[show abstract][hide abstract] ABSTRACT: Operating ITER in the reference inductive scenario at the design values of Ip = 15 MA and QDT = 10 requires the achievement of good H-mode confinement that relies on the presence of an edge transport barrier whose pedestal pressure height is key to plasma performance. Strong gradients occur at the edge in such conditions that can drive magnetohydrodynamic instabilities resulting in edge localized modes (ELMs), which produce a rapid energy loss from the pedestal region to the plasma facing components (PFC). Without appropriate control, the heat loads on PFCs during ELMs in ITER are expected to become significant for operation in H-mode at Ip = 6–9 MA; operation at higher plasma currents would result in a very reduced life time of the PFCs.
[show abstract][hide abstract] ABSTRACT: The first detailed measurements of ion-impurity dynamics for NBI-heated ELMy H-modes at the edge of the JT-60U tokamak are reported. We investigated the ability of external momentum/power input to modify and control the radial electric field, Er, and pedestal structures. The relationship between Er and pedestal structures of ion-impurity density, ni, and temperature, Ti, during the ELMing H-mode phase for various momentum input directions (i.e. co-, balanced- and counter-NBI) and input powers from perpendicular NBI are compared with the ELM-free phase. The observed trend is that the edge Er-well width increases in the co-NBI discharge, while the Er value at the base of the Er-well becomes more negative in the counter-NBI discharge. The scale length for both ni and Ti in the pedestal is ~2 cm and values are ~1 for both ELM-free and ELMing phases with different magnitudes of Er (and/or Er shear). Characteristics of the turbulent density fluctuation, in addition to a uniform toroidal MHD oscillation (i.e. n = 0), during both ELM-free and ELMing phases are also reported.
[show abstract][hide abstract] ABSTRACT: The dependence of the ion-temperature-gradient scale length on the hydrogen isotope mass was examined in conventional H-mode plasmas in JT-60U tokamak. While identical profiles for density and temperature were obtained for hydrogen and deuterium plasmas, the ion conductive heat flux necessary for hydrogen to sustain the same ion temperature profile was two times that required for deuterium, resulting in a clearly higher ion heat diffusivity for hydrogen at the same ion-temperature-gradient scale length. On the other hand, the ion-temperature-gradient scale length for deuterium is less than that for hydrogen at a given ion heat diffusivity.
[show abstract][hide abstract] ABSTRACT: The relation between toroidal rotation velocities (Vt) in the core and edge regions is investigated in H-mode plasmas with a small external torque input from the viewpoint of momentum transport. The toroidal rotation velocity in the core region (core-Vt) gradually varies on a timescale of ~20 ms after a rapid change in the toroidal rotation velocity in the edge region (edge-Vt) at the L–H transition. This timescale of ~20 ms is consistent with a transport timescale using the momentum diffusivity (χ) and convection velocity (Vconv). In steady state, a linear correlation between the core- and edge-Vt is observed in H-mode plasmas when the ion pressure gradient (∇Pi) is small. This relation between core- and edge-Vt is also explained by momentum transport. The Vt profiles with a large ∇Pi are reproduced in the core region of r/a ~ 0.2–0.7 by adopting a residual stress term 'Πres = αkχ∇Pi' proposed in this paper. Here r/a is the normalized plasma radius and αk1 is a radial constant. Using this formula, Vt profiles are reproduced over a wide range of plasma conditions. Parameter dependences of the edge-Vt are investigated at a constant ripple loss power, ripple amplitude and plasma current. A reduction in the CTR-rotation is observed with decreasing ion temperature gradient (∇Ti). Here CTR refers to the counter-IP direction.
