Detachment dynamics and disturbance rejection in the TCV X-Point Target divertor

Abstract

The X-Point Target (XPT) is an alternative divertor configuration with a secondary X-point in the divertor leg. We investigate its detachment dynamics and disturbance rejection capacity on the TCV tokamak, comparing it with a long-legged single-null (SN) divertor. We employ a system identification approach using multi-sine perturbations to D₂ fuelling in Ohmic L-mode scenarios and to N₂ seeding and electron cyclotron resonance heating (ECRH) power in auxiliary-heated L-mode scenarios, using the CIII emission front position as a proxy for the detached state evolution. For D₂ fuelling perturbations at 4-31 Hz, the gain of the front response at the secondary X-point is at least a factor of 3 lower than upstream of it, where the response is similar to that of the SN. This indicates that the reduced detachment sensitivity of the secondary X-point in an XPT configuration previously observed in quasi-steady state experiments, extends to control-relevant timescales as an enhanced disturbance rejection capacity. In auxiliary-heated scenarios, the CIII front in the XPT remains nearly stationary near the secondary X-point under N₂ seeding and ECRH power perturbations, whereas the CIII front in SN discharges responds strongly. However, due to complex SOL profiles in the region close to the secondary X-point of the XPT configuration, this reduction in CIII front response may not imply disturbance rejection of the detached state as a whole. The enhanced disturbance rejection at the secondary X-point could provide a passive buffer against disturbances, simplifying the task of exhaust controllers to maintain the detached state throughout a discharge cycle.

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Cite

BibTeX RIS

BibTeX

@misc{winkel2026detachment,
  title = {{Detachment dynamics and disturbance rejection in the TCV X-Point Target divertor}},
  author = {M. Winkel and K. Verhaegh and B. Kool and K. Lee and M. Carpita and D. Brida and G. Derks and O. Février and R. I. Morgan and O. Pan and A. Perek and C. Theiler and M. Zurita and M. van Berkel and the TCV team and the EUROfusion tokamak exploitation team},
  howpublished = {arXiv},
  year = {2026},
  month = nov,
  doi = {10.48550/arxiv.2606.23432},
  eprint = {2606.23432},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2606.23432},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Detachment dynamics and disturbance rejection in the TCV X-Point Target divertor
AU  - M. Winkel
AU  - K. Verhaegh
AU  - B. Kool
AU  - K. Lee
AU  - M. Carpita
AU  - D. Brida
AU  - G. Derks
AU  - O. Février
AU  - R. I. Morgan
AU  - O. Pan
AU  - A. Perek
AU  - C. Theiler
AU  - M. Zurita
AU  - M. van Berkel
AU  - the TCV team
AU  - the EUROfusion tokamak exploitation team
PY  - 2026
DA  - 2026/11/02
PB  - arXiv
DO  - 10.48550/arxiv.2606.23432
UR  - https://arxiv.org/abs/2606.23432
AB  - The X-Point Target (XPT) is an alternative divertor configuration with a secondary X-point in the divertor leg. We investigate its detachment dynamics and disturbance rejection capacity on the TCV tokamak, comparing it with a long-legged single-null (SN) divertor. We employ a system identification approach using multi-sine perturbations to D_2 fuelling in Ohmic L-mode scenarios and to N_2 seeding and electron cyclotron resonance heating (ECRH) power in auxiliary-heated L-mode scenarios, using the CIII emission front position as a proxy for the detached state evolution. For D_2 fuelling perturbations at 4-31 Hz, the gain of the front response at the secondary X-point is at least a factor of 3 lower than upstream of it, where the response is similar to that of the SN. This indicates that the reduced detachment sensitivity of the secondary X-point in an XPT configuration previously observed in quasi-steady state experiments, extends to control-relevant timescales as an enhanced disturbance rejection capacity. In auxiliary-heated scenarios, the CIII front in the XPT remains nearly stationary near the secondary X-point under N_2 seeding and ECRH power perturbations, whereas the CIII front in SN discharges responds strongly. However, due to complex SOL profiles in the region close to the secondary X-point of the XPT configuration, this reduction in CIII front response may not imply disturbance rejection of the detached state as a whole. The enhanced disturbance rejection at the secondary X-point could provide a passive buffer against disturbances, simplifying the task of exhaust controllers to maintain the detached state throughout a discharge cycle.
ER  -

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