Transient impurity events in JET with the new ITER-like wall

Sertoli, M.; Flannegan, J C; Cackett, A.; Hodille, E.; Vries, Peter de; Coffey, I.; Sieglin, B.; Marsen, S.; Brezinsek, S.; Matthews, G. F.; Coenen, J. W.; Contributors, Jet‐Efda

doi:10.1088/0031-8949/2014/t159/014014

Cited by 17 publications

(6 citation statements)

References 13 publications

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“…This corresponds to a fully evaporated W-droplet of r ∼ 80µ m. The impact on the plasma is minor despite an increase in radiated power. No disruption was caused by any of theses events which in principle are similar to impurity events observed with the ILW [29,30,31]. The actual droplet size leaving the surface can not be determined due to the long lifetime of droplets leaving the local observation volume, and potentially passing thourgh divertor and main plasma [1,32].…”

Section: Resultsmentioning

confidence: 90%

ELM induced tungsten melting and its impact on tokamak operation

Coenen

Arnoux

Базылев

et al. 2015

Journal of Nuclear Materials

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Équipe 107 : Physique des plasmas chaudsInternational audienceIn JET-ILW dedicated melt exposures were performed using a sequence of 3MA/2.9T H-Mode JET pulses with an input power of P-IN = 23 MW, a stored energy of similar to 6 MJ and regular type I ELMs at Delta W-ELM = 0.3 MJ and f(ELM) similar to 30 Hz. In order to assess the risk of starting ITER operations with a full W divertor, one of the task was to measure the consequences of W transients melting due to ELMs. JET is the only tokamak able to produce transients/ ELMs large enough (>300 kJ per ELM) to facilitate melting of tungsten. Such ELMs are comparable to mitigated ELMs expected in ITER. By moving the outer strike point (OSP) onto a dedicated leading edge the base temperature was raised within similar to 1 s to allow transient ELM-driven melting during the subsequent 0.5 s. Almost 1 mm (similar to 6 mm(3)) of W was moved by similar to 150 ELMs within 5 subsequent discharges. Significant material losses in terms of ejections into the plasma were not observed. There is indirect evidence that some small droplets (similar to 80 mu m) were ejected. The impact on the main plasma parameters is minor and no disruptions occurred. The W-melt gradually moved along the lamella edge towards the high field side, driven by j x B forces. The evaporation rate determined is 100 times less than expected from steady state melting and thus only consistent with transient melting during individual ELMs. IR data, spectroscopy, as well as melt modeling point to transient melting. Although the type of damage studied in these JET experiments is unlikely to be experienced in ITER, the results do strongly support the design strategy to avoid exposed edges in the ITER divertor. The JET experiments required a surface at normal incidence and considerable pre-heating to produce tungsten melting. They provide unique experimental evidence for the absence of significant melt splashing at events resembling mitigated ELMs on ITER and establish a unique experimental benchmark for the simulations being used to study transient shallow melting on ITER W divertor PFU

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Section: Resultsmentioning

confidence: 90%

ELM induced tungsten melting and its impact on tokamak operation

Coenen

Arnoux

Базылев

et al. 2015

Journal of Nuclear Materials

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“…Such fast influxes should not be confused by events caused by so-called UFOs or transient impurity events. Although such events were often found to be present in ILW discharges [16], fewer caused disruptions, compared to carbon wall operations in 2007 to 2009 [12].…”

Section: Disruption Causes With the Iter-like Wallmentioning

confidence: 96%

The influence of an ITER-like wall on disruptions at JET

et al. 2014

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In order to preserve the integrity of large tokamaks such as ITER, the number of disruptions has to be limited. JET has operated previously with a low frequency of disruptions (i.e. disruption rate) of 3.4% [P.C. de Vries, et al., Nucl. Fusion 51 (2011) 053018]. The start of operations with the new full-metal ITER-like wall at JET showed a marked rise in the disruption rate to 10%. A full survey was carried out to identify the root causes, the chain-of-events and classifying each disruption, similar to a previous analysis for carbonwall operations. It showed the improvements made to avoid various disruption classes, but also indicated those disruption types responsible for the enhanced disruption rate. The latter can be mainly attributed to disruptions due to too high core radiation but also due to density control issues and error field locked modes. Detailed technical and physics understanding of disruption causes is essential for devising optimized strategies to avoid or mitigate these events.

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“…Reproduced with IAEA permission © IAEA [1999] from ITER Physics Expert Group (1999). Du et al 2019), or from enhanced scrape off layer cross-field transport caused by reaching the density limit (Greenwald 2002), or by an influx of high-Z impurities that radiate the thermal energy through line emission (Izzo 2006;Sertoli et al 2014). The loss of thermal energy leaves behind a cold, often impurity-rich plasma that still carries the equilibrium current; the current then begins decaying as the heightened resistivity converts magnetic to thermal energy.…”

Section: Thermal Quench Dynamicsmentioning

confidence: 99%

MHD stability and disruptions in the SPARC tokamak

et al. 2020

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SPARC is being designed to operate with a normalized beta of $\beta _N=1.0$ , a normalized density of $n_G=0.37$ and a safety factor of $q_{95}\approx 3.4$ , providing a comfortable margin to their respective disruption limits. Further, a low beta poloidal $\beta _p=0.19$ at the safety factor $q=2$ surface reduces the drive for neoclassical tearing modes, which together with a frozen-in classically stable current profile might allow access to a robustly tearing-free operating space. Although the inherent stability is expected to reduce the frequency of disruptions, the disruption loading is comparable to and in some cases higher than that of ITER. The machine is being designed to withstand the predicted unmitigated axisymmetric halo current forces up to 50 MN and similarly large loads from eddy currents forced to flow poloidally in the vacuum vessel. Runaway electron (RE) simulations using GO+CODE show high flattop-to-RE current conversions in the absence of seed losses, although NIMROD modelling predicts losses of ${\sim }80$ %; self-consistent modelling is ongoing. A passive RE mitigation coil designed to drive stochastic RE losses is being considered and COMSOL modelling predicts peak normalized fields at the plasma of order $10^{-2}$ that rises linearly with a change in the plasma current. Massive material injection is planned to reduce the disruption loading. A data-driven approach to predict an oncoming disruption and trigger mitigation is discussed.

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Transient impurity events in JET with the new ITER-like wall

Cited by 17 publications

References 13 publications

ELM induced tungsten melting and its impact on tokamak operation

ELM induced tungsten melting and its impact on tokamak operation

The influence of an ITER-like wall on disruptions at JET

MHD stability and disruptions in the SPARC tokamak

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