[show abstract][hide abstract] ABSTRACT: The effects of toroidal field (TF) ripple on the edge pedestal characteristics were examined in the TF ripple scan experiments at the plasma current Ip of 1.1 MA in JET and JT-60U. The TF ripple amplitude δR was defined as a value averaged over the existing ripple wells at the separatrix on the outer midplane. By the installation of ferritic inserts (FIs), δR was reduced from 1% to 0.6% at 3.2T (0.5% at 2.2 T) in JT-60U. In JET, δR was varied from 0.08% to 1% by feeding different currents to the odd and even set of coils out of 32 TF coils. The pedestal pressure pped was similar for the cases before and after the installation of FIs in JT-60U. Similarly, no clear difference in pped was also observed in the variation of δR in JET. The core and edge toroidal rotation clearly shifted in the counter-direction by increased δR. However, there were no changes in the spatial profiles of electron density, electron temperature and ion temperature. By the installation of FIs in JT-60U, the ELM frequency fELM decreased by ~20%, while the ELM energy loss increased by 50–150%. The increased ELM loss power by 30% suggests a reduction of inter-ELM transport with the reduced δR. In JET, fELM increased only slightly with increased δR while the edge toroidal rotation frequency decreased as δR increased. From the inter-machine similarity experiment at 1.1 MA, TF ripple less than 1% does not strongly affect the pedestal pressure. However, in the single TF ripple scan at the higher Ip of 2.6 MA in JET, it clearly decreases with the increased δR, accompanying with a strong density pump out at large TF ripple. These results suggests that the effect of TF ripple on H-mode properties becomes stronger in the plasmas with higher Ip or lower edge collisionality of ripple diffusion.
[show abstract][hide abstract] ABSTRACT: The paper compares the transport properties of a set of dimensionless identity experiments performed between JET and JT-60U in the advanced tokamak regime with internal transport barrier, ITB. These International Tokamak Physics Activity, ITPA, joint experiments were carried out with the same plasma shape, toroidal magnetic field ripple and dimensionless profiles as close as possible during the ITB triggering phase in terms of safety factor, normalized Larmor radius, normalized collision frequency, thermal beta, ratio of ion to electron temperatures. Similarities in the ITB triggering mechanisms and sustainment were observed when a good match was achieved of the most relevant normalized profiles except the toroidal Mach number. Similar thermal ion transport levels in the two devices have been measured in either monotonic or non-monotonic q-profiles. In contrast, differences between JET and JT-60U were observed on the electron thermal and particle confinement in reversed magnetic shear configurations. It was found that the larger shear reversal in the very centre (inside normalized radius of 0.2) of JT-60U plasmas allowed the sustainment of stronger electron density ITBs compared with JET. As a consequence of peaked density profile, the core bootstrap current density is more than five times higher in JT-60U compared with JET. Thanks to the bootstrap effect and the slightly broader neutral beam deposition, reversed magnetic shear configurations are self-sustained in JT-60U scenarios. Analyses of similarities and differences between the two devices address key questions on the validity of the usual assumptions made in ITER steady scenario modelling, e.g. a flat density profile in the core with thermal transport barrier? Such assumptions have consequences on the prediction of fusion performance, bootstrap current and on the sustainment of the scenario.
[show abstract][hide abstract] ABSTRACT: The effect of core pressure gradient just inside the top of the pedestal on the energy loss due to type-I edge localized modes (ELMs) is studied. An analysis of the experimental data from JT-60U shows that the ELM energy loss normalized by the pedestal stored energy increases with the pressure gradient inside the top of the pedestal normalized by the pedestal pressure gradient. The dependence of normalized ELM energy loss on the normalized pressure gradient inside the top of the pedestal is similar to that predicted by the integrated code TOPICS-IB. The stability of linear ideal MHD modes is analysed using experimental profiles. It is found that the steep core pressure gradient inside the top of the pedestal broadens the eigenfunction profiles of unstable modes inwards. The TOPICS-IB simulation predicted that this broadening can enhance the ELM energy loss.
