E. DE LA LUNA et al.

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IMPACT OF ELM CONTROL IN JET EXPERIMENTS ON H-MODE TERMINATIONS WITH/WITHOUT CURRENT RAMPDOWN AND IMPLICATIONS FOR ITER.

E. de la Luna1, A. Loarte2, F. Rimini3, P. de Vries2, F. Koechl3, C. Reux4, P. Lomas3, P. Buratti5, P. Carvalho6, V. Parail3, E. R. Solano1 and JET contributors* EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK 1Laboratorio Nacional de Fusión, CIEMAT, 28040 Madrid, Spain. 2ITER Organization, Route de Vinon-sur-Verdon, 13067 St Paul Lez Durance, France. 3Culham Centre for Fusion Energy, Culham Science Centre, Abingdon, OX14 3DB, UK 4CEA, IRFM, F-13108 Saint Paul-lez-Durance, France. 5Unità Tecnica Fusione - ENEA C. R. Frascati - via E. Fermi 45, 00044 Frascati (Roma), Italy 6Instituto de Plasmas e Fusão Nuclear, IST, Universidade de Lisboa, Lisboa, Portugal *See author list of X. Litaudon et al., Nuclear Fusion 57 (2017) 102001 First author e-mail address: elena.delaluna@ciemat.es

Abstract

The dynamics of a slow H-mode rampdown (to mimic the power rampdown scenario foreseen for ITER) have been systematically studied in JET during both the current (Ip) flat-top and rampdown phases, in order to explore the conditions under which W accumulation develops and how it can be controlled using external actuators that are known to affect the impurity transport, such as central electron heating (ICRH in JET) or ELM control (vertical kicks and pellet injection). The experiments have shown that maintaining ELM control during the exit from the H-mode phase is a very effective way to avoid W accumulation and to achieve a smooth and well-controlled termination in both Ip scenarios. The density decays more slowly than the plasma current causing the Greenwald density fraction to increase during the Ip rampdown. The results have been modelled with the JINTRAC suite of codes in order to validate existing transport models and provide improved physics basis for ITER predictions.

1. INTRODUCTION

An important aspect of ITER operation will be the termination of the high confinement H-mode, which requires the plasma current (Ip) and the energy content to be ramped down in a controlled and robust way. Previous H-mode termination studies in JET and other devices focused on aspects related to flux consumption and vertical stability control [1]. It is only recently that attention has been given to issues related to impurity accumulation and its control during this phase, which is particularly problematic in a Be/W wall environment, such as JET [2]

or ITER [3]. It is known from existing experiments that, as the heating power is reduced and the plasma approaches the transition from the high to low (H-L) confinement regime, extended intermittent ELM-free phases are produced. This situation is prone to tungsten (W) accumulation, which can cause a radiative collapse of the discharge and eventually lead to a disruptive termination. This is an important consideration for ITER since its design allows only for a small number of disruptions. The situation regarding W peaking and its control is particularly challenging for the exit phase of the ITER QDT=10 H-mode. In ITER the radial position control must be maintained during fast variations of the total stored energy (i.e. H-L transitions) to keep the plasma away from the inner wall, which is facilitated by keeping the plasma in H-mode during the Ip rampdown for as long as possible [4]. On the other hand, studies for ITER have show that a slow transition to L-mode can be prone to W accumulation [3]. With these key issues in mind, a set of dedicated experiments was conducted in JET, specifically designed to examine the evolution of plasma parameters during the H-mode termination phase, the conditions under which W accumulation develops and how it can be controlled with external actuators that are known to affect impurity transport, such as central electron heating (for core impurity control) or active ELM control (for edge impurity control). JET has the right W source for ITER (divertor and not main wall as in AUG), ICRH (for central electron heating), pellet pacing and vertical kicks [5] (fast vertical plasma displacements at an adjustable frequency) for actively controlling the ELM frequency and also has the capability to do NBI normal/tangential injection and change the particle source and torque, which offers a unique opportunity to study the required processes for ITER. Note that both pellets and kicks (for Ip<10 MA) are part of the ELM control strategy in ITER [6]. Density control is also an important aspect of the ITER rampdown. Since the Greenwald density (nGW=Ippa2) scales with Ip, the density must be reduced during the rampdown phase to stay below the density limit ne < nGW and avoid disruptions. Moreover, the evolution of the core density has a significant influence on the behaviour of the alpha heating power during the rampdown phase, affecting also the power flow across the separatrix required to maintain the plasma in the H-mode regime. An improved understanding of how the plasma density evolves during the H-mode termination, which is generally prescribed ad-hoc in ITER simulations, is then

