Page 306 - Fundamentals of Magnetic Thermonuclear Reactor Design

286 Fundamentals of Magnetic Thermonuclear Reactor Design

l Injector arrangement at a safe distance from the plasma (no problems with
injector components irradiation).
l Neutral beam distribution independence of the magnetic field.
l Shutter at the injector outlet precludes the need of atmospheric gas delivery
to tokamak for maintenance and repair purposes. It can also be used for tri-
tium localisation.

Further development of the injector system aims to address the following
problems:
l Increasing the functional components’ durability;
l Increasing the accelerated ion current density;
l Reducing particle losses in the accelerator’s ion-optical duct and in the neu-
trals transport channel (the main losses are associated with the ion peeling-
off in the acceleration duct (∼30%), beam divergence (>20%), the halo
effect (∼15%), and the fast atom re-ionisation (∼5%)); and
l Neutralisation efficiency improvement through the use of metallic vapour jet
targets and plasma-based neutralisers. Currently, the efficiency of negatively
charged ion beam neutralisation is ∼60%.

9.3.2 Electron Cyclotron Resonance Heating
In this plasma heating option, the introduced power is absorbed either at ECR
frequency or at its harmonics. ECR frequency and the corresponding wave-
length are determined by the magnetic field:
−1
–1
2
f ≈ 28 ⋅ B [GH, T]; ω= f = 1.76 ⋅10 11 ⋅ Bt[s ,T]; λ ≈ B [cm,T]
fce≍28⋅ ce t Z ce ce ce t
Bt [GHz, T]; wce = 2πfce=1.76×10- The magnetic field values typical of present-day tokamaks correspond
11⋅Bt [s−1,T]; λce≍Bt−1 [cm, T] to the millimetre-wavelength range. For example, for ITER, f ≈ 150 GHz,
ce
12
−1
w ≈ 10 s ; λ ≈ 2 mm. Generation of electromagnetic millimetre-waves
ce
ce
by the ECR method in a strong magnetic field is achieved with gyrotrons. Ul-
trahigh frequency (UHF) radiation is generated by a relativistic electron beam
travelling in helical trajectories in a magnetic field.
ECR heating allowed plasma electrons in the T-10 tokamak to be heated to
10 keV for the first time. This accomplishment gave a powerful impetus to the
wide application of gyrotrons in tokamaks and stellarators. Pulse duration in
LHD stellarator at a power of 200 kW was brought to 1000 s. ITER requires
gyrotron modules with a unit power of around 1 MW allowing a heating time
of 1000 s. Gyrotrons have a wide application in magnetic fusion reactor en-
gineering. This includes plasma heating during plasma current plateau, non-
inductive current generation, gas breakdown assistance, discharge start-up and
subsequent plasma column formation, as well as control of current and electron
temperature profiles.
Electromagnetic energy is removed from the UHF generator through wave-
guides filled with compressed air to avoid short-cutting. Special measures