[show abstract][hide abstract] ABSTRACT: Mechanisms of plasma rotation effects on edge magnetohydrodynamic (MHD) stability are investigated numerically by introducing energies that are distinguished by physics. By comparing them, it is found that an edge-localized MHD mode is destabilized by the difference between the eigenmode frequency and the equilibrium toroidal rotation frequency, which is induced by rotation shear. In addition, this destabilizing effect is found to be effective in the shorter wavelength region. The effect of poloidal rotation on the edge MHD stability is also investigated. Under the assumption that the change in equilibrium by poloidal rotation is negligible, it is identified numerically that poloidal rotation can have both stabilizing and destabilizing effects on the edge MHD stability, which depends on the direction of poloidal rotation. A numerical analysis demonstrates that these effects of plasma rotation in both the toroidal and poloidal directions can play important roles in type-I edge-localized mode phenomena in JT-60U H-mode plasmas.
[show abstract][hide abstract] ABSTRACT: The detailed characteristics of the precursor of the type I edge localized mode (ELM) have been studied in JT-60U using diagnostics with high temporal and spatial resolution such as a microwave reflectometer, electron cyclotron emission (ECE) heterodyne radiometer and grating polychromator. Coherent density and temperature precursors have been observed before the collapse phase of type I ELM. The growth rate of the precursor is evaluated to be γ/ωA ~ 10−3 for several edge pedestal conditions. From the phase delay between ECE signals measured at two toroidal locations and the frequency of the precursor, the toroidal mode number is experimentally evaluated as n = 8–10 or 14–16 assuming that the precursor rotates toroidally with the same toroidal rotation speed of carbon impurity. It is found that the dominant n varies with each ELM under the same plasma condition. The ratio of the pressure gradient inside the pedestal (∇pin) to the pressure gradient within the pedestal (∇pped) has been confirmed as an important parameter in determining the ELM energy loss (ΔWELM) normalized to the pedestal stored energy (Wped), ΔWELM/Wped. From the comparison of the reduction rate in the ion temperature profile due to ELMs, a larger reduction rate within the pedestal and a wider ELM affected area are observed in the plasma with larger ∇pin/∇pped. When the plasma near the top of the pedestal on the high-field side is heated by an electron cyclotron wave (ECW) power of 1.57 MW, the ΔWELM/Wped is reduced by ~35%, together with an increase in the ELM frequency. The increasing rate of the ELM frequency with the heating power is about four times larger in the ECW injection case than the natural power dependence observed in the neutral beam injection case.
[show abstract][hide abstract] ABSTRACT: Two integrated core/scrape-off layer/divertor transport codes TOPICS-IB and JINTRAC with links to MHD stability codes are coupled with models of pellet injection to clarify effects of the pellet on the behaviour of edge-localized modes (ELMs). Both codes predict the following two triggering mechanisms. Energy absorption by the pellet and its further displacement due to the E × B drift, as well as transport enhancement by the pellet, are found to be able to trigger the ELM. The ablated cloud of pellet absorbs the background plasma energy and causes a radial redistribution of pressure due to the subsequent E × B drift. Further, the sharp increase in local density and temperature gradients in the vicinity of ablated cloud causes the transient enhancement of heat and particle transport. Both mechanisms produce a region of increased pressure gradient in the background plasma profile within the pedestal, which triggers the ELM. The mechanisms have the potential to explain a wide range of experimental observations.
[show abstract][hide abstract] ABSTRACT: We have observed a fishbone-like mode in high-β plasmas above the ideal β limit without a conducting wall. The mode frequency chirps down, and its initial value is close to the precession frequency of trapped fast ions produced by perpendicularly injected neutral beams. This mode can often induce the resistive wall mode (RWM). The mode is observed mainly in the high-βN plasma where an ideal kink-ballooning mode (IKBM) and the RWM are marginally stable. Since this mode and the RWM were simultaneously observed, the mode is attributed to the interaction between the trapped fast ions and a marginally stable IKBM stabilized by a conducting wall. Actually, the MARG2D analysis shows that the IKBM is wall-stabilized and has a real frequency of the same order as the observed mode frequency. From these results, the observed mode was named 'energetic particle driven wall mode (EWM)'. Moreover, the EWM can change the edge localized mode (ELM) behaviour. For example, it was observed that the ELM was synchronized with the EWM. An energy loss due to the EWM-triggered ELM is smaller, and its frequency becomes higher compared with the usual ELM.
