Discharges in the JET tokamak where the safety factor profile is identified as the critical factor for triggering internal transport barriers

Joint European Torus discharges which demonstrate the critical role the safety factor profile, q, can play in the formation of internal transport barriers (ITB) are examined. In these discharges, the target parameters, including the E x B flows, were kept virtually the same, except for the q profile. In a discharge with a nonmonotonic q, an ITB was triggered whereas a discharge with monotone q made no such transition. Thus, there is strong evidence that the q profile was the critical factor for the triggering of an ITB. Possible interpretations of this finding are discussed.     

Modeling of a lower-hybrid current drive by including spectral broadening induced by parametric instability in tokamak plasmas

By incorporating parametric instabilities of lower hybrid (LH) waves into a ray-tracing Fokker-Planck code, accurate simulations of the LH deposition profiles are provided, which are useful for interpreting the long-lasting internal transport barriers (ITBs) sustained by lower hybrid current drive (LHCD) on JET (Joint European Torus). Utilizing the new model, the simulation of the q-profile evolution results in agreement with that provided by the motional Stark effect reconstructed equilibria. Low magnetic shear (s approximately equal to 0) is produced by LHCD in a layer close to the ITB radial foot.     

JET quasistationary internal-transport-barrier operation with active control of the pressure profile

Quasistationary operation has been achieved on the Joint European Torus tokamak in internal-transport-barrier (ITB) scenarios, with the discharge time limited only by plant constraints. Full current drive was obtained over all the high performance phase by using lower hybrid current drive. For the first time feedback control on the total pressure and on the electron temperature profile was implemented by using, respectively, the neutral beams and the ion-cyclotron waves. Although impurity accumulation could be a problem in steady state ITBs, these experiments bring some elements to answer to it.     

Poloidal rotation dynamics, radial electric field, and neoclassical theory in the jet internal-transport-barrier region

Results from the first measurements of a core plasma poloidal rotation velocity (upsilontheta) across internal transport barriers (ITB) on JET are presented. The spatial and temporal evolution of the ITB can be followed along with the upsilontheta radial profiles, providing a very clear link between the location of the steepest region of the ion temperature gradient and localized spin-up of upsilontheta. The upsilontheta measurements are an order of magnitude higher than the neoclassical predictions for thermal particles in the ITB region, contrary to the close agreement found between the determined and predicted particle and heat transport coefficients [K.-D. Zastrow, Plasma Phys. Controlled Fusion 46, B255 (2004)]. These results have significant implications for the understanding of transport barrier dynamics due to their large impact on the measured radial electric field profile.     

On the bootstrap effect in large tokamaks with lower hybrid driven current

The amplification of lower hybrid (LH) driven current of various profiles has been found. The results strongly depend on the plasma temperature profile, on the LH current profile, and on the magnitude of the particle diffusion flux.This work was started during the visit of the author in the I. V. Kurchatov Institute of Atomic Energy, Moscow. The author thanks IAE for hospitality, and V. Parail, G. V. Pereverzev, A. Smoljakov and R. Klíma for many useful and critical discussions.     

HL-2A装置的LHCD运行区域与驱动电流分布控制

讨论了低杂波电流驱动（LHCD）实验中电流驱动效率、电流分布控制与等离子体参数和入射波谱的关系,以及波的可近性对确定功率沉积分布的作用。讨论了控制电流密度分布的方法及在HL－2A装置上实现中心负剪切位形的可能性。     

Transition between internal transport barriers with different temperature-profile curvatures in JT-60U Tokamak plasmas

A spontaneous transition phenomena between two states of a plasma with an internal transport barrier (ITB) is observed in the steady-state phase of the magnetic shear in the negative magnetic shear plasma in the JT-60U tokamak. These two ITB states are characterized by different profiles of the second radial derivative of the ion temperature inside the ITB region (one has a weak concave shape and the other has a strong convex shape) and by different degrees of sharpness of the interfaces between the L mode and the ITB region, which is determined by the turbulence penetration into the ITB region.     

