Experimental observations of fast-ion losses on KSTAR

Junghee Kima,b, Jun Young Kimb, T. N. Rheea, M. Isobec, K. Ogawac, K. Shinoharad, M. García-Muñoze, Y. M. Jeona, S. H. Kimf, and S. W. Yoona

a National Fusion Research Institute, 34133 Daejeon, Korea

b Korea University of Science and Technology, 34113 Daejeon, Korea c National Institute for Fusion Science, 509-5292 Toki, Japan

d Japan Atomic Energy Agency, 311-0193 Naka, Japan e Universidad de Seville, 41012 Seville, Spain

f Korea Atomic Energy Research Institute, 34057 Daejeon, Korea

Email address: kimju@nfri.re.kr

1. Introduction Confinement of the fast ions generated by the auxiliary heating (NBI, ICRH) and the fusion

products is essential to realize the commercial fusion reactor. In reality, some amount of fast-ions, which can be the source of the fusion reaction, can be lost, depending on the fast ion distribution. Although the relative number of lost fast-ions is small, the impact of the fast-ion loss on the fusion devices could cause significant damage [1] on the first-wall due to its high energy. Moreover fast ion loss can lead to degradation of fusion performance which is related to achievement of advanced operation scenario [2], hence understanding of fast ion loss mechanism becomes important task for the future fusion plasma research.

Like other fusion devices, experimental investigations on KSTAR tokamak have shown various internal activities such as MHD instabilities affect the fast-ion loss [3-5]. In addition to this, many experimental studies have shown the applied non-axisymmetric magnetic perturbations are able to vary fast-ion loss intensity and patterns [6,7].

The paper is composed of the following contents. Introduction of this study and the fast-ion loss detector are described in the first and section briefly. Third and fourth sections exhibit the experimental observations on the fast-ion losses which are associated with the edge activities such as ELMs, RMP as well as core MHD instabilities such as tearing mode and fishbone. Last section summarizes the overall contents and addresses the future works.

2. Fast ion loss detector (FILD) A scintillator-based fast ion loss detector (FILD) [8] in KSTAR has been operated since

2011 experimental campaign to measure the energy (Larmour radius) and the pitch-angle of the escaping fast-ions. In particular, the shape of the front surface of the FILD head is resembled with the plasma boundary shape so as to reduce the heat load on the detector head. This design prevents saturation of the heat-induced ionoluminescent emission. Additional 16-channel PMT (photo-multiplier tube) system via fiber-optic array has been used to observe the correlation between the fast-ion loss and the fast MHD activities. (Figure. 1)

3. Fast ion loss associated with the edge activities A. Edge localized mode (ELM) Almost fast ions escaping from the bulk plasma are responding to the edge activities such

as profile changes or the edge-localized modes (ELMs) since the orbits of the escaping fast-ions travel along the edge and SOL (scrape-off layer) region. One of the most active phenomena at the boundary plasma is the edge-localized mode (ELM). Like on other major fusion devices, strong correlation with the ELM burst is observed on the FILD signal [9].

The distinctive point seen in the ELM-associated fast-ion loss is the spread of the pitch-angle distribution and the multiple peaks in the PMT signal during a single type-I ELM burst. Multiple-peaks during a single ELM burst (right on the Figure. 2) may be the signature of the interaction between the fast-ion orbits and the ELM filaments. The broaden pitch-angle distribution captured by the scintillator plane may be composed of the multiple pitch-angle spots of the bombarding fast-ions for 5 msec of integration time (left on the Figure. 2). Temporal difference between adjacent peaks in the FILD PMT signal (500 kHz of sampling rate) is about 0.3 – 0.5 msec. It is planned to use the ultrafast camera to identify the pitch angle of each small scintillation spot on the phase space map (FILD) by comparing with the timing of ELM filamentations.

Figure 1 Design of the KSTAR FILD head is shown on the top figure, and the fast measurement system based on the PMT was added and the positions of the measurement segments corresponding to each optical-fiber channel on the scintillator plate were calibrated.