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critical to controlling and predicting the rampdown in ITER. To that end, two sets of experiments were performed in JET, one set with the power rampdown programmed at constant Ip and a second set where the heating power and the plasma current were ramped down simultaneously, allowing the impact of the plasma current on the evolution of plasma parameters to be examined. Simulations of the results presented here have already started [7] using the JINTRAC suite of codes [8] aimed at validate the existing transport models and provide improved physics basis for ITER predictions. An overview of the experimental and modelling results obtained are presented and the implications for ITER discussed.

2. DESCRIPTION OF EXPERIMENT

To investigate the dynamics of the slow (ITER-like) H-mode termination phase, a series of experiments were carried in JET where the NBI power was gradually rampdown to mimic the slow rampdown scenario foreseen for ITER [4]. The H-mode exit phase was then examined under a variety of conditions that include a range of gas puff and power levels (NBI and ICRH) applied in combination with ELM control methods. Since NBI particle and momentum input is known to affect the core impurity transport in stationary H-mode phases [7,9], the same combination of normal and tangential beams was chosen in all discharges where the impact of ELM control was tested, so that the effect of ELM control on the W accumulation process could be isolated. All discharges were run with a plasma current Ip=2 MA and toroidal magnetic field BT= 2.2 T, resulting in a q95 of 3.2, with dominant NBI heating (PNBI=10-14 MW) and fuelling by D gas. ICRH heating (<2 MW) in the hydrogen minority scheme was used during the initial heating phase and also during the NBI ramp down phase to provide central heating for mitigation of core W accumulation [10]. The selected plasma shape was a lower single null, low triangularity (dav=0.22) configuration, with a divertor geometry optimized for maximum pumping (with both strike points in the divertor corners), which is known to facilitate access to good H-mode performance in JET-ILW [5]. An example of the typical evolution of the plasma parameters in these experiments is shown in Fig. 1. Here two discharges with similar heating power (PNBI=13.5 MW, PICRH=1.5 MW) but different gas puff levels are compared. Stationary H-mode with good normalized confinement (H98=1) was established using constant heating power and gas injection before the NBI power was slowly ramped down in 3 s. To better compare these cases, the time origin in Fig. 1 is set to the start of the power rampdown. The gas puff was stepped down to values <0.6´1021 e/s just before the beginning of the NBI ramp down phase to minimize the gas fuelling during the H-mode termination. One can see that ELMs become more compound (large ELMs followed by short period of smaller ELMs) and long ELM free phases develop as the pedestal

temperature (not shown) decreases during the power rampdown until it reaches a critical value, below which the plasma can not sustain the Type I ELMy H-mode and a transition to L-mode occurs, often preceded by a period of Type III ELMs. This period can be very short (~100 ms) as in #89232, or lasts for over ~1 s, as in #90221, leading to a slower decrease in the stored energy. Phases with Type III ELMs prior to the H-L transition are a common observation during the H-mode termination phase in JET, even in conditions where the heating power is switched off abruptly [4]. The duration of the Type III ELMy H-mode phases is determined by the edge power flow (Pnet) margin above the H-mode power threshold at the time of the last Type I ELM and therefore it tends to be longer in conditions with slowly varying Pnet as shown in Fig 1 for the discharge at higher gas (#90221). The edge power flow is defined as Pnet=Ploss-Prad-dW/dt, where PIN includes the ohmic power and absorbed additional heating power, Prad is the radiated power in the plasma bulk and dW/dt is the time derivative of the plasma energy. In these experiments a radiative collapse is induced in a reproducible manner during the exit from the H-mode by operating at relatively low gas puff (<1.3 ´1022 D/s) during the flat-top phase, typically 25% below the minimum gas dosing required to maintain stable conditions against impurity accumulation. Operation at low gas typically leads to

Fig. 1. Comparison of two discharges ((2 MA, q95=3.2, low d) at similar heating power (PNBI=13.5 MW, PICRH=1.5 MW) but different gas dosing: #90221 (1.81022 D/s) and #89232 (1.2×1022 D/s). Only the discharge at low gas develops W accumulation during the H-mode termination phase (slow NBI rampdown at constant Ip in this case).