Stability of internal transport barriers to ideal MHD ballooning modes

Internal transport barriers (ITB) in tokamaks can form near a minimum in the q profile, q(min), where magnetic shear is weak. We have analyzed their stability to short wavelength (n>1, where n is the toroidal mode number) ideal MHD ballooning modes, by considering the s-alpha model equilibrium. We show that the ballooning transformation fails in regions of low shear but that one can then adopt a complementary approach based on the recurrence relation describing the toroidal coupling of radially localized modes on adjacent rational surfaces. Inclusion of the stabilizing effects of favorable average curvature or finite-n using this technique leads to stable high-pressure ITB configurations. The theory also shows the advantages of operating with low-order rational values of q(min).     

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.     

Dynamics of the transport barrier formation on the FT-2 tokamak caused by lower hybrid heating

Experiments at the FT-2 tokamak had demonstrated effective plasma LH heating, which was accounted for by both direct absorption of RF power and plasma transport suppression. The improved core confinement accompanied by Internal Transport Barrier (ITB) formation was observed. The RF pulse switch off is followed by triggering of LH transition and the External Transport Barrier (ETB) formation near the last closed flux surface. The present paper is devoted to a much more detailed study of the radial electric fieldE _r behaviour in the region of ITB and ETB and its influence on the tokamak microturbulence in these regions. The new experimental data were obtained by spatial spectroscopic technique using additional pulse helium puffing in hydrogen plasma. Simultaneously microscale plasma oscillations in the frequency band (0.01–2) MHz are observed with local enhanced microwave scattering diagnostics and by x-mode fluctuation reflectometry. Experiments demonstrate that the improved confinement is associated with the modification of microturbulence by the shear of theE×B poloidal velocity. This conclusion is also confirmed by the data obtained by Langmuir probes in the edge plasma. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002”. The study was performed with the support of the Ministry of General and Professional Education of RF (TOO-7.4-2797), INTAS-01-2056 and the RFBR Grants 00-02-16927, 01-02-17926 and 02-02-17684.     

Quasisteady high-confinement reversed shear plasma with large bootstrap current fraction under full noninductive current drive condition in JT-60U

A quasisteady reversed shear plasma with a large bootstrap current fraction ( approximately 80%) has been obtained for the first time in the JT-60U tokamak. The shrinkage of reversed shear region was suppressed by the bootstrap current peaked at the internal transport barrier (ITB) layer and the ITBs at a large radius were sustained, which, by combination with an H-mode edge pedestal, resulted in a high confinement or 2.2 times the H-mode scaling for 6 times energy confinement time or 2.7 s. Furthermore, a full noninductive current drive was obtained by the bootstrap current and the beam driven current.     

Modification to poloidal charge exchange recombination spectroscopy measurement in JT-60U tokamak

With consideration of the effects of the atomic process and the sight line direction on the charge exchange re-combination spectroscopy （CXRS）, a code used to modify the poloidal CXRS measurement on Tokamak-60 Upgrade （JT-60U） in Japan Atomic Energy Research Institute is developed, offering an effective tool to modify the measurement and analyse experimental results further. The results show that the poloidal velocity of ion is overestimated but the ion temperature is underestimated by the poloidal CXRS measurement, and they also indicate that the effect of observation angle on rotation velocity is a dominant one in a core region （r/a 〈 0.65）, whereas in an edge region where the sight line is nearly normal to the neutral beam, the observation angle effect is very small. The difference between the modified velocity and the neoclassical velocity is not larger than the error in measurement. The difference inside the internal transport barrier （ITB） region is 2-3 times larger than that outside the ITB region, and it increases when the effect of excited components in neutral beam is taken into account. The radial electric field profile is affected greatly by the poloidal rotation term, which possibly indicates the correlation between the poloidal rotation and the transport barrier formation.[第一段]     

Characteristics of electron heat transport of plasma with an electron internal-transport barrier in the large helical device

Associated with the transition from ion root to electron root, an electron internal transport barrier (ITB) appears in the large helical device, when the heating power of electron cyclotron resonance heating exceeds the threshold power. The incremental thermal diffusivity of electron heat transport chi(inc)(e) in the ITB plasma is much lower than that in the plasma with the heating power below the threshold, and the thermal diffusivity chi(e) decreases with increasing of heating power [dchi(e)/d(P/n(e))<0] in helical ITB plasmas.     

