#Authors Ondřej Grover, Mikhail Gryaznevich, Michal Odstrčil, Jan Stockel, Vojtěch Stránský, Vojtěch Svoboda, Gabriel Vondrášek, Gennadii Vorobjev

#Motivation * Detail investigation of the radiofrequency discharge can help to learn confinement conditions of microwave generated plasma when the toroidal magnetic field is lower then resonant conditions for given frequency. This can be of interest for the ITER planned experiment, where the inital regime of it’s scenario is planned with toroidal magnetic field two times lower then nominal. * Secondly, for the GOLEM operation with the HTS poloidal field coils in the superconducting regime, the modifications of the discharge scenario is required. To reduce AC losses during current ramp-up in HTS coils, reduction in the current ramp-up speed is needed, as this will result in a slower ramp-up of the current in coils. One of possible solutions may be reduction in the loop voltage needed for the plasma breakdown. It is well known that RF pre-ionisation is a powerful tool to achieve this goal

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To reduce the loop voltage required for breakdown of the working gas, a pre-ionization has to be used. The GOLEM tokamak is equipped by an electron gun. Alternatively, microwaves (MW) are used for this purpose based on a low-cost magnetron with frequency 2.45GHz, which provide the MW power at a level of ~1 kW.

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The apparatus for the microwave pre-ionization, Picture of the apparatus and the electrical scheme of power supplies are shown.

Vacuum and MW breakdown driven plasma shot

The RF power was injected in a short (<10ms) pulse during the ramp-up of the toroidal magnetic field Bt and the breakdown occurs at Bt = 0.0875 T, which corresponds to localization of the Electron Cyclotron Resonance layer (for f = 2.45 GHz) inside the tokamak vessel.

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Loop voltage characteristics of vacuum shot (black) and plasma discharge performed by means of microwave (red) - breakdown occurs when ECR layer is in the center of the vessel

MW versus ElGun preionization @ 10 mPa

A series of discharges, where the breakdown was achieved either by electron gun or microwave pre-ionization was performed.

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Loop voltage at breakdown versus the toroidal magnetic field. Comparison of the microwave preionization (red symbols) and the electron gun preionization (black symbols) at the pressure of the working gas 10 mPa.

It is seen that the MW preionization is more efficient when the EC resonance layer is inside the tokamak vessel, while the electron gun preionization acts better at low magnetic fields.

MW versus ElGun preionization - Working gas survey

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