[show abstract][hide abstract] ABSTRACT: A complex multistage transition of the edge radial electric field is observed in JT-60U H-mode phase without edge localized mode. An interesting feature is that the poloidal rotation velocity of the carbon impurity ions changes in the later H-phase without a comparable change in the main ion pressure gradient, indicating a change in the parallel momentum (and particle) balance channel.
[show abstract][hide abstract] ABSTRACT: Effects of the edge collisionality on ELM characteristics have been investigated in the grassy ELM regime on JT-60U. Both in the high and low q regions (q95 > 6 and q95 ~ 4.3), ELM amplitude becomes higher at high edge collisionality (density). This collisionality dependence is opposite to the dependence observed in the type I ELM regime, where the ELM amplitude decreases with increasing edge collisionality. The stability analysis suggests that the grassy ELM at q95 ~ 4.3 can be triggered by the infinite-n ballooning mode localized inside the pedestal, when peeling–ballooning modes are stable. In the case of higher collisionality, peeling–ballooning modes can also be unstable, and as a result a larger ELM can be triggered. Since both stability boundaries for infinite-n ballooning mode and peeling–ballooning modes are located near the operational point, a small change in the pedestal condition such as a local pressure gradient and/or a bootstrap current can determine the most unstable mode. This mechanism is considered as a possible candidate for the appearance of a mixture ELM of grassy ELMs and large ELMs.
[show abstract][hide abstract] ABSTRACT: Propagation of plasma ejected by type-I edge localized mode (ELM) has been measured in scrape-off layer (SOL) of the JT-60U tokamak, using optical system of motional Stark effect (MSE) diagnostics as beam emission spectroscopy (BES) diagnostics through a new technique developed. This MSE/BES system measures Dalpha emission from heating neutral beam excited by collisions with the ejected plasma, as well as background light (e.g., bremsstrahlung). While spatio-temporal change in the beam emission gives information on propagation of the ejected plasma, the background light that is observed simultaneously in all spatial channels veils the information. In order to separate the beam emission and the background light, a two-wavelength detector is newly introduced into the MSE/BES system. The detector observes simultaneously at the same spatial point in two distinct wavelengths using two photomultiplier tubes through two interference filters. One of the filters is adjusted to the central wavelength of the beam emission for the MSE diagnostics, and the other is outside the beam emission spectrum. Eliminating the background light, temporal change in the net beam emission in the SOL has been evaluated. Comparing conditionally averaged beam emission with respect to 594 ELMs in a discharge at five spatial channels (0.02-0.3 m outside the main plasma near equatorial plane), radial velocity of the ELM pulse propagation in SOL is evaluated to be 0.8-1.8 km/s (approximately 1.4 km/s for least-mean-squared fitting).
The Review of scientific instruments 04/2010; 81(4):043502. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: An L/R model that predicts the current decay time from the circuit equation is essentially used for the design of the International Thermonuclear Experimental Reactor. In order to verify the validity of the L/R model in the determination of current decay time during disruption, the plasma current decay time in the JT-60U tokamak is studied using experimental plasma resistance and inductance. The plasma resistance during the initial phase of current quench is estimated from the electron temperature profile measured using the electron cyclotron emission diagnostic system and by measuring the He I line emission intensity ratios and plasma inductance is estimated by the Cauchy-Condition surface method using magnetic sensor signals. Further, the radiation-induced disruptive plasma discharges with massive neon gas puffing are also analysed. The observed area-normalized current decay times have a weak dependence on the electron temperature, particularly in a small decay time region (5–10 ms m−2). The observed decay times are lesser by one order of magnitude than the decay times estimated by the L/R model. However, a novel model for decay time prediction, which takes into account the time derivative of the plasma inductance, wields results that are extremely consistent with the experimental decay time.