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive (LHCD) alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in D_α emission and a spontaneous rise in main plasma density. Good lower hybrid (LH) coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D₂ gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive (CD) efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray tracing code GENRAY are reported.     

Stability of tokamak plasmas with internal transport barriers against high n ideal magnetohydrodynamic ballooning mode

A ballooning mode equation for tokamak plasma, with the toroidicity and the Shafranov shift effects included, is derived for a shift circular flux tokamak configuration. Using this equation, the stability of the plasma configuration with an internal transport barrier （ITB） against the high n （the toroidal mode number） ideal magnetohydrodynamic （MHD） ballooning mode is analysed. It is shown that both the toroidicity and the Shaftanov shift effects are stabilizing. In the ITB region, these effects give rise to a low shear stable channel between the first and the second stability regions. Out of the ITB region towards the plasma edge, the stabilizing effect of the Shaftanov shift causes the unstable zone to be significantly narrowed.     

EAST H模等离子体中离子内部输运垒特性的初步实验研究

为了研究EAST上H模等离子体中离子内部输运垒(ITB)的特性,利用电荷交换复合光谱诊断,分析了离子ITB形成和稳态阶段等离子体离子温度和环向旋转速度的时空演化。结果表明,在离子ITB形成和稳定期间,ITB肩部附近(R=1.928m)的离子温度梯度增加时,ITB区域(R=1.984m)的离子温度梯度会有所降低,反之亦然。考虑到在离子ITB形成前,芯部区域不同半径位置的离子温度梯度同时增加或减小,得到了在R=1.984m处判断离子ITB是否形成的归一化离子温度梯度的阈值。     

On the Nonlinear Processes During Generation of Current Drive

The manifestations of the induced scattering effects in the generation process of current driven by lower-hybrid (LH) waves are studied taking into account the radiative-resonant interactions. The influence extent of the LH wave modulational instability on current drive is estimated. It is shown that the induced scattering of LH waves on plasma particles leads to a change of the LH waves spectrum and through this leads to an essential influence on fast electrons generation rate and the steady-state current drive. The modulational instability of LH waves can provide a “spectral gap” filling in the case of sufficiently strong LH wave pumping.     

Suppression of plasma turbulence during optimized shear configurations in JET

Correlation of density turbulence suppression and reduced plasma transport is observed in the internal transport barrier (ITB) region of JET tokamak discharges with optimized magnetic shear. The suppression occurs in two stages. First, low frequency turbulence and ion transport are reduced across the plasma core by a toroidal velocity shear generated by intense auxiliary heating. Then with the ITB formation, high frequency turbulence and electron transport are reduced locally within the steep pressure gradient region of the ITB.     

Probing internal transport barriers with heat pulses in JET

The first electron temperature modulation experiments in plasmas characterized by strong and long-lasting electron and ion internal transport barriers (ITB) have been performed in JET using ion cyclotron resonance heating in mode conversion scheme. The ITB is shown to be a well localized narrow layer with low heat diffusivity, characterized by subcritical transport and loss of stiffness. In addition, results from cold pulse propagation experiments suggest a second order transition process for ITB formation.     

Polarization and spatial-mode behavior of short-wavelength VCSELs

We have investigated the packaging introduced feedback and resulting variations in the characteristics of Vertical-cavity surface-emitting lasers (VCSELs). An integrated test and characterization method was developed for both the packaged and bare dies as well as on-wafers chips. Using an ‘integrated test bench (ITB)’ for characterization, packaged and ‘un-packaged’ VCSELs from multiple sources were studied. ITB was used to monitor and measure simultaneously several beam-parameters under various bias conditions including both dc and high-speed pulsed pumping at room-temperature, or at higher-/lower-temperatures that are thermo-electrically controlled. Typical beam parameters included state of polarization (SOP), transverse (spatial) modes, wavelength spectra, and output power. A common observation was that majority of devices are affected by the presence of optical windows capping the package. A shift in longitudinal mode was noticeable in packaged versus ‘un-packaged’ (window-removed) devices. But, SOP and transverse modes in various devices are affected severely and in an unpredictable fashion.