Page 307 - Fundamentals of Magnetic Thermonuclear Reactor Design

Plasma Heating Systems Chapter | 9 287

(including the use of quasi-optic components and parasitic mode filters) are tak-
en for waveguides to enhance the efficiency of energy transfer from the gyrotron
to the plasma to 70%–90% through 10–40-m-long ducts. The power per area at
2
the beam inlet window is up to 30–50 MW/m . The fact that the generator is at
a safe distance from the plasma allows reduction of the UHF system irradiation.
Outlet windows of gyrotrons are made of synthetic polycrystalline diamond that
has high thermal conductivity and small dielectric loss. Introduction of UHF ra-
diation to the plasma is provided by an ordinary wave from the chamber’s exter-
nal loop or by an extraordinary wave from the internal loop. Heating efficiency
is optimised by varying the angle between the wave propagation direction and
the magnetic field orientation, and varying the wave focusing conditions.
The gyrotron system includes the following:

l The magnetic system including the superconducting and the warm sole-
noids;
l System for canalising energy from the gyrotron to the tokamak, comprised
of ‘mirror’ transition lines and super dimensional waveguides;
l Antenna modules for introduction of UHF radiation to the plasma;
l Set of high-voltage modulators equipped with a device protecting gyrotrons
against breakdowns; and
l The control and diagnostic system.
A summary description of two gyrotron systems for ECR plasma heating is
presented next, one developed for the T-15 tokamak, and another intended for
ITER [4].
The total power of the T-15 UHF heating system is 5 MW. The system in-
cludes 24 self-contained power generators with a power of 200–300 kW, using
1–2 s pulses and a wavelength of 3.6 mm. Three solenoids, including one super-
conducting and two warm ones, generate a magnetic field. A bellows arrange-
ment is used to connect the waveguide to the gyrotron’s outlet port, allowing
the axes of the waveguide and the gyrotron port to be aligned without breaking
the vacuum.
The UHF energy transmission system is comprised of superdimensional
waveguides, enabling minimisation of losses, sizes and radiation towards outer
space. They are grouped into two bundles of 12 waveguides. The waveguide
duct has only two 90-degree bends to minimise the magnetic field distortions.
The duct is turned using a mirror corner. The waveguides are filled with dry
nitrogen or elegas to enhance the electrical strength. UHF energy is introduced
into the chamber using a system of rotary-focusing and focusing mirrors.
The ITER ECR plasma heating system has a design power of 20 MW. It
requires at least 20 modules, which are to be fabricated and delivered in equal
parts by the European Union, Russia and Japan [4].
The tests of a Russian gyrotron module, operating at a 170 GHz fre-
quency, gave the following results. A continuous operation lasted more
than 100 s at a rated power of 1 MW and energy conversion efficiency of