P(M

W)

Gas

15

0

2

510

04

(1022

D/s

)

PRAD,bulk

PIN

WM

HD

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4

(MJ)

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Dα,

inne

r x10

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HL

# 89232 (low gas)# 90221 (high gas)

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t-t(start of power rampdown) (s) -1 0 1 2 3

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c)

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an increase in W content of the plasma, as both the inward influx of impurity from the divertor (higher at lower ELM frequency) and the W source (increased W sputtering at higher divertor temperature) increase, which contribute to the development of a peaked W concentration profile later in the discharge. To maintain stationary conditions during this initial phase, vertical kicks at 43 Hz (at least twice as large as the frequency of the natural ELMs) were applied continuously till 0.5 s before the beginning of the rampdown, which ensures an effective impurity flushing by ELMs and prevents W accumulation. We find that kicks reliably trigger ELMs in these plasma conditions, with an ELM triggering efficiency of 70-98%, depending on pedestal parameters, in agreement with previous results [5]. This allows for stable and robust H-mode operation at low gas puff, while still maintaining good H-mode confinement. This recipe was used in the discharge #89232 shown in Fig. 1. An abrupt increase in the core radiated power can be seen 2.3 s after the start of the power rampdown, a clear indication of the on-going W accumulation, which accelerates the transition to L-mode and leads to a rapid loss of the thermal energy. Note that W accumulation develops despite the application of central ICRH heating (<2 MW). It is in this scenario where the effectiveness of the different impurity control actuators has been tested. For all discharges included in this study, confinement and normalised pressure at the start of the rampdown are at or close the ITER targets, with H98=0.9-1 and bN in the range of 1.7 to 2.2 (higher at lower gas). The time-averaged edge power flow values, normalized to the predicted L-H threshold power [11] (PthMARTIN08), lie in the range of 1.2-2.4, with radiated power fractions varying from 15% to 40% (lower at higher gas). Line averaged density ranged from 3.5×1019 m-3 to 5.5×1019 m-3, corresponding to a Greenwald density fraction of 0.5-0.7 (fGW=<ne>/nGW). It is worth noting that PthMARTIN08 does not represent well the actual threshold power in JET-ILW, which can be higher or lower than the existing scaling depending on the divertor geometry [12]. Existing results indicate that the scaling underestimates the net power required for H-mode access by ~15% [13] for the divertor geometry used in these experiments, so that the values of Pnet/PthMARTIN08 in these experiments could actually be lower. It should be point out that all discharges with a radiation collapse in this dataset present signs of locked mode with poloidal and toroidal mode numbers m=2 and n=1, which are often precursors of a disruption, but not all of them terminate in a disruption. We find that even a small amount of ICRH power (< 2 MW) applied during the L-mode phase after the radiation collpase results in a significant increase of the core temperature, even after several minor thermal quenches, which prevents the current quench, allowing the soft landing of the discharges.

