Status of the 1-MW, 140-GHz, CW gyrotron for W7-X

1\. Introduction
High frequency gyrotrons with high output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion experiments. Electron cyclotron resonance heating (ECRH) has proven to be an important tool for plasma devices especially for stellarators, as it provides both net current free plasma start up from the neutral gas and efficient plasma heating. For the stellarator Wendelstein 7-X now under construction at IPP Greifswald, Germany, a 10 MW ECRH system is foreseen. A European collaboration has been established between Forschungszentrum Karlsruhe (FZK), IPP Garching / Greifswald, IPF Stuttgart, CRPP Lausanne, CEA Cadarache and TED Vélizy, to develop and build the 10 gyrotrons each with an output power of 1 MW for continuous wave (CW) operation.
 
2\. Design
The design parameters of the gyrotron are summarized in Table 1. A temperature limited magnetron injection gun without intermediate anode (diode type ) is used.
The cylindrical cavity is designed for operation in the TE28,8 mode. It is a standard cavity with a linear input downtaper of 2.5° and a non-linear uptaper with the initial angle of 3°. ... mehrThe transitions between tapers and straight section are smoothly rounded over a length of 4-6 mm to avoid mode conversion. The TE28,8-cavity mode is transformed to a Gaussian TEM00 output mode by an advanced rippled-wall mode converter and a three mirror system. The output window unit uses a single, edge cooled CVD-diamond disk with an outer diameter of 106 mm, a aperture of 88 mm and a thickness of 1.8 mm.
 
3\. Experiment
The output power of the first series tube turned out to be almost linearly dependent on the electron  beam current at constant magnetic field. An output power of 1 MW at 40 A and 1.15 MW at 50 A was measured in short pulse operation (~ms) with efficiencies of 31 % and 30%, respectively (without depressed collector operation). This behaviour shows the good quality of the electron emitter which had been proven by an optical inspection and by the homogeneous temperature distribution before installation of the emitter ring into the gyrotron.
RF-field distribution measurements (perpendicular to the output RF-beam direction) were performed at different positions with respect to the window. The beam is shifted by about 12 mm in horizontal direction. The Gaussian content was calculated from the measurements to be 97.5 %.
In a range between 5.52 – 5.56 T of the magnetic field at the cavity, no maximum for the output power was found. The power increased slightly with increasing magnetic field. For maximum output power, the accelerating voltage (corresponding to the energy of the electrons inside the cavity) was adjusted and followed nicely the law that the ratio between magnetic field and the relativistic factor g has to be constant. Increasing the voltage beyond this value leads to an excitation of neighbouring modes. The measurements were performed at a constant beam current of 40 A, but with optimising the electron beam radius.
A strong dependence of the output power has been found for different electron beam radii in the cavity. The desired mode can only be excited in a small range between 10.25 mm and 10.43 mm. At lower beam radii, arcing occurs, at higher radii a wrong mode is excited. The optimum value of the beam radius decreases slightly with decreasing cavity field.
In long pulse operation, the power was measured calorimetrically with a RF-load which was placed about 6 m away from the gyrotron window. The highest output power inside the load for a three minute pulse was measured to be 906 kW. Including the calorimetrically measured stray radiation the total output power was 922 kW with an efficiency of 45 % (depressed collector operation).
At FZK, the pulse length at full power is limited to three minutes, but at reduced electron beam current (< 30 A) longer pulses can be achieved. The pressure increase during the 30 minute pulse (1839 s) is less than a factor of two ending up at about 6·10-9 mbar.
After the successful tests at FZK, the tube was delivered to IPP Greifswald for tests at highest output power and a pulse length of 30 minutes. A directed output power of 865 kW was measured inside the load, and a total output power of about 920 kW was estimated taking the losses in the transmission line into account (world record in energy content).
The second series tube arrived at FZK end of November. First very short pulse experiments (1 ms) yielded an output power of 960 kW.
 
4\. Conclusions
The first series tube of the 1-MW, 140 GHz, CW gyrotrons for W7-X passed the site acceptance test successfully. In short pulse operation the power was measured to be 1 MW, and for three minutes the power was 922 kW at an efficiency of 45 %. At IPP the pulse length could be extended to 30 minutes with the same output power.