Plans for the electron cyclotron heating system on J-TEXT

. A new 105 GHz/500 kW/1 s electron cyclotron resonance heating (ECRH) system has been designed and being constructed on J-TEXT Tokamak. This system mainly consists of a microwave source, a transmission line, a launcher and other auxiliary units. Based on corrugated waveguides, the wave from the gyrotron can be efficiently transmitted with HE 11 mode to the steerable quasi-optical launcher for injection. The transmission efficiency is about 85%, and the injection angle of the wave can be adjusted by the flat mirror of the launcher. Commissioning of this electron cyclotron heating system is scheduled to be done at 2019.

Also, ECRH is proved to be helpful for current profile control, MHD stability, transport study, etc [7]. To fulfill the requirements of heating power on J-TEXT tokamak, an ECRH system with one 105 GHz/500 kW/1s gyrotron is being built.
The ECRH system under design mainly consists of a high power microwave source, a transmission line based on corrugated waveguide, and a launcher connected to J-TEXT tokamak. The tokamak has a major radius of 1.05m and a minor radius of 0.25~0.29m. The center-line toroidal field for normal discharge is about 2T.
Considering the parameters above and requirements of our experiments, a 105GHz/500kW/1s gyrotron has been chosen as the microwave source. A transmission line is constructed based on circular corrugated waveguides to transmit electron cyclotron (EC) waves from gyrotron to the port of the tokamak with HE11 mode, which is proved to be an efficient way in reducing the loss of power. The launcher is designed according to the expected poloidal and toroidal injection angles and actual limitation of space.
This paper consists of following parts. The details of microwave source are given in Section 2. Section 3 introduces the design of the transmission line. Section 4 gives the parameters of launcher. Conclusions are shown in Section 5.

Microwave source
As the high power microwave source of ECRH system, the gyrotron delivers EC waves of megawatt level power at frequency varied from 28~170 GHz. For ECRH system, operating parameters of the whole system are mainly determined by parameters of the gyrotron. According to the theory of EC wave propagation and absorption in plasma, the frequency of gyrotron f is determined by the toroidal magnetic field BT, as follows [8]: Where fce is the electron cyclotron resonance frequency, e is the electron charge, me is the electron mass, and n is the number of the harmonic.
For the J-TEXT tokamak, the maximum toroidal magnetic field is 2.4 T at present [9]. Considering the needs of physics experiment and values, the 105 GHz/500 kW/1 s gyrotron, which is manufactured by GYCOM, has been chosen for J-TEXT tokamak. The parameters of this type gyrotron is shown in Table 1. When the EC wave is injected from the low field side of the J-TEXT port 4, the second harmonic of extraordinary mode (X2) will be absorbed at the toroidal magnetic field of about 1.88 T.  Table 2.

Transmission line
A transmission line (TL) is responsible for transmitting the EC waves from gyrotron to launcher. The TL must meet the following requirements: high power capability, low attenuation, low probability of arcing, low difficulty in measuring the power of EC waves.

Launcher
As shown in Section 3, the EC wave is transmitted to the port 4 (233mm × 219mm) on J-TEXT tokamak. A quasi-optical launcher has been designed to inject the beam into plasma with the desired poloidal and toroidal direction. The structure of the designed 105GHz launcher is shown in Fig. 2.
In Fig. 2, the wave is firstly focused by the ellipsoidal mirror, and then it will be reflected to plasma by the steerable flat mirror. The poloidal and toroidal angles of the incident beam can be adjusted with the steerable flat mirror by two separate poloidal and toroidal drivers. The steering mechanism of the flat mirror is shown in Fig. 3.
Considering the space limitation of port 4 and the steering mechanism, the poloidal and toroidal rotation angles are ±15°. More details about the design of ellipsoidal mirror and flat mirror can be found in Ref. [12].

Conclusion
To expand the operation range of J-TEXT tokamak, a 105GHz/500kW/1s ECRH system is being developed.
The system is based on the Gycom gyrotron, the transmission line with corrugated waveguides, and a quasi-optical launcher. Currently, the design of this system has been finished and all the subunits are on the way. The gyrotron will be delivered at the end of 2018, and the first commissioning of the ECRH system will be done at the middle of 2019. This ECRH system will be mainly used for plasma heating, transport, MHD stability, disruption avoidance, etc. We plan to carry out experiments with the 105 GHz ECRH system at the beginning of 2020.