3. IMPACT OF ELM CONTROL ON H-MODE TERMINATION

It is known from present experiments (e.g. JET [14], AUG [15]) that the use of ELM control, that is the increase of the ELM frequency (fELM) over that of spontaneous ELMs, can play an important role in preventing the contamination of the plasma by W in metal wall devices. Increasing fELM effectively reduces the production and influx of W impurities from the divertor into the core region through the edge transport barrier (ETB), which is the first step to avoid radiation peaking. The demonstrated capability of kicks in JET to reliably increase fELM without negatively affecting the plasma confinement [5] makes them the ideal tool for these studies. Results in JET demonstrate that ELM control with kicks is effective not only in stationary ELMy H-mode plasmas at powers well above the H-mode threshold power [5], but also during the exit from the H-mode phase. This is illustrated in Fig. 2 where two discharges with/without ELM control during the rampdown phase are compared. In this case the NBI power and the plasma current (2MA, 0.25M/s) were ramped down simultaneously using a full-bore plasma (at constant plasma size and shape). In #90653 (in red) kicks (at 43 Hz) were maintained throughout the Ip rampdown. BT was kept constant (2.2 T) to allow ICRH during the Ip rampdown, causing q95 to increase from 3.2 to 5. The location of the strike points remained unchanged during the H-mode termination. In this example, PNBI=10.5 MW and PICRH=1.7 MW (switched off at t=11.5s). With the use of kicks during the main heating phase, the ELM frequency remains relatively constant and the initial plasma conditions at the start of the rampdown are very similar for both discharges, with a radiated power fraction of ~30%, a central line averaged density of 4×1019 m-3 (fGW=0.55), H98=1.0 and bN=1.7. One can see that for the uncontrolled termination (#90652, in red) ELMs return to their natural frequency (~20 Hz) immediately after the kicks are switched off at t=11.5s (see Fig. 2(e)), coinciding with the start of the rampdown. In this case, a strong core radiation is observed during the H-mode termination phase, leading to a fast collapse of the H-mode pedestal (marked as H-L-H transitions in Fig. 2(e)), during which the pedestal density drops by almost 40%. This sudden decrease in edge density temporarily increases the pedestal temperature (not shown) and the plasma returns to a steady period of Type III ELMs until the final H-L transition occurs. In contrast, in the discharge with ELM control (#90653, in blue) long ELM free phases are avoided and the build-up of impurities in the core is prevented, even without ICRH. As a result, the plasma remains in the Type I ELMy H-mode regime for a longer period, causing a slower decrease of the plasma energy. In these experiments, the plasma inductance (not shown) for the controlled termination varied from li=0.85 at the beginning of the Ip rampdown to 1.3 at the end of the heating phase, well below the requirement for ITER of li<1.6. Note that the fastest drop in plasma energy content during the rampdown (marked with an arrow in Fig. 2(b)), which causes a similar change in bpol

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amplitude and decay rate in both discharges (not shown), takes place at a lower current level for the controlled termination. These fast transients coincide with the first H-L transition at t=12.84s for the uncontrolled termination and with the back transition to L-mode at t=14.04s for the controlled one. At those points the thermal energy in the plasma is reduced by 25% and 65% and Ip reaches ~10% and ~25% of its flat top value, the higher values for the controlled termination. Reaching the H-L transition early in the ITER high QDT H-mode termination can result in un uncontrolled radial inward motion of the plasma column, which may lead to contact with the main wall. Moreover, the appearance of such fast transients in ITER could lead to large heat particle and fluxes to the divertor that could cause significant W sputtering and, in the worst cases, may even produce melting of the W divertor [6]. The results shown here confirm that the use of ELM control, not only during the main heating phase (to mitigate ELMs), but also throughout the H-mode termination in the rampdown phase is a mandatory requirement for ITER [4]. It is important to note that while the decrease in plasma temperature during the rampdown (Fig. 2(c)) is determined by the reduction in the edge power flow (faster for the discharge with W accumulation), the behaviour of the density (Fig. 2(d)) is dominated by the ELM regime and the associated confinement. In the absence of ELM control, fELM decreases continuously as the power is ramped down, which reduces the time-averaged edge particle transport [7].

This results in a steady increase of the pedestal density that continues till the first H-L transition at t=12.72s, after which the edge density clamps while the central density continues to increase, leading to an increase of density peaking over time that is eventually interrupted by the H-L transition at t=14s. In contrast, with ELM control (#90653), the peaking of the temperature (Te) and density (ne) profiles remains largely unchanged as the stored energy slowly decreases during the rampdown until the plasma undergoes a H-L transition. In Fig. 3 the

