arXiv subjects

G. Derks

Publications and source records attributed to G. Derks.

6 recordsLinked to original sources

arXiv2026

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

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.

physics.plasm-ph↗

arXiv2025

Divertor shaping with neutral baffling as a solution to the tokamak power exhaust challenge

Exhausting power from the hot fusion core to the plasma-facing components is one of the biggest challenges in fusion energy. The MAST Upgrade tokamak uniquely integrates strong containment of neutrals within the exhaust area (divertor) with extreme divertor shaping capability. By systematically altering the divertor shape, this study shows the strongest evidence to date that long-legged divertors with a high magnetic field gradient (total flux expansion) deliver key power exhaust benefits without adversely impacting the hot fusion core. These benefits are already achieved with relatively modest geometry adjustments that are more feasible to integrate in reactor designs. Benefits include reduced target heat loads and improved access to, and stability of, a neutral gas buffer that 'shields' the target and enhances power exhaust (detachment). Analysis and model comparisons shows these benefits are obtained by combining multiple shaping aspects: long-legged divertors have expanded plasma-neutral interaction volume that drive reductions in particle and power loads, while total flux expansion enhances detachment access and stability. Containing the neutrals in the exhaust area with physical structures further augments these shaping benefits. These results demonstrate strategic variation in the divertor geometry and magnetic topology is a potential solution to one of fusion's biggest challenges: power exhaust.

physics.plasm-ph↗

arXiv2024

Investigations of atomic \& molecular processes of NBI-heated discharges in the MAST Upgrade Super-X divertor with implications for reactors

This experimental study presents an in-depth investigation of the performance of the MAST-U Super-X divertor during NBI-heated operation (up to 2.5 MW) focussing on volumetric ion sources and sinks as well as power losses during detachment. The particle balance and power loss analysis revealed the crucial role of Molecular Activated Recombination and Dissociation (MAR and MAD) ion sinks in divertor particle and power balance, which remain pronounced in the change from ohmic to higher power (NBI heated) L-mode conditions. The importance of MAR and MAD remains with double the absorbed NBI heating. MAD results in significant power dissipation (up to $\sim 20 \%$ of $P_{SOL}$), mostly in the cold ($T_e < 5$ eV) detached region. Theoretical and experimental evidence is found for the potential contribution of $D^-$ to MAR and MAD, which warrants further study. These results suggest that MAR and MAD can be relevant in higher power conditions than the ohmic conditions studied previously. Post-processing reactor-scale simulations shows that MAR and MAD can play a significant role in divertor physics and synthetic diagnostic signals of reactor-scale devices, which are currently underestimated in exhaust simulations. This raises implications for the accuracy of reactor-scale divertor simulations of particularly tightly baffled (alternative) divertor configurations.

physics.plasm-ph↗

arXiv2010

Localized mode interactions in 0-pi Josephson junctions

A long Josephson junction containing regions with a phase shift of pi is considered. By exploiting the defect modes due to the discontinuities present in the system, it is shown that Josephson junctions with phase-shift can be an ideal setting for studying localized mode interactions. A phase-shift configuration acting as a double-well potential is considered and shown to admit mode tunnelings between the wells. When the phase-shift configuration is periodic, it is shown that localized excitations forming bright and dark solitons can be created. Multi-mode approximations are derived confirming the numerical results.

nlin.PS↗

arXiv2008

Stability analysis of $π$-kinks in a 0-$π$ Josephson junction

We consider a spatially non-autonomous discrete sine-Gordon equation with constant forcing and its continuum limit(s) to model a 0-$π$ Josephson junction with an applied bias current. The continuum limits correspond to the strong coupling limit of the discrete system. The non-autonomous character is due to the presence of a discontinuity point, namely a jump of $π$ in the sine-Gordon phase. The continuum models admits static solitary waves which are called $π$-kinks and are attached to the discontinuity point. For small forcing, there are three types of $π$-kinks. We show that one of the kinks is stable and the others are unstable. There is a critical value of the forcing beyond all static $π$-kinks fail to exist. Up to this value, the (in)stability of the $π$-kinks can be established analytically in the strong coupling limits. Applying a forcing above the critical value causes the nucleation of $2π$-kinks and -antikinks. Besides a $π$-kink, the unforced system also admits a static $3π$-kink. This state is unstable in the continuum models. By combining analytical and numerical methods in the discrete model, it is shown that the stable $π$-kink remains stable, and that the unstable $π$-kinks cannot be stabilized by decreasing the coupling. The $3π$-kink does become stable in the discrete model when the coupling is sufficiently weak.

nlin.PS↗

arXiv2002

Transverse instability and its long-term development for solitary waves of the (2+1)-Boussinesq equation

The stability properties of line solitary wave solutions of the (2+1)-dimensional Boussinesq equation with respect to transverse perturbations and their consequences are considered. A geometric condition arising from a multi-symplectic formulation of this equation gives an explicit relation between the parameters for transverse instability when the transverse wavenumber is small. The Evans function is then computed explicitly, giving the eigenvalues for transverse instability for all transverse wavenumbers. To determine the nonlinear and long time implications of transverse instability, numerical simulations are performed using pseudospectral discretization. The numerics confirm the analytic results, and in all cases studied, transverse instability leads to collapse.

nlin.CD↗