[show abstract][hide abstract] ABSTRACT: A series of experiments have been carried out in 2008 at JT-60U and JET to find common characteristics and explain differences between internal transport barriers (ITBs). The identity experiments succeeded in matching the profiles of most dimensionless parameters at the time ITBs were triggered. Thereafter the q-profile development deviated due to differences in non-inductive current density profile, affecting the ITB. Furthermore, the ITBs in JET were more strongly influenced by the H-mode pedestal or edge localized modes. It was found to be difficult to match the plasma rotation characteristics in both devices. However, the wide range of Mach numbers obtained in these experiments shows that the rotation has little effect on the triggering of ITBs in plasmas with reversed magnetic shear. On the other hand the toroidal rotation and more specifically the rotational shear had an impact on the subsequent growth and allowed the formation of strong ITBs.
[show abstract][hide abstract] ABSTRACT: Properties of the toroidal momentum diffusivity (χ) and the convection velocity (Vconv) in JT-60U H-mode plasmas have been obtained by plasma parameter scans such as the plasma current, neutral beam heating power and electron density. The toroidal momentum diffusivity increases with increasing heat diffusivity (χi) over a wide range of radii (r/a = 0.2–0.6) and χ/χi ~ 0.7–3 at the half radius (r/a = 0.5). The inward convection velocity (−Vconv) increases with increasing χ, and −Vconv/χ ~ 0.5–2 (m−1) at r/a = 0.5. It is found that the ratio χ/χi increases with increasing ion temperature and decreases with increasing electron density. These tendencies are observed in other radial positions of r/a = 0.3, 0.4 and 0.6. Moreover, the ratio −Vconv/χ at r/a = 0.4, 0.5 and 0.6 increases with increasing ion and electron temperatures or temperature gradients.
[show abstract][hide abstract] ABSTRACT: Rseduction of heat loading appropriate for the plasma facing components such as the divertor is crucial for a fusion reactor. Power handling by large radiative power loss has been studied in long pulse ELMy H-mode discharges on JT-60U (τd = 30–35 s). Case 1 is argon (Ar) seeding into standard ELMy H-mode plasmas, where large radiation loss in the confined region of the main plasma caused a change in ELM characteristics from Type-I to Type-III. Case 2 is a combination of Ar and nitrogen (Ne) gas seeding into Type-I ELMy H-mode plasmas with an internal transport barrier (ITB). For case 1, large radiation loss both from the main plasma and from the divertor was produced, and operation of Type-III ELMs was preferable to a reduction in ELM energy loss fraction (WELM/Wdia) to 0.15%. Both transient and steady-state heat loadings were reduced. Relatively good energy confinement (HH98y2 = 0.87 − 0.75) with large frad (Prad/Pabs > 0.8) and divertor plasma detachment was sustained continuously for 13.5 s. For case 2, with reduced Ar seeding to the main plasma and increased divertor radiation with Ne seeding, the ELMy H-mode plasma with an ITB had better energy confinement (HH98y2 = 0.95 − 0.8), which was sustained continuously for 12 s. The radiated power was increased primarily in the divertor ( ), which was produced both by seeded Ne ions and by carbon influx due to transient (ELM) and steady-state heat loadings in the attached divertor. Reduction in the heat loading was not enough, thus enhancement of the radiated power in the divertor will be necessary for the formation of the divertor detachment.
