Engineering Challenges of Wendelstein 7-X Mechanical Monitoring During Second Phase of Operation

Figures & data

Fig. 1. Toroidal displacements under low iota 2.5-T EM loads in meters. (Last reference regime commissioned on June 28, 2018. Finite Element GM version 6.04: 5 × 1.4-M nodes.)

TABLE I Operation Phases, Achievements, and Future Goals

Operation Phase	OP1.1 (2015–2016)	OP1.2a/b (2017–2018)	OP2 (2021+)
Operation phase main characteristic	Limiter configuration plasma	Inertially cooled divertor	Water-cooled high heat flux divertor, steady state
Maximum energy in one pulse, MJ	4	200	18 000
Maximum pulse, s duration	6	100	1800
Discharge programs^Citation5,Citation6	~940	~1500 + ~1600
Magnet system energizing	>130 2 “main” 2.5-T load configurations	>200 20 “main” 2.5 T and 100 variations 2 × 2.7 T and transitions between regimes	Operation sequence to be decided:^Citation4 ~1000 2.5-T “main” cycles ~100 3-T cycles
Fraction of design loads	70% with some exceptions	70% for all components	100%
Mechanical sensor availability	~95%	~90% (2.6% blacklisted; 7% questionable)	–

Fig. 2. Fragment of MS global FE model with indication of main support types.⁷

Fig. 3. Comparison between numerical prediction and measurements for coil mutual displacements between NPC type 5 coils and PLC type B during ramping up low iota coil currents. (a) Finite element prediction, (b) typical monitoring supervision results, and (c) post processing of monitoring results to show average signal and error bars.

Fig. 4. Finite element prediction for PLC type B coil case von Mises stress (recalculated from strain gauges rosette⁴) during ramping up low iota coil currents. The location of the sensor is shown in Fig. 3; the two lines represent two half-modules similar to Fig. 3.

Fig. 5. Typical example of FE model of welded lateral support.

TABLE II Types of Analyses for OP1.2a and OP1.2b

Part of Second Phase of Operation	Restart from Cooldown or Other EM Load	ANSYS Perturbation Analysis (Linearization)
OP1.2a	27	0
OP1.2b	54	104

Fig. 6. Schematic view of linearization.

Fig. 7. Filtering of configuration proposals from physicists after improvements following OP1.1 and OP1.2a.

Fig. 8. Example of MS structural limitations for 2.5-T regimes proposed for NBI operation with minimum fast ion impact on in-vessel components.¹⁶

TABLE III Fast Discharge Events from High Coil Current Levels with Their Consequences

Operation Phase	Fast Discharge Date	Configuration	NPCs (kA)	PLCs (kA)	Consequence and Date, if Specified
OP1.1	2015/16	Multiple events for limiter and standard			Bolt prestress degradation up to 15 MPa
OP1.2a	2017
	July 6	Limiter	10	+3.9	July 7: problem with PLC AAC31 (four strain gauges, no explanation)
	Aug. 24	High iota	12	−8.3	Aug. 30: problem with sensor AAD11HH901 (“crack”-like event); blacklisted for OP1.2b due to clear failure behavior during warming-up/cooldown cycle afterward
	Nov. 28	Inward shift	10	−3.1/+6.3
	Dec. 5	Reversed standard	−10	0
OP1.2b	2018
	June 27^a	Standard case	13.5	0
	June 28	Partial low iota	8.0	−6
	Aug. 16^a	Modified standard	13.1	−0.7	Aug. 17: degradation of bolt preload (3 MPa)

^aUnexpected event.

K. RISSE et al., “The Magnet System of Wendelstein 7-X Stellarator in Operation,” Fusion Eng. Des., 136, 12 (2018); https://doi.org/10.1016/j.fusengdes.2017.12.008.

 Web of Science ®Google Scholar

T. RUMMEL et al., “Wendelstein 7-X Magnets: Experiences Gained During the First Years of Operation,” Fusion Sci. Technol. 75 (2019); https://doi.org/10.1080/15361055.2019.1629248.

 Web of Science ®Google Scholar

J. FELLINGER et al., “Preparation for Commissioning of Structural Sensors of Wendelstein 7-X Magnet System,” Fusion Eng. Des., 98–99, 1048 (Oct. 2015); https://doi.org/10.1016/j.fusengdes.2015.06.041.

 Web of Science ®Google Scholar

V. BYKOV, “Engineering Challenges of W7-X: Improvement of Numerical Modeling and Mechanical Monitoring After Commissioning and First Phase of Operation,” Fusion Sci. Technol., 72, 546 (Nov. 2017); https://doi.org/10.1080/15361055.2017.1352427.

 Web of Science ®Google Scholar

S. AKASLOMPOLO et al., “Predictive Modelling of NBI Ion Wall Loads in the W7-X Stellarator,” Nucl. Fusion, 58, 8, 082010 (2018); https://doi.org/10.1088/1741-4326/aac4e5.

 Web of Science ®Google Scholar