Figure 2 Fast-ion loss signal is shown on the left: CCD camera (scintillator screen, 5msec integration image) and the right: PMT signal (top: Dα signal during a type-I ELM burst, middle: magnetics, bottom: FILD PMT signal from the low-pitch angle side of the scintillator screen).

B. Fast ion loss change in response to the resonant magnetic perturbation (RMP) It has been found that the intended (resonant) edge magnetic perturbation can mitigate or

suppress the ELM bursts in KSTAR [10]. However it is known and found that symmetry breaking of the equilibrium lead to the localized enhancement of the fast-ion loss to the wall [6,7,9]. Symmetry breaking of the fast ion loss pattern in KSTAR has been observed by the single FILD through the experiments by means of changing the polarity of the RMP coil current (perturbed Br direction) [11] as shown in Figure. 3. Left one (shot# 9056) is the case of the inward Br application, hence the FILD signal strength during RMP-on phase drops to almost zero-level. On the other hand, outward Br perturbation enhances the fast-ion loss on the FILD position as presented on the right part (shot# 9092).

Numerical simulations of the fast-ion orbits and the superposition (red dots) of the magnetic equilibrium and the applied magnetic perturbations express the Poincare plots shown in Figure. 4 have been performed by the 3-D orbit-following Lorentz-orbit code (LORBIT) [12]. As depicted in left two figures in Figure. 4, only rare fast-ion orbits (blue dots in the Poincare section) are intersecting with the FILD head. On the right two figures in Figure 4, outward Br perturbation increases the number of intersection between the orbits and the FILD head, which is consistent with the results shown in Figure. 3. Mechanism of the fast-ion loss (mainly prompt loss) in the presence of the toroidally asymmetric field is as follows. RMP produces the edge stochasticity, and then the orbits travelling through the edge plasma are perturbed and drifted to the bad-curvature region via stochastic ripple loss [13] as a candidate mechanism. If the RMP field strength is weak, stochastic layer width is thin and it does not affect to the fast-ion orbit whose normal gyro-radius is 3 – 4 cm. Drifted orbits by the stochastic field structure faces the relatively significant radial perturbation field when the orbit approaches the RMP coil. If the magnetic perturbation direction near the FILD head location is outward, the orbits drift toward

Figure 3 Toroidally asymmetric fast-ion loss pattern is shown. Left figure represents the inward Br perturbation case and the suppressed FILD signal, and the right one (outward Br perturbation) shows the local enhancement of the fast-ion loss.

the outside. However, drifted orbits can be pushed into the inward direction when the drifted orbits experience the inward Br perturbation.

Edge magnetic perturbation changes the pitch-angle distribution of the lost fast-ions. Broadening of pitch-angle distribution is observed via the FILD signal frequently. Figure 5 (b) and (d) represent the calculated pitch-angle distributions corresponding to the FILD scintillator images of non-RMP and RMP-on phase respectively.

To investigate effect of various RMP spectra on the fast-ion loss, the ‘mixed’ RMP configuration has been applied as presented in Figure 6. In case of shot number 11111 (inward Br at the FILD head position), we applied four pulses of mixed RMP (top: +--+ / middle: +-+- / bottom: ++--) configuration trying two cases: (1) middle coil only (n=2) with constant current, (2) all coils with constant current, (3) top and bottom coils (n=1 with non-resonant perturbation) with constant current and (4) constant middle coil current plus top and bottom coil current scan. For the given RMP polarity, the fast ion loss rate in FILD-1 decreased to the noise level immediately except third pulse (Top/Bottom coils only) as soon as RMP is turned on. For shot number 11458, plasma parameters similar with 11111 are reproduced, but the RMP coil current polarity is opposite to that of shot number 11111 case. Polarity of RMP coil current was changed by 180 degree (top: -++- / middle: -+-+ / bottom: --++) and the fast ion loss rate measured by the FILD-1 increased for all four RMP pulses as the RMP is turned on. The biggest enhancement of loss rate was observed during the second pulse (all the coils) and the application of middle-coil without top/bottom coils shows big increase of loss rate, too. In a comparison of

#11111 and 11458, it has been observed that the toroidally asymmetric fast ion loss can be regulated by changing RMP polarity. Also in the mixed RMP case, the middle-coil looks more effective to perturb the fast-ion orbit. Similarly, degradation of fast-ion confinement is observed through the solid-state neutral particle analyser (NPA) [14] when the n=2 magnetic perturbation using only the middle-coil is applied. NPA diagnostic result implies the perturbation using the n=2 middle coil can perturb the core fast-ion orbits, resulting in degradation of energy level and confinement of core fast-ions. This degradation might affect the anomalous low pitch-angle strip seen in the FILD scintillator map of shot number 11458 (Figure 6).