density and temperature profiles during the H-mode termination for the two discharges shown in Fig 2 are plotted at selected time intervals (marked by vertical lines in Fig. 2(e-f)). Here r=yp1/2 is used as the radial coordinate, where yp is the normalized poloidal flux. The discharge with the uncontrolled termination (#90652) shows clear signs of the strong W accumulation, that is the hollow Te profile and the remarkable peaking of the density profile, localized in a very narrow region close to the magnetic axis (r<0.3, better observed by the LIDAR system in JET). It is worth noting that while the core and edge Te drop gradually with the decrease in Pnet in the discharge with ELM control (#90653), the density profile shows no significant changes during the rampdown. This is correlated with the relatively constant fELM maintained by the kicks (~85% ELM triggering efficiency) throughout this phase (see next section). The influence of ELMs on the W accumulation process has been recently modelled with the JINTRAC suite of codes. Integrated core+edge+SOL simulations [7], including a recently developed discrete model for the description of ELMs, have

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)(1

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c)

d)

f)

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Type III ELMsH-L-H

H-L

H-L

kicks

kicks (43 Hz)

11 12 13 14 15Time (s)

Fig. 2. Rampdown of a full size H-mode plasma (low d, 2 MA, q95=3.2, PNBI=10.5 MW, PICRH=1.7 MW) with (#90653) and without (#90652) ELM control. Shown parameters are: (a) Additional heating (dashed lines) and radiated power, (b) plasma stored energy, (c) central Te, (d) core (dashed line) and edge ne and (e-f) WI emission signal from the inner divertor.

T e (k

eV)

0

3

5

0 0.2 0.4 0.6 0.8 1.0

T e (k

eV)

L-mode

Type I ELMs

ρρ

#90652 (uncontrolled)

#90653 (with kicks) 0 0.2 0.4 0.6 0.8 1.00

1

2

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4

5

After ELM-free

Type I ELMsTransition to Type III ELMs

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n e (1

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ECE HRTS LIDAR

n e (1

019 m

-3)

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6

8

0 0.2 0.4 0.6 0.8 1.0

4

Fig. 3. ne (Thomson Scattering) and Te (ECE) profiles for the two discharges shown in Fig.2 (selected times marked as vertical lines in Fig.2(e-f)

E. DE LA LUNA et al.

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shown that the lengthening of the inter-ELM periods leads to an uncontrolled increase of the W concentration in the edge region, which progressively moves closer to the plasma centre. What follows in the uncontrolled H-mode terminations is well described by neoclassical transport [7]. The initial increase in central radiation caused by the reduction in fELM as the power is ramped down reduces the core Te gradient, which causes an increase of the inward directed W neoclassical convection (vneo.µÑni/ni-0.5´ÑTi/Ti), triggering a process of self accelerating W accumulation, with the associated pronounced rise in Prad. Moreover, the collapse of the pedestal after the transition to Type III ELMs (or H-L back transition) leads to the formation of increased density gradients in the ETB, reducing the neoclassical temperature screening efficiency, thus contributing to the on-going core W build-up once the sawteeth activity disappears.

4. PEDESTAL PARAMETERS BEHAVIOUR DURING POWER AND CURRENT RAMPDOWN IN JET-ILW

As discussed in the introduction, a key question addressed in these studies is the effect of the plasma current in the evolution of the plasma density during the H-mode termination. We start by examining the strong correlation that exists between the ELM regime and the plasma density as the H-mode approaches the H-L transition. This is documented in Fig. 4 for three representative discharges: #90222 (with medium gas ~1.7×1022 D/s), #90223 (with low gas~1.1×1022 D/s), #90224 (with low gas but with kicks throughout the NBI rampdown). The three discharges have similar initial conditions (PNBI=10.5, PICRH=1.6 MW, Prad/Pin=30% and H98=0.9-1) but only the one at low gas (in orange) develops core W accumulation. In this case the NBI rampdown was performed at constant Ip and the ICRH power was maintained throughout the H-mode exit phase. Plotted in Fig. 4 is the ELM frequency, the pedestal density and the density peaking (defined as

