ECH Experiments and Stray Radiation Study in Wendelstein 7-X in the Context of Steady-State Operation of Fusion Devices D. Moseev1, H.P. Laqua1, T. Stange1, S. Marsen1, H. Braune1, K.J. Brunner1, V. Erckmann1, J. Fellinger1, D. Hathiramani1, W. Kasparek2, J.W. Oosterbeek1, R.C. Wolf1, U. Wenzel1, F. Wilde1,3, and the Wendelstein 7-X team1 1Max-Planck-Institut für Plasmaphysik, Greifswald, Germany 2University of Stuttgart, Stuttgart 3Karlsruhe Institute of Technology, Karlsruhe, Germany

Outline Introduction: Overview of OP1.1 Wendelstein 7-X ECRH on W7-X Overview of OP1.1 Measurements of non-absorbed microwaves: first-pass absorption diagnostic and sniffer probes Understanding of stray radiation measurements – power balance modelling Off-axis heating in OP1.1 O2 ECRH in OP1.1 Stray radiation in the machine ports Conclusions

Introduction: Wendelstein 7-X

Introduction: ECRH facility at W7-X More than 4(8) MW of ECRH 6(10) gyrotrons at 140 GHz 40 m long quasi-optical transmission line

Overview of OP1.1: pulse lenght

Limitation: loss of microwave absorption 14.12.2015 @ 14:11:36 UTC

Overview of OP1.1: pulse lenght

What are we afraid of? Unabsorbed power is one of the main problems with microwave heating. Diagnostics and in-vessel elements can sustain large long-lasting loads of stray radiation (unpolarized, isotropic microwave radiation). The real danger is a directed beam load as a result of a shine-through or reflection. This kind of load is very localized with large energy density flux.

ECA diagnostic

Sniffer probes Directed beam loads are difficult to measure: refraction, beam broadening, reflections, etc… Related quality to the first pass absorption – directed beam absorption, measured by stray radiation modelling. Sniffer probes are absolutely calibrated!

Do we understand what stray radiation measurement represnt? Multiresonator model: Isotropic, uniform, and arbitrary polarized in every resonator The level of stray radiation is defined by power balance In every volume: p – stray radiation energy flux, W/m2 Pinputk – input power in the volume k Index j runs through different surfaces Aj with different microwave absorption ηj (color-coded). pi stray radiation power density in neighbor volume i with interface area Si H. P. Laqua et al., 28th EPS conference, 2001

Estimation of microwave absorption Effective absorption area of the stellarator measurement const

Off-axis heating in OP1.1

Deposition around normalized radius of 0.4 Off-axis heating in OP1.1 Off-axis heating can be used for transport studies Profile manipulations Etc. Should we worry about the microwave absorption for non-central deposition? Deposition around normalized radius of 0.4 Limit: 50 kW/m2

Single-pass absorption of O2 ECRH X2 ECRH is almost completely absorbed at low Te – very good heating technique for densities below 1.2∙1020 m-3. For higher densities ordinary polarization should be used (O2) – poorer absorption

Multipass absorption of O2 ECRH Test of O2-scenario in OP1.1: Setup a multipath scenario for 4 of the 6 gyrotron beams 3 beam paths allow up to 95% absorption Plane graphite tile Curved steel paneel ECRHbeam

O2 ECRH in OP1.1 Limit: 50 kW/m2

Stray radiation in the ports Maximum allowed ΔT = 40 °C of the bellows, in 30 min. experiments corresponds to stray radiation load of around 12 kW/m2 S. Marsen et al., submitted to NF

Stray radiation in the ports Simulation of stray radiation penetration with 1 MW of unabsorbed mm-wave power into the AEE – port in the launcher module. Bellow is endangered! AEE

Conclusions In OP1.1 several hundreds of experiments were terminated due to the loss of mm-wave absorption – very reliable sniffer interlock system. Stray radiation measurements in W7-X are interpretable and agree with theoretical predictions. X2 on- and off-axis experiments show nearly full microwave absorption. Single pass absorption experiments of O2 microwaves demonstrates absorption of more than 70%, which agrees with TRAVIS predictions of 71%. Multi-pass O2 heating scheme could sustain plasma and 95% absorption demonstrated. For steady-state operation, port bellows can be overloaded with stray radiation, actions needed.

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