High Magnetic Field Side Research Articles

NMR and magnetic studies on 7 K anomaly in Tl 3H(SO 4) 2

Measurements of the spin-lattice relaxation time, NMR absorption line and magnetization have been carried out on the Tl 3H(SO 4) 2 crystal below 50 K. The anomaly at around 7 K was: (1) the spin-lattice relaxation times of 1H and 205Tl nuclei increase steeply with decreasing temperature below 7 K, (2) the NMR absorption lines below 7 K shift to the high-magnetic field side in comparison with that above 7 K, and (3) the 1H NMR line width exhibits a drastic increase of the line width with decreasing temperature below 7 K. These results indicate that the magnetic dipole fluctuation of the proton changes at 7 K. On the other hand, there are no remarkable anomalies of magnetic susceptibility at around 7 K. From these results it is deduced that the anomaly at around 7 K is caused by the change in quantum mechanical process of the proton from proton tunneling to zero-point vibration of hydrogen in the hydrogen bond with the decrease of temperature.

Gas and Pellet Injection Systems for JT-60 and JT-60U

Designs and operations of the gas system and pellet injection systems for JT-60 and JT-60U are described. A gas injection valve that is a key component of the gas injection system was developed using a multilayer piezoelectric element. The maximum flow rate of this system is 43.3 Pa⋅m3/s. The valve has mechanism for adjustment at atmospheric side meaning that a repair and an adjustment can be conducted without ventilation inside a vacuum vessel. It was confirmed that the effect of magnetic field and temperature change on the valves in the JT-60U environment was negligible.In JT-60U, two systems of pellet injector – a pneumatic drive and a centrifugal one – were developed. The pneumatic type attained a pellet velocity of 2.3 km/s, which was the world record at the time in 1988. On the other hand, the centrifugal one was developed in 1998. This injector can eject trains of up to 40 cubic (2.1 mm3) pellets at frequencies of 1 to 10 Hz and speed of 0.1 to 1.0 km/s. A guide tube for a magnetic high field side injection (HFS) (top) was also developed in 1999. The pellet injection experiment with the HFS system started in 2000. In addition, another guide tube for HFS(mid) injection was newly developed and installed in March 2001. These systems are working well.

High density operation at JET by pellet refuelling*

Several approaches are used at JET to achieve operation at high densitywith good energy confinement. One of them is the injection of solid fuelpellets to realize efficient particle refuelling by deposition deepinside the plasma column. The new pellet launch system capable oflaunching from the torus magnetic high field side was investigated forits capability to fulfil this task in conventional ELMy H-modedischarges. Optimized pellet scenarios were developed for plasmaconfigurations with IP = 2.5 MA, Bt = 2.4 T, averagedtriangularity ⟨δ⟩≈0.34 and mainly neutral beam heatingat a level of approximately 17 MW. The accessible operational range wasextended by the pellet tool with respect to gas puff refuelling. Forexample, H-mode conditions could be maintained at densities beyond theGreenwald level. Plasma energy confinement was observed to becomedensity independent at high densities. When avoiding confinementdeterioration due to pellet triggered MHD activity or parasitic pelletborn gas in appropriate pulse schedules, enhanced particle inventorywith more peaked density profiles was achieved while maintaining theplasma pressure profile. First attempts indicate that pellet advantages canbe combined with the benefits achieved by higher triangularity. Veryhigh particle refuelling efficiencies were found for pellets injectedinto `ITER-like' discharges with an upper triangularity ⟨δu⟩ of0.53.

Design and implementation of a high speed guiding system for inboard pellet refuelling

Cryogenic pellet injection from the magnetic high field side (HFS) of a tokamak is the currently favoured scheme to refuel steady state H-mode discharges. However, in currently operating tokamaks access to the HFS has to be gained via strongly curved guide tubes severely restricting injection speeds and leading to shallow plasma penetration where a large portion of the injected material is lost in the edge gradient layer within a few milliseconds after injection. Therefore, at ASDEX Upgrade an optimized pellet guiding system for speeds close to 1000 m/s has been designed and put into operation. The 17 m long track consists of: (i) a funnel installed to catch pellets scattered by the accelerator; (ii) a preliminary teflon tube section; and (iii) a guiding track transferring the pellets in a diagnostic port into the plasma. Using a looping geometry the system is designed to keep semi-permanent contact between pellet and track by steering the pellets in a controlled trajectory. Emphasis is also put on efficient evacuation of all sections to avoid pressure build-up in the system due to the Leidenfrost effect. In first proof-of-principle plasma discharges a scan of pellet injection at v=240–880 m/s and plasma parameter studies at v=560 m/s were completed.

