3.13 Neutral Beam Heating and Current Drive System
HU Chundong HU Liqun WANG Shaohu
3.13.1 System Parameters
We proposed that NBI specification of pulse length is larger than 100s. In the first step,
neutral beam power is 4 MW, then plus 4 MW in the future. The NB H&D system consists
of two injectors. One beam line is co- injection with the power of 2 MW, another beam line
is counter- injection with 2 MW in Fig.1.
Fig.1 Direction of EAST neutral beam injector
3.13.2 System Design
An elevation view of conceptual design each beam line for EAST is shown in Fig.2.
Beam line components include ion source, neutralizer, bending magnet, ion dump, and
cryopump. The ion source will produce 100A, 60 keV H+ (or D+) beam, three extractor are
envisioned. The energetic ions are neutralized by electron capture in a gas neutralizer.
Unneutralized ions are swept out of the beam with a magnetic field. The gas flow to each
beam line is estimated to be a few tens torr liters per second, and a cryopump system has
Fig.2 Elevation view of the EAST neutral beam injector
been designed to handle this gas. The maximum allowable pressure in the beam line is
dictated by the requirement that the gas flow to the EAST Tokamak from all beam lines
must be less than 1 torr liter, furthermore, the pressure in the drift region between the
sweep magnet and the entrance aperture must be kept low so as to minimize reionization of
the neutral beam.
3.13.3 Component Design
220.127.116.11 Ion source
In configuration, the ion source is similar to the multipole bucket source used in JET,
The plasma generator uses many filament pairs mounted on the back plate. The body,
formed of OFHC copper with the magnet positioning plates on the outer surface, is cooling
water channels on the sidewall in Fig.3.
Fig.3 Scheme of ion source
The extractor is multi-aperture three electrodes system. The electrode nearest the
plasma is at the positive potential VA. This can be varied from 30 to 60 keV. The center
electrode is held at the negative potential VD = (- VA / 15). The third electrode is at earth.
Plasma electrode and negative grid are cooling with pure water. We hope to get H+ current
100 A, and min. divergence at perveance optimum 1.7 º.
18.104.22.168 Source magnetic shielding
A large rectangular enclosure of magnetic material will be constructed around the
source to serve both as a magnetic shield and a high voltage safety enclosure.
22.214.171.124 Neutralizer gas cell
The ion beam is neutralized by background hydrogen gas in the chamber. For our
beam energies we require a target thickness of 0.15 torr.cm to approach equilibrium. The
construction of the gas cell is surrounding 0.64 cm thick copper with 1 cm diameter
water-cooling lines. The inside diameter of the cell is 50 cm and length is 1.5 meters in
Fig.4. Since the cell can project only approximately 1 meter into the vacuum enclosure, the
remaining 0.5 meters is incorporated in the source alignment assembly. The gas cell is
fixed and bolted to the end of the vacuum enclosure with water inlets and outlets through
the edge of the mounting flange.
Fig.4 Neutralizer gas cell position
126.96.36.199 Bending magnet
A bending magnet is located behind neutralizer. The magnet’s function is to remove
charged particles from the beam in a controlled manner. The ion deflection system is a C
shaped magnet operating in the 90° deflection mode. The air gap is 50 cm and the required
field is 1.5 kG, with 1% uniformity.
188.8.131.52 Residual ion dump
The ions are deflected and dumped on the three plates of the residual ion dump (RID)
by a magnetic field. In order to reduce the water flow requirements and the complex, high
cost fabrication of swirl-tube targets, the ion dumps were redesigned to an inertial type.
The design is predicated on an a maximum power flux of 1.5 kW/cm2 . This power flux
was brought about primarily by making the targets V–shaped, falling away from the
incoming beam. It was also necessary to bend the ion beams through a slightly greater
angle and therefore increase the angle of incidence for the highest power beam on its dump
area. This is illustrated in Fig.5.
Fig.5 Ion-dump with magnetic field
184.108.40.206 Vacuum system
The vacuum gain has two steps: one is initial pump-down of the beam line from 1
atmosphere to 10-6 torr; another is used cryopump to 10-8 torr.
The neutral beam emerging from the neutralizer end should propagate through high
vacuum (< 10-5 torr) to minimize reionization losses. Since the gas throughput for
producing 100 A is the order of a few tens of torr liters per second, the cryopump used in
the main vacuum for NBI of EAST pump speed is about 800,000 l/s. Gas pressure in the
drift tube is reduced about 30% by the drift tube LHe cryocondensation pump, this pump
needs a speed of about 40,000 l/s. So large of LHe and LN2 supply system is required.
A calorimeter is placed just past the bending magnet. The function of the calorimeter
described here is primarily to provide a beam stop for beam operation when the torus
plasma is not present. Such operation will be required for conditioning and tuning up the
sources. The calorimeter will also provide a measure of the total beam energy. The total
energy in each beam will be measured by monitoring the rise in temperature of the water,
which cools beam stop.
Since the calorimeter will sometimes be used when the bending magnets are not on,
and since the beam power losses cannot be predicted with certainty, the design heat loads
are based upon source design extracted current density. This gives the highest possible
intensity at the calorimeter for conservative design.
3.13.4 Electrical and Control System
The power supply system will provide the electrical power, controls, and monitors to
permit independent conditioning and operation of the sources. The design will maximize
safety, reliability, and ease of maintenance and minimize required floor space. Each of
these power supply sub-systems will independently provide appropriate filament, arc, accel,
and suppressor grid power to its associated NB source. Each will perform the electrical
functions of primary line power conditioning, transformation and rectification, overload
protection, monitoring, and control necessary for conditioning and operating a NB source.
As for 4 MW neutral beam production, total power about 15 MW is required. For long
pulse operation (>100s), power supply uses semiconductor switches (IGBTs) for fast
switching. Data acquisition and control for NBI is an integrated network of computer. PLC
system is used for the control of the cryopump, vacuum and gas feed system.