ne(r=0.2)/ne(r=0.9)) during the power rampdown as a function Pnet/PthMARTIN08. The analysis covers the entire H-mode phase, from 0.5s before the start of the power rampdown till the back transition to L-mode. One can see that the discharge with higher gas (#90222) the pedestal density evolves in a very similar manner to that described for the uncontrolled termination in Fig. 2: a gradual increase as the ELM frequency of the Type I ELMs decreases and eventually a rollover after the transition to the Type III ELMy H-mode regime, during which the pedestal density

decreases continuously, more rapidly for the uncontrolled

termination, until the H-L transition occurs. Without ELM control the density peaking first

decreases due to an increase in edge density as the fELM decreases. But while with high gas the density peaking recovers its initial value during the Type III ELMy H-mode phase, for the uncontrolled termination it rapidly increases as the impurity and particle content in the plasma centre continuously raise driven by the neoclassical inward convection. In contrast, controlling fELM with kicks avoids the density rise during H-mode exit phase and keeps the density peaking rather constant. As shown in Fig. 4, the back H-L transitions in these experiments (in discharges without W accumulation) occurs at a ratio of Pnet/PthMARTIN08 ~0.5 (~0.50-0.65 for the complete dataset). This observation would seem to suggest a strong hysteresis in the power threshold, specially if one considers that the edge power flow required to access H-mode in JET-ILW for the divertor corner configuration used in these experiments appears to be ~15% higher than the predictions by the existing scaling (based on data obtained at BT=3 T [13]). It is worth noting that a non-negligible amount of plasma radiation is always present during the H-L back transition and this has not been included in the H-mode power threshold scaling. But it may also simply indicate that there are other factors not related to the power margin over the H-mode threshold determining the back transition to L-mode, such as local edge plasma parameters and their gradients. In general, we find that the HL transition

Fig. 4. Variation of (a) fELM, (b) pedestal density and (c) density peaking as the edge power flow is reduced during the slow NBI rampdown phase at constant Ip for three selected discharge (see text).

#90222 (medium gas:ΓD=1.7 x 1022 D/s)

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occurs within a narrow range of Te,ped (0.35-0.4 keV), independently of ne,ped (a clear example is shown later in Fig. 6). Unfortunately, there is no experimental data on the L-H transition for the particular combination of BT and divertor geometry used in these experiments, which precludes drawing firm conclusions on this subject. It must be mentioned that previous dedicated studies have shown a lack of hysteresis in JET-ILW [16]. Further detailed experiments are therefore needed to determine if power hysteresis exists during the slow H-mode power rampdowns explored here. The evolution of the pedestal parameters during the slow H-mode termination studied here is shown in Fig. 5(a). Here the trajectories of the pedestal ne and Te during the exit from the H-mode with H98=1 at the start of the rampdown till the H-L transition, with and without ELM control, are compared for two set of discharges: one of them where the power rampdown was carried out at constant Ip while in the other one Ip was ramped down together with the power. We can see that, for the uncontrolled terminations and as long as the Type I ELMs are maintained, the increase in pedestal density as the power decreases is accompanied by a decrease in the pedestal temperature, in such a way that the pedestal pressure (Pe,ped) remains constant. After the transition to Type III

ELMs, both particle and energy confinement decreases and Pe,ped is seen to decrease rapidly, mainly driven by a fast drop in the edge density, till eventually the H-L transition occurs. With ELM control, however, the edge temperature decreases at nearly constant density during the power rampdown, demonstrating that the use of ELM control actually changes key features of the termination. As shown in Fig. 5(b), if one remove the variation in Ip, by normalizing both Te,ped and ne,ped to it, the differences between Ip constant/ramp-down during the Type I ELMy H-mode phase disappears and the maximum Pe,ped achieved during this phase in the uncontrolled discharges scales with Ip2. This reveals that, at least for spontaneous Type I ELMs, the ballooning limit applies for these JET plasmas because the pressure decreases with Ip2 regardless of the increasing q95 in this phase. For ITER this may be different if pedestals are nearer the kink/peeling limit where an Ip´BT pressure limit scaling would be expected. An additional observation can be made regarding the impact of plasma current on the evolution of pedestal parameters. As can be seen in Fig. 5(a), the density decay slightly faster for the discharges with ELM control at constant Ip and this is a reproducible effect in the limited set of discharges examined here. This arises from the different ELM dynamics found in the H-mode termination experiments performed at constant Ip or with the Ip ramped down (see Fig. 6).