Figure 4 Poincare section plots of the fast-ion orbits (blue) overlaid on the perturbed magnetic field (red).

Another factor to change the fast-ion loss rate is toroidal rotation. Almost experiments have shown the relation between the fast-ion loss rate and the toroidal rotation level. To investigate the effect of the toroidal rotation on the screening of the external perturbation field and the fast ion loss rate, electron cyclotron heating (ECH) has been used to change the rotation speed while the edge magnetic perturbation is applied. In the example of shot number 11465, middle coil with n=2 configuration was turned on during plasma current flat-top. Then 110 GHz and 170 GHz ECH have been applied to regulate toroidal rotation velocity with keeping the middle-coil-assisted edge magnetic perturbation. Through this experiment, both in L- and H-modes,

Figure 5 Change in the lost fast-ion’s pitch-angle distributions are plotted. (a) and (b) represent the one of general RMP-off cases (two NBI sources). (c) and (d) show the broadened pitch-angle distribution under the n=1 RMP.

Figure 6 Fast ion loss rate measured by FILD-1 during mixed (n=1 & n=2) RMP. As RMP coil current polarity is altered, local fast-ion loss rate is changed from ‘suppression’ to ‘enhancement’.

toroidal rotation dependency on the fast-ion loss has been observed clearly as depicted in Figure 7. Radial electric field, which could interplay with the toroidal plasma rotation, at the L- and H-modes is able to affect the fast-ion loss rate because of non-ambipolar transport. In-depth analysis including plasma rotation is being performed with the MARS-F code [15].

4. Fast ion loss associated with the core MHD activities In addition to the fast-ion loss associated with the edge activities, fast-ion losses associated

with the core MHD instabilities has been also observed. It is difficult that the confined fast-ion orbits perturbed in the core region are lost. Instead, redistribution of the fast-ion density profile is occurred. However in some cases interactions with the core activities cause the fast-ion loss. Simple drift of the orbit cannot fully explain these losses. For the trapped fast-ions existing in the core, it is easy to escape to the outside of the plasma when the toroidal precession frequency of the orbit has integer ratio with island rotation frequency. Pitch-angle of the escaping fast-ion orbit can be varied since the banana tip position is moving when the magnetic moment is conserved (adiabatic invariant) [16]. In particular, orbit behaviour of the passing fast-ions is sensitive to the magnetic island position. If the m/n = 2/1 or 3/2 island position is close to the plasma boundary (i.e. q-profile is broad.), core fast-ions can be lost easily. Experimental results depicted in Figure 8 shows the evidence that the fast-ion loss is associated with the tearing mode. FILD spectrogram shows the same frequency track of the magnetics. Gradual increase of the FILD signal, capturing a specific part in the scintillator map to eliminate the original prompt loss spot, implies the fast-ion loss intensity is proportional to the width of the island. In

particular low pitch-angle (below 40°) fast-ions are susceptible to loss since the injection angle

Figure 7 Fast-ion loss rate decreases as toroidal rotation velocity increases. The relation between the fast ion loss rate and the toroidal velocity seems to follow the exponential function (from the log-plot) and the loss patterns in L- and H-mode regime are well distinguished.

of the NBI on KSTAR is quite tangential, which means it is fairly difficult to generate the high pitch-angle (close to 90° (full perpendicular velocity component)) fast-ion orbits.

Figure 8 Tearing mode-associated fast-ion loss signal is presented. FILD spectrogram is strongly correlated with the tearing mode signature on the Mirnov coil signal spectrogram. In addition, FILD signal intensity is proportional to the island width.

Figure 9 Fast-ion loss signal whose spectrogram is correlated with that of magnetics.

