1. 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

2. 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

3. Introduction: Wendelstein 7-X

4. 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

5. Overview of OP1.1: pulse lenght

6. Limitation: loss of microwave absorption
14:11:36 UTC

7. Overview of OP1.1: pulse lenght

8. 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.

9. ECA diagnostic

10. 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!

11. 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

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

13. Off-axis heating in OP1.1

14. 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

15. 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

16. 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

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

18. 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

19. 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

20. 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.

21. Backup