A First 2 MW-Class (136)/170/204 GHz Multi-Frequency Gyrotron Pre-Prototype for DEMO: Design, Construction and Key Components Verification

DEMO is planned to be the first DEMOnstration fusion power plant that will proof the production of electricity by facilitating the Tokamak concept.
The fusion reaction takes place in a magnetically confined plasma that consists of Deuterium and Tritium.
A temperature of up to 120 million Kelvin is required for the reaction.
Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) is a possible method to heat and to control the plasma.
The sources are high-power vacuum gyrotrons that produce microwave power at MW-levels in the microwave and sub-THz range.
The European (EU) DEMO requirements associated with this work are adapted from those of ITER, from 1 MW to 2 MW output power per unit, from 170 GHz single-frequency operation to 136/170/204 GHz multi-frequency / multi-purpose operation and the ability to tune the frequency within a bandwidth of $\pm$ 10 GHz around the center frequency in steps of 2-3 GHz.
In this work, the first 2 MW 170/204 GHz dual-frequency coaxial-cavity short-pulse pre-prototype has been designed and built on the basis of an existing 2 MW 170 GHz coaxial-cavity short-pulse pre-prototype.
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The Magnetron Injection Gun, the coaxial cavity and the quasi-optical output coupler are adapted to the magnetic field profile of a new 10.5 T SC magnet and to the operation at 204 GHz.
Existing components, such as the cathode shall be used.
The TE$_{40,23}$ mode is identified as the operating mode at 204 GHz.
The mode fits to the parameters that are already determined by the TE$_{34,19}$ mode at 170 GHz.
A new coaxial cavity is designed using an adapted systematic procedure.
This design provides an output power of 2.1 MW at 204 GHz, which is an increase of about 20 %.
The design allows an output power up to 2.6 MW at 170 GHz.
In addition, a new 170/204 GHz dual-frequency quasi-optical output coupler has been developed providing a Gaussian mode content of > 95 %.
Finally, simulations at 136 GHz show a principle operation satisfying the DEMO requirements.

Frequency step-tunability around the given center frequencies has been investigated for the first time considering the insert loading constraint during the mode selection process. A novel mode series is presented that includes a jump in the azimuthal mode index from the nominal TE$_{m,p}$ mode to the TE$_{m+2,p-1}$ mode. The proposed mode series reduces the deviation of the relative caustic radii by a factor of two compared to classical approaches. Simulations are performed to tune the gyrotron in steps from high to low frequency and reverse. The deviation of the output power is in maximum 5 % at 170 GHz and 18 % at 204 GHz.

An update of the frequency measurement system for gyrotron operation at KIT is performed to verify gyrotrons and their key components operating above 200 GHz.
They are upgraded from the current frequency limitation of 175 GHz to at least 260 GHz.
Particularly, the quasi-optical mode generator is optimized.
It is used to validate the quasi-optical output coupler.
The mechanically controlled linear drivers are replaced by high-precision computer-controlled linear drivers.
A goniometer has been added for a full electronically adjustment.
A novel automated measurement procedure with mode evaluation algorithms is implemented.
The optimizations reduce the time needed to excite the desired gyrotron mode with higher quality from months to a few days.
The excited cavity modes operating at 170 GHz and 204 GHz feed the 170/204 GHz dual-frequency quasi-optical output coupler for verification in cold measurement.
A comparison shows an excellent agreement with simulations.
The TE$_{40,23}$ mode at 204 GHz is the mode with the highest eigenvalue ever excited in cold measurements.

Finally, a study on alternative mode series is performed considering an extended wall loading constraint of up to 2.5 kW/cm$^2$.
These mode series benefit from low deviation of the relative caustic radius.
One identified mode series has a beam radius and an insert radius, which is almost identical to the existing mode series.
The result is that the most complicated and costly components to manufacture (insert and MIG) can be reused and do not need to be modified.
A fast verification of the proposed mode series using several existing components is proposed.