Effect of nitrogen seeding on the energy losses and on the time scales of the electron temperature and density collapse of type-I ELMs in JET with the ITER-like wall

Abstract

The baseline type I ELMy H-mode scenario has been re-established in JET with the new tungsten MKII-HD divertor and beryllium on the main wall (hereafter called ITER-like wall, JET-ILW). The first JET-ILW results show that the confinement is degraded by 20-30% in the baseline scenarios compared to the previous carbon wall JET (JET-C) plasmas. The degradation is mainly driven by the reduction in the pedestal temperature. Stored energies and pedestal temperature comparable to the JET-C have been obtained to date in JET-ILW baseline plasmas only in the high triangularity shape using N 2 seeding. This work compares the energy losses during ELMs and the corresponding time scales of the temperature and density collapse in JET-ILW baseline plasmas with and without N 2 seeding with similar JET-C baseline plasmas. ELMs in the JET-ILW differ from those with the carbon wall both in terms of time scales and energy losses. The ELM time scale, defined as the time to reach the minimum pedestal temperature soon after the ELM collapse, is ≈2ms in the JET-ILW and lower than 1ms in the JET-C. The energy losses are in the range ∆W ELM /W ped ≈7-12% in the JET-ILW and ∆W ELM /W ped ≈10-20% in JET-C, and fit relatively well with earlier multi-machine empirical scalings of ∆W ELM /W ped with collisionality. The time scale of the ELM collapse seems to be related to the pedestal collisionality. Most of the non-seeded JET-ILW ELMs are followed by a further energy drop characterized by a slower time scale ≈8-10ms (hereafter called slow transport events), that can lead to losses in the range ∆W slow /W ped ≈15-22%, slightly larger than the losses in JET-C. The N 2 seeding in JET-ILW significantly affects the ELMs. The JET-ILW plasmas with N 2 seeding are characterized by ELM energy losses and time scales similar to the JET-C and by the absence of the slow transport events.

Correlation analysis for energy losses, waiting times and durations of type I edge-localized modes in the Joint European Torus

“…A very similar conclusion is true for the relative energy lossW ELM /W tot , which shows that an increased energy loss is due to a higher fraction of STEs. This is in accordance with recent studies wherein it was seen that the STEs carry a significant proportion of the energy of the total ELM event [22]. STEs are absent in the JET CW database analyzed in this work.…”

“…The time window (delimited by t a and t b ) is chosen dynamically, with t a taken as 3/4 of the time till the next ELM and t b taken as 1/3 of the time since the last ELM. Dynamic selection of the time window compensates for the varying timescales of ELM energy loss between JET CW and JET ILW plasmas [22]. Furthermore, in order to offset inaccuracy arising due to eddy currents in the vacuum vessel and small radial plasma motion following an ELM, a time interval of 3 ms has been allowed after an ELM in which the data is not used for energy loss estimation.…”

“…JET ITER-like wall ELMs are sometimes followed by an extended collapse phase, called the slow transport event (STE) [22], the presence of which has been proposed to be related to divertor/scrape-off layer (SOL) conditions [23] [24] and to a change in recycling behavior in a W divertor [25] [26] . These STEs are analogous to the second phase of ELM collapse observed at ASDEX Upgrade (AUG) [24].…”

Insights into type‐I edge localized modes and edge localized mode control from JOREK non‐linear magneto‐hydrodynamic simulations

“…The simulations shown in the following are based on a typical ASDEX Upgrade H‐mode equilibrium with an edge safety factor q 95 = 5.9. The experimentally observed type‐I ELM crashes take about 2 ms corresponding to the so‐called long ELMs . A pre‐ELM equilibrium reconstruction (ASDEX Upgrade discharge #33616 at 7.2 s) performed by the CLISTE code is used for initial conditions, which is already unstable such that the simulations only allow the investigation of the ELM crash itself but not the inter‐ELM phase.…”

Global and pedestal confinement and pedestal structure in dimensionless collisionality scans of low-triangularity H-mode plasmas in JET-ILW

“…Moreover, the pedestal temperature T e ped is higher than 400-500 eV, while in JET-ILW the Type III ELMs occur at T e ped lower than 300 eV [4,13]. Finally, the ELM energy losses are large compared to the Type III ELMs, with a value (W ELM /W ped > 8-10%) relatively consistent with Type-I ELM losses [14,65]. The details of the ELM behaviour with collisionality will be discussed elsewhere.…”