Noise limits on ITER plasma vertical stabilization system imposed by tungsten divertor monoblock thermal fatigue

• Published in: Fusion engineering and design 2020-12, Vol.161, p.111861, Article 111861
• Main Authors: Miller, M.A., Pitts, R.A., Bonnin, X., Carli, S., Escourbiac, F., Gribov, Y., Kavin, A.A., Lukash, V.E., Khayrutdinov, R.R., Komarov, V., van Vugt, D.
• Format: Article
• Language: English
• Subjects: Acceptance criteria; Acceptance tests; Coils; Cyclic loads; Diagnostic systems; Displacement; Divertor; Evolution; Fatigue; Feedback; Finite element method; Flux density; Frequencies; Heat flux; ITER; Liquid cooling; Noise; Palmgren-Miner rule; Plasma; Plasma currents; Power gain; Recrystallization; Stabilization; Surface temperature; Temperature distribution; Thermal analysis; Thermal fatigue; Time dependence; Tungsten monoblock; Vertical orientation; Vertical stability
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2020.111861

•Noise in tokamak reactor plasma vertical position drives oscillations in divertor strike points.•Strike point movements drive thermal fatigue in actively cooled tungsten monoblocks.•Fatigue accumulation assessed with Palmgren-Milner’s rule and Rainflow counting.•RMS noise in speed of plasma vertical position must be less than 0.4 ms−1 in ITER.•Natural tokamak vertical plasma oscillations beneficial for divertor power handling. Tokamak plasmas with vertically elongated cross-sections are unstable due to vertical displacements. Feedback stabilization of the ITER plasma current centre will be performed using the speed of its vertical displacements, dZ/dt, as input. Inevitable noise in the diagnostic signal for dZ/dt leads to “noisy” components in the feedback voltage and current in the stabilizing coils. This leads to noisy components in the vertical position of the plasma current centre and the divertor strike points. For burning plasma conditions on ITER, these strike point displacements may be a concern for thermal fatigue at the water cooling interface of the tungsten monoblocks constituting the divertor targets. A study is presented here in which this concern is examined for the first time. The baseline 15 MA scenario (fusion power of 500 MW with a 500 s flattop and fusion power gain, QDT = 10) is simulated with the DINA code, assuming low frequency noise in the dZ/dt diagnostic signal. The noise has uniform spectrum with a given root mean square (RMS) value ( =0.6 ms−1 or 0.2 ms−1) in the frequency band (0, 1 kHz). The results of these DINA simulations are combined with dissipative divertor plasma solutions obtained with the SOLPS-ITER plasma boundary code to provide a time dependent divertor target heat flux density profile. The latter is then imposed on a finite element model of the target monoblocks to assess the temporal evolution of the 3D temperature field in the block, including the Cu-W and Cu-CuCrZr joints at the cooling interface. These joints represent the points of the Cu and CuCrZr materials that see the largest temperature changes and are thus at greatest risk of failure. To evaluate an acceptance criterion on the non-cyclic and non-uniform thermal loads at the joints, an approach is developed which combines a Rainflow counting technique with Palmgren-Miner’s rule for fatigue accumulation. Analysis of the temperature evolution at the Cu-W joint shows that the RMS value of noise  ∼0.6 ms−1 is unacceptable from the point of v