[show abstract][hide abstract] ABSTRACT: In order to understand the physics of the ELM trigger and determine the ELM size, the fast ELM dynamics of type I and grassy ELMs have been studied in JT-60U, using new fast diagnostics with high spatial and temporal resolutions such as a lithium beam probe (Δt ~ 0.5 ms) and a charge exchange recombination spectroscopy (Δt ~ 2.5 ms), which can measure the electron density and the ion temperature, respectively. The evolution of the ion pressure profile in the pedestal region has been evaluated for the first time by detailed edge profile measurements. Then, the dynamics of the density, the ion temperature and the ion pressure in the ELM cycle has been investigated. The co-rotating plasmas are compared with the counter (ctr)-rotating plasmas for the understanding of the toroidal rotation effects. Type I ELMs observed in co-rotating plasmas exhibit a larger and wider ELM affected area (Δnped/nped ~ 30%, radial extent >15 cm) than ctr-rotating plasmas (Δnped/nped ~ 20%, radial extent ~10 cm). Just before a type I ELM crash, the pedestal ion pressure and its maximum gradient in co-rotating plasmas are 20% and 12% higher than those in ctr-rotating plasmas, respectively. It is found that the radial extent of the ion pressure gradient at the pedestal region in co-rotating plasmas is 14% wider than that in ctr-rotating plasmas. The experimental results suggest that the ELM size is connected with the structure of the plasma pressure in the whole pedestal region. As for the dynamics of grassy ELMs, the collapse of density pedestal is smaller (<20%) and narrower (~5 cm) than those of type I ELMs, as observed in the collapse of the electron temperature pedestal. Thus, it is confirmed that both conductive and convective losses due to grassy ELMs are small.
[show abstract][hide abstract] ABSTRACT: Recent JT-60U experimental results towards the establishment of advanced tokamak (AT) operation are reviewed. We focused on the further expansion of the operational regime of AT plasmas towards higher βN regime with wall stabilization. After the installation of ferritic steel tiles in 2005, the high power heating in a large plasma cross-section in which the wall stabilization is expected has been possible. In 2007, the modification of power supply of NBIs improved the flexibility of the heating profile in long-pulse plasmas. The investigation of key physics issues for the establishment of steady-state AT operation is also in progress using new diagnostics and improved heating systems. In weak magnetic shear plasma, high βN ~ 3 exceeding the ideal MHD limit without a conducting wall ( ) is sustained for ~5 s (~3τR) with RWM stabilization by a toroidal rotation at the q = 2 surface. External current drivers of negative-ion based NB and lower-hybrid waves together with a large bootstrap current fraction (fBS) of 0.5 can sustain the whole plasma current of 0.8 MA for 2 s (1.5τR). In reversed magnetic shear plasma, high βN ~ 2.7 (βp ~ 2.3) exceeding with qmin ~ 2.4 (q95 ~ 5.3), HH98(y,2) ~ 1.7 and fBS ~ 0.9 is obtained with wall stabilization. These plasma parameters almost satisfy the requirement of ITER steady-state scenario. In long-pulse plasmas with positive magnetic shear, a high βNHH98(y,2) of 2.6 with βN ~ 2.6 and HH98(y,2) ~ 1 is sustained for 25 s, significantly longer than the current diffusion time (~14τR) without neoclassical tearing modes (NTMs). A high G-factor, (a major of fusion gain), of 0.54 and a large fBS > 0.43 are suitable for ITER hybrid operation scenario. Based on the plasma for ITER hybrid operation scenario, the high βN of 2.1 with good thermal plasma confinement of HH98(y,2) > 0.85 is sustained for longer than 12 s at and frad > 0.79. Physics studies for the development of AT plasmas, physics studies of H-mode, pedestal and ELM characteristics and physics studies on impurity transport, SOL/divertor plasmas and plasma–wall interactions are also in progress. The active NTM stabilization system using modulated ECCD, which is synchronized to rotating island, has been developed and the efficiency of modulated ECCD in m/n = 2/1 NTM stabilization has been demonstrated. The intrinsic toroidal rotation driven by the ion pressure gradient and by the ECH is confirmed. The dedicated H-mode and pedestal experiments indicate two scalings, H-factor evaluated for the core plasma as and pedestal width scaling of . New fast diagnostics with high spatial and temporal resolutions reveals the different structures of pedestal pressure between co- and counter-rotating plasma, resulting in different ELM sizes determined by the radial penetration depth of the ELM crash. The tungsten accumulation becomes more significant with increasing toroidal rotation in the counter-direction.