Fig. 5. Trajectories of (a) Te,ped vs. ne,ped and (b) normalized to Ip, from H-mode with H98~1 to L-mode, as power slowly decreases, with (open symbols) and without ELM (closed symbols) control, at constant Ip and with Ip simultaneously ramped down. Note that only the Type I ELM phase is included in (b) for the uncontrolled terminations.

Fig. 6. Impact of ELM control (kicks) on plasma density during the power rampdown at fixed Ip (#90469, black) and with Ip rampdown (#90653, blue). #90652 (without kicks) is also included in (d). The period where kicks trigger ELMs and the triggering efficiency is shown in (e,f)

t - t (start of NBI rampdown) (s)-1 0 1 2 3

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E. DE LA LUNA et al.

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We find that kicks reliably trigger ELMs with an efficiency of 75-85% (higher for the Ip rampdown case) during the initial phase of the rampdown (for a longer period in #90653) but, as the plasma approaches the H-L transition and Te,ped decreases (and edge collisionality increases) the ability of a kick to trigger an ELM disappears (see Fig. 6(e,f)), in agreement with earlier experiments [5]. As a result, the Type I ELMs are gradually replaced by smaller and faster Type III ELMs in the discharge at constant Ip (in black) that remain until the plasma undergoes the H-L transition. However, in the case of the Ip rampdown (in blue), small and regular naturally occurring Type I ELMs at <fELM>~40Hz appear after kicks stops triggering ELMs and last till almost the end of the H-mode. This experimental finding is consistent with earlier experiments in JET-C where a transition from Type III to Type I ELMs was observed in discharges where the plasma current was ramped down [17]. More analysis is required to identify how the edge MHD stability is modified by the changes in edge current and/or magnetic shear during this phase. The reduced confinement of the Type III ELMy H-mode regime leads to a loss of pedestal pressure in #90649 (in black) that is manifested in a significant decrease of the plasma density, with both the edge (not shown) and core density decreasing simultaneously during this phase. This is in contrast with the Ip rampdown case (in blue), where the density profile is kept nearly unchanged as long as Type I ELMs are maintained (see Fig. 3). Whether the density behaviour shown here during the Ip rampdown extrapolates to ITER or not requires detailed analysis as the penetration for recycling neutrals is modelled to be much less effective than in JET. Note that, despite the differences in ELM behaviour prior to the HL transition, the two discharges in Fig. 6 terminate smoothly with no signs of W accumulation. This highlights the importance of avoiding long ELM-free phases during the initial phase of the rampdown for efficient impurity control with ELM pacing. The evolution of the plasma density shown in Fig. 5 has important consequences for the Ip rampdown in ITER. Similar to previously reported observations in JET-C [18], we find that the plasma density decays more slowly than the plasma current and, as a result, the Greenwald density fraction slowly increases during the Ip rampdown (see Fig. 6(d)). In this example, fGW raises above its flat-top value of 0.58 to ~0.80 with ELM control (#90653) and up to 0.95 in the uncontrolled termination (#90652), as the plasma current is reduced. As already shown in previous studies [18], the longer the plasma stays in H-mode (for controlled terminations) the larger the increase in fGW. More experiments and additional modelling work are required to better understand the ELM dynamics and the associated confinement changes during the Ip rampdown in view to develop operational scenarios that provide the required impurity control without the unwanted increase in fGW during this phase. To summarize the results obtained in these experiments, the dependence of the H-mode duration (defined as DtH-mode=tHL-tstart-of-rampdown) during the rampdown on the initial plasma conditions is plotted in Fig. 7. We choose Pnet/PthMARTIN08 for this comparison because, although it might be inaccurate for JET-ILW (the actual Pth could

be larger), this is the normalization typically used for ITER predictions. We find that DtH-mode reduces as the power margin above the H-mode threshold at the beginning of the rampdown decreases. This is particularly relevant for ITER since high QDT ITER plasmas are expected to operate at a relatively low margin above the H-mode threshold power. The H-mode terminations are generally longer with ELM control (DtH-mode~3s is equivalent to 10 energy confinement times of the initial H-mode plasma with H98=1), even at low heating power above the predicted H-mode threshold. Similarly, experiments with high gas injection delays the transition to H-mode but, as shown before, density raises during the H-mode termination. Attempts to use pellet pacing for ELM control has resulted, so far, in terminations with low radiation levels but poor density control (due to the extra gas introduced by pellet injection) and further investigation is required to assess the effectiveness of this ELM control approach.

