
arc evaporator and renewed after each pulse. The maximum pumping speed is
5
−4
10 L/s. A background pumping up to ∼10 Pa is performed using a turbomo-
lecular pump.
The ion beam is neutralised on gas flowing out of the ion source. The pro-
cess is controlled by selecting the neutraliser length and the working gas pres-
.
−2
15
sure. The gas target thickness inside the injector is 5 10 cm . Ions that fail to
recharge are turned ∼90 degrees and sent to an ion receiver.
The only difference with injectors operating on the negatively charged ion
principle is ion source.
The main advantages of the NBI plasma heating in the fusion reactor engi-
neering context include the following: