6.7 ICRF heating system for HT-7U

In order to satisfy the requirements of heating plasma on HT-7U project, ion cyclotron resonant frequency (ICRF) heating system will be built. Base on HT-7U parameters, the frequency range 30MHz-110MHz has been chosen. And 3MW and 6MW ICRF heating system will be available at the first and second phases separately. The following plasma heating experiments can be done at HT-7U:

1.H and D fundamental and harmonic frequency heating

2. Ion Bernstein Waves (IBW) heating

3. Fast wave and IBW current drive.(FWCD and IBWCD)

A summary of ICRF usable in the frequency range is summarized in Table 1.

Table 1.

Heating Mode / B（T） / Frequency (MHz)
ωCH / 2—3.5 / 30—54
2ωCH / 1—3.5 / 30—107
2ωCD / 2—3.5 / 30—54
2ωCT / 3—3.5 / 30—36

Design

The design features for HT-7U ICRF heating system are:

RF output power 3MW

Frequency range 30-110MHz

Operation mode Continue Wave

Transmission line 9 inch coaxial transmission line

Matching system triple liquid stub tuners

Antenna 4 loop antennas, one fold wave guide antenna

The ICRF heating system is including RF transmitter, transmission line, matching system, feedthrough, antenna, plasma loading measurement unit, data acquisition unit, high voltage power supply system, phase shifter, and DC breaker. Each unit is designed for continue wave (CW) operation. Fig.1 is shows the schematic view of the 3MW ICRF heating system.

6.7.1 RF transmitter

Three are two RF transmitters to output RF power up to 3MW. Each transmitter can have 1.5MW output, and frequency can be changed from 30-110MHz easily. In order to do fast matching the impedance of the antenna during plasma discharge, the transmitter should have ±2MHz -3db working bandwidth. The diagram of the transmitter is show at Fig.2. The system includes RF generator, waveform controlling, pre power amplifier, control and protection system, high voltage power supply, and three stages high power amplifiers.

The waveform controlling system based on a computer control has an ability to control RF transmitter to output the arbitrary waveform and to change the RF frequency quickly.

The pre power amplifier is a wide-band transistor amplifier. The output RF power is around 200w. And the powerful tetrodes (TH561, TH781 and EIMAC 8973) are used for three stages high power amplifiers. Each amplifier can output 15kW, 200kw and 1.5Mw separately . The output RF power from each tetrode is shown at table 2

tetrode / Output RF Power / Frequency range
TH561 / 15 kW / 300MHz
TH781 / 150 kW / 200MHz
Eimac 8973 / 1500 kW / 30-110MHz

Table 2

After three-stage amplifier to amplify the RF signal, 1.5MW RF power is ready for heating experiment.

6.7.2 High Voltage Power supply

An amplifier using the tetrode should have power supplies. A tetrode needs anode, control grid, screen grid and cathode power supplies. In order to reduce the parasitic oscillation of the amplifier, the screen grid is grounded and a ferrite material is used for the final power amplifier (FPA, using eimac 8973 tetrode). In such situation, the cathode is biased a negative voltage. Table 3 is shown the voltage of the power supply for the different tetrodes.

tetrode / Anode voltage(kV) / control grid(v) / screen grid(v) / Cathode(v)
Eimac 8973 / 22-25 / (-450)-(-850) / 0 / (-1000)-(-2500)
TH781 / 18-22 / (-300)-(-500) / 1000-2000 / 0
TH561 / 0.6-1 / (-150)-(300) / 700-1300 / 0

Table 3

Because eimac 8973 is a expensive tetrode, there is a crow bar system in FPA power supply to protect the eimac 8973 tetrode damaged from arcing inside tetrode. The protection procedure is designed in three levels. The first level is to switch off the RF signal due to FPA’s anode current is reach the maximum current (100A). The second level is to switch off RF signal and FPA anode’s high voltage power supply due to the anode current is over the maximum current. The third level is to switch off RF signal, FPA anode’s high voltage power supply and to turn on the crow bar system to protect the eimac 8973 tetrode due to the FPA’s anode current is more than twice as the maximum current. The diagram of the power supply for FPA is shown in Fig.3. And Fig.4 is shown the schematic circuit diagram of power supplies.

6.7.3  Transmission line

ICRF heating system for HT-7U is a high output RF power and can be running in continue wave (CW) mode. To satisfy high power transfer from the transmitter to the antenna, the transmission line size is 9 inch. Its outer conductor diameter is 230 mm, and inner conductor diameter is 100 mm. The characteristic impedance is 50 W. Between inner and outer conductor, the dry nitrogen gas is filled at the pressure of 3 atm. To satisfy the CW operation, the inner and the outer conductor must be cooled by the pure water. a DC breaker must be used in order to isolate the grounds between RF transmitter and the antenna,.

6.7.4  The matching system

In the Ion Cyclotron Resonance Frequency (ICRF) range, the characteristic impedance of the coaxial transmission lines and the output impedance of the transmitter are 50 W and the plasma loading of the loop antenna is in the 0.5 – 5 W range. It is necessary to match the antenna loading to 50W by adding a matching system. Normally a stub tuner is used for traditional matching system. The stub tuner is a short-circuited coaxial transmission line with a changeable length. Because the metal contact finger of conventional stub tuner is seriously damaged sometimes, we develop a liquid stub tuner for HT-7U. It utilizes the difference of radio frequency wavelengths in gas and in liquid due to the different relative dielectric constants. The liquid (i.e. oil silicon) is filled between inner conductor and outer conductor. By using a pump to control the liquid level, the parameters of this matching system can be changed. Since there is no mechanically moving parts, it will work more reliable. Due to the CW operation, the inner and outer conductor of the liquid stub tuner must be cooled by pure water. There are two triple liquid stub tuners for HT-7U 3MW ICRF heating system. A schematic diagram of a liquid stub tuner is shown in Fig.5. And Fig.6 is shown the triple liquid stub tuners.

Fig.6. The triple liquid stub tuners

6.7.5  phase shifter

The phase shifter is specially designed for FWCD. To use a phase shifter, the phase different between two ICRF antennas can be changed. The phase difference can be changed from 00 to 3600. Because of CW operation and high power RF transfer, the phase shifter needs water-cooling and 3 atm. nitrogen gas filled also. Fig.7 is shown the schematic diagram of the liquid phase shifter.

6.7.6  feedthrough

In order to transfer RF power to antenna, a feedthrough must be used to separate the vacuum. Because of a high power RF transfer, 9 inch in diameter of a feedthrough is chosen. The ceramic part of the feedthrough is difficult to make it in china, it is considered to buy this feedthrough in other laboratory.

6.7.7  Antenna

The loop antenna and fold wave guide antenna are chosen. There are two ports for ICRF heating in HT-7U.According to the purpose of the high power ICRF heating and Fast Wave Current Driven, each port integrate two antennas. The phase between antenna can be change in 0-3600. The transmission line is configured to provide the flexibility of adjusting the phasing of the straps for heating (0,0,0,0), (0,p,0,p) or for current drive (0,p/2,p,3p/2). Because of CW operation, the faraday shell and strap of the antenna must be cooled by pure water. The surface of the antenna should be coated by B2C to reduce the impurity radiation during the high power ICRF heating. In order to adjust the coupling of the antenna to plasma, the antenna can be moved a little in radial direction.