5. DISCUSSION AND CONCLUSIONS

The experiments in JET-ILW have successfully demonstrated for the first time that the use of ELM control (with kicks so far) is an effective method to avoid W accumulation during the H-mode termination phase and to achieve a smooth and well-controlled termination. With ELM control the long ELM free phases typically observed as the plasma approaches the H-L transition can be avoided, which prevents the uncontrolled build-up of impurities in the core and the ensuing radiation collapse. As a result, the plasma remains in type I ELMy H-mode for a longer period, which is advantageous for the ITER rampdown. It is found that ELM control via vertical kicks provides not only impurity control but also density control, which is also a key aspect in the ITER

1.2 1.4 1.6 1.8 2.0 2.2 2.4Pnet /Pth

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Fig. 7. H-mode duration, from the start of the power rampdown till the H-L transition, vs. the time- averaged Pnet/Pth

MARTIN08 at the start of the rampdown

IAEA-CN-123/45

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rampdown scenario. But the density is found to decrease more slowly than the plasma current leading to an increase of the Greenwald density fraction during the Ip rampdown. These experiments show that the plasma density evolution during the H-mode termination phase critically depends on how the ELM regime, and its associated confinement, changes as the plasma approaches the H-L transition. We have also found experimental evidence that the Ip rampdown itself can affect the ELM behaviour through the influence of the edge current and /or magnetic shear in the pedestal stability. An improved understanding of the ELM dynamics and the associated particle transport as the plasma current is reduced is therefore required to correctly predict the ITER’s rampdown. So far, rampdown experiments in JET-ILW were performed using a full-bore plasma. The ITER rampdown scenario, on the other hand, includes a strong reduction of the plasma elongation to maintain vertical stability. This will be the subject of further experimental studies in JET. A discrete model to describe ELMs [7] has been used for the first time in JINTRAC simulations allowing the different aspects involved in the control of impurities by ELMs (i.e. W sputtering, W expulsion and its transport trough the scrape-off layer and the ETB) to be examined in detail. Both experimental and modelling results clearly show that the impact of ELMs on particle and impurity transport must be considered to make reliable predictions for the ITER rampdown. In addition to the ELM control studies, other mechanisms affecting the core impurity transport during H-mode termination, such as central electron heating (ICRH), were also investigated. In these experiments, attempts at preventing core W accumulation during the rampdown with ICRH alone were not successful, mainly due to the limited available coupled ICRH power (1-2 MW) and possibly excessive H minority concentration during this phase (D gas is strongly reduced) that is known to decrease the ICRH heating efficiency [10]. Recent modelling results [7] suggest that higher level of ICRH (~4 MW) could play a critical role in avoiding core W accumulation and this, in principle, could reduce the ELM control requirements during the H-mode termination. More experiments are needed to confirm these predictions. In general, good agreement was found between the JINTRAC integrated core+edge+SOL simulations [7] and the results obtained in the experiments reported here, which give confidence in the prediction of the W transport behaviour for ITER but more experiments and additional modelling work are required to better understand the extrapolability of the obtained results to ITER. Dedicated experiments are planned for the next JET campaign in plasmas at higher Ip/BT, where higher ICRH power is expected to be available, aiming at further optimizing the use of the core and edge W control actuators available in JET during the Ip rampdown phase (with/without reduced plasma elongation) in view to develop a more robust and safe termination for ITER.

ACKNOWLEDGEMENTS

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the EURATOM research program 2014-2018 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commision. This research was supported in part by grant FIS2017-85252-R, Ministerio de Ciencia, Innovación y Universidades, Spain.

REFERENCES

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