Description
A self-burning magnetically confined fusion plasma requires the confinement of the generated alpha particles until their energy is transferred via collisions to the DT fuel. Wendelstein 7-X is the first stellarator optimised for this purpose but requires for experimental proof an average plasma beta (ratio of kinetic to magnetic pressure) of β > 4 %, which will not be achievable with the available heating power at the standard magnetic field of 2.5 T. A simple method to increase β is to reduce the magnetic field, as the magnetic pressure decreases more than the kinetic pressure in this case. Even though the electron cyclotron resonance heating system of W7-X is designed for second harmonic heating at 140 GHz, the less absorbing third harmonic (X3) can be used at around 1.7-1.8 T in combination with special reflectors in the vacuum vessel allowing up to 3 passes through the plasma for almost complete absorption. However, the plasma breakdown is not possible with the third harmonic, which required a customised plasma startup scenario with a detuned gyrotron at 101 GHz and a separate multipath scenario with even 6 passes through the plasma. In addition, up to 4 MW of neutral beam injection are necessary to increase the plasma beta and shift the resonance into the plasma centre for successful X3-takeover. Already in the first campaign with X3 operation, a total heating power of 10 MW (ECRH + NBI) was sufficient to achieve a volume averaged plasma beta of 2.8 % at W7-X with central beta values of the order 10 %.