Another example of the core activity is beam-induced fishbone. Figure 9 represents the m/n = 1/1 fishbone-associated fast-ion loss signal on the spectrograms. Perturbation location of the fishbone is at the deep core region (near the q=1 surface), hence the resonant interaction between the orbit precession/circulation and the fishbone (kink) frequency is the main mechanism of the fast-ion loss. Other m/n = 1/1-induced MHD activity such as sawtooth crash in H-mode with on-axis NBI on KSTAR have not shown the clear signature of the fast-ion loss outside the edge except the low qedge experiments which the location of the core MHD activity is close to the edge region.

5. Summary Various fast-ion loss cases in KSTAR are introduced. Two representative edge-activity-

induced fast-ion loss phenomena are observed, which are the ELM-induced loss and the RMP-associated losses. As seen on both the FILD scintillator map and the PMT signal, multiple pitch-angle may be due to interaction with the ELM filaments that can produce significant density fluctuation or magnetic fluctuation near the fast-ion orbits exploring the edge/SOL region. RMP-associated fast-ion loss has specific change in the loss intensity and the pitch-angle. Main mechanism of the fast-ion loss change associated with the applied edge magnetic perturbation is composed of stochastic ripple loss due to the edge stochasticity and the drift by the radial perturbation field direction. It is considered that the magnetic stochastic layer can form the ‘short-circuit’ along the radial direction between the edge and the SOL, causing direct loss (radial drift) to the wall. Although the edge stochasticity does not seem to cause toroidally localized loss, radial perturbation field outside the stochastic region can form the symmetry breaking of the fast-ion loss pattern. Fast-ion orbits passing through the stochastic region cannot be on the single flux surface and expand the radial variation width. This change in fast-ion orbit result in increase of radial displacement and then the orbit experiences the radial perturbation field direction, causing the toroidally asymmetric loss pattern. This process is being verified by means of field-line tracing numerically. In addition to the fast-ion loss from the edge, core MHD instability-induced fast-ion losses are observed. Principal origin of the loss of the core fast-ions is the resonant interaction between the fast-ion orbit precession and the rotating islands. In particular, drifting orbit caused by the presence of the tearing mode islands is sensitive to the loss when the q-profile shape is broad. Loss intensity increases as the island width of the tearing mode increases since the number of fast-ion orbits intersecting the magnetic island increases when they meet the bigger islands. Besides, fishbone-induced fast-ion loss is observed. Resonant interaction with the m/n = 1/1 fishbone instability may be a source of the loss. On the other hands, another 1/1 activity such as sawtooth crash has not exhibited the clear loss. From these observations, resonant interaction between the confined fast-ions and the core MHD instabilities is the main mechanism for the loss of the core fast-ions generated from the tangential on-axis NBI on KSTAR.

General trend from the experimental observations of the fast-ion loss intensity under the magnetic perturbation on KSTAR has revealed the slight increase of the loss rate as well as symmetry breaking of the loss pattern. This observation has been mainly investigated by the vacuum simulations. In order to estimate the loss rate quantitatively with higher accuracy, consideration of the plasma-response in the 3-D fast-ion orbit simulation is required and is being combined with the 3-D equilibrium produced by the HINT2 code [17, 18]. Further sophisticated analyses with the MHD code such as MARS-F are being performed with the kinetic profiles including the toroidal rotation profile.

In addition, fast-ion loss database which has relation with the magnetic field strength and the plasma current level is being established in order to estimate the fast-ion diffusion coefficient so that it can verify the performance of the advanced operation scenarios (i.e. High-

performance operation accompanying the Alfvén eigenmodes could deteriorate the confinement due to anomalous fast-ion loss [19]). Acknowledgement This work was supported by Ministry of Science, ICT, and Future Planning under KSTAR project and by the National Research Foundation of Korea (NRF) under Contract No. 2011-0018719. It was also partly supported by the JSPS-NRF-NSFC A3 Foresight Program in the field of Plasma Physics (NSFC: No. 11261140328, NRF: No. 2012K2A2A6000443). The authors would like to thank Dr. H.H. Lee for his calculation of magnetic island width.

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