A repetitive pellet injection system for JT-60U

A centrifugal pellet injector and a guide tube for magnetic high-field side (HFS) injection were developed in JT-60U. The injector can eject trains of up to 40 cubic (1.9 mm) 3 deuterium pellets at frequencies of 1–10 Hz and velocities of 0.2–1.0 km/s. Pellet injection experiments into ohmic heating (OH) plasma were performed. From plasma density measurement, central fueling and enhanced fueling rate compared to gas puffing were observed for magnetic low-field side (LFS) injections. The results of HFS injection through the guide tube connected to the upper port of the vessel showed that the pellet velocity exceeding 220 m/s caused pellet destruction inside the tube.

ORNL mock-up tests of inside launch pellet injection on JET and LHD

In experiments on ASDEX-Upgrade and DIII-D tokamaks, the injection of D 2 pellets from the magnetic high-field side of the plasma resulted in deeper pellet penetration and improved fueling efficiency. Based on those successful experiments, fusion researchers at the Joint European Torus and the Large Helical Device decided to implement inside launch pellet injection. These injection schemes require the use of curved guide tubes to route the pellets from the acceleration devices to the inside launch locations, and the pellets are subjected to stresses from centrifugal and impact forces in traversing the tubes. Before the installations on the large experimental fusion devices, mock-ups of the guide tubes were constructed and tested at the Oak Ridge National Laboratory to determine the pellet speed limit for reliable operation without pellet fracturing. In laboratory testing of the mock-ups, it was found that the pellet speed had to be limited to a few hundreds of meters per second for intact pellets. In this paper, the test equipment and experimental results are described.

Refuelling performance improvement by high speed pellet launch from the magnetic high field side

Limited available pellet velocities have so far restricted the refuelling performance of efficient launch schemes from the tokamak magnetic high field side (HFS). Although pellet injection during H mode has resulted in more peaked density profiles and enhanced performance with respect to gas puff refuelling, prompt particle and energy losses caused by pellet induced ELM bursts have still limited the extension of the operational area. Now, the preliminary version of a new optimized pellet injection set-up at ASDEX Upgrade allows for significantly higher injection speeds when launching pellets from the magnetic HFS. Intact pellets with velocities up to vP = 560 m/s were successfully injected instead of the vP = 240 m/s available with the previous set-up. The velocity increase results in a deeper pellet penetration and seems to follow the vP1/3 scaling derived from a multimachine study using conventional pellet launch from the torus outside. The inward shift of the pellet particle deposition profile with respect to the penetration depths turned out to be approximately the same for both launch velocities. The respectively achieved deeper particle deposition inside the plasma column reduced the particle and energy loss rates during the immediate post-pellet phase. Thus, further enhancement of tokamak operation in the high density regime seems feasible by means of high speed pellets launched from the torus inner side.

Localized measurements of electron cyclotron current drive using MSE spectroscopy on the DIII-D tokamak

A new method of current drive analysis is demonstrated that compares the measured pitch angles from motional Stark effect (MSE) spectroscopy with simulations of the expected MSE response to localized ECCD. By comparing simulations of the MSE signals with measurements, the best fit of the ECCD profile is determined. The ECCD efficiency ascertained in this manner decreases as the current drive location moves to larger minor radius owing to electron trapping effects. The ECCD efficiency is shown to be highest when the driven current is on the high magnetic field side of the plasma, as expected from theory. The width of the ECCD profile is also in good agreement with theory; previous reports of a broader than expected ECCD profile are shown to have been caused by insufficient radial resolution of the magnetic equilibrium reconstruction.

GHz帯における高周波キャリア型薄膜磁界センサの動作特性解析

This paper discusses the properties of a high-frequency carrier-type thin-film magnetic field sensor in a GHz band. Calculations based on bias susceptibility theory and Maxwell’s equation indicated that the properties of the sensor in the GHz band were meinly determined by ferromagnetic resonance. The high-sensitivity area shifted to the high magnetic field side when the carrier frequency was increased. The properties are greatly different from those in megahertz band. Therefore, the high-sensitivity area of the sensor can be selected by using saturation magnetization or an anisotropy field to determine the resonance frequency.

Improved core fueling with high field side pellet injection in the DIII-D tokamak

The capability to inject deuterium pellets from the magnetic high field side (HFS) has been added to the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. It is observed that pellets injected from the HFS lead to deeper mass deposition than identical pellets injected from the outside midplane, in spite of a factor of 4 lower pellet speed. HFS injected pellets have been used to generate peaked density profile plasmas [peaking factor (ne(0)/〈ne〉) in excess of 3] that develop internal transport barriers when centrally heated with neutral beam injection. The transport barriers are formed in conditions where Te∼Ti and q(0) is above unity. The peaked density profiles, characteristic of the internal transport barrier, persist for several energy confinement times. The pellets are also used to investigate transport barrier physics and modify plasma edge conditions. Transitions from L- to H-mode have been triggered by pellets, effectively lowering the H-mode threshold power by 2.4 MW. Pellets injected into H-mode plasmas are found to trigger edge localized modes (ELMs). ELMs triggered from the low field side (LFS) outside midplane injected pellets are of significantly longer duration than from HFS injected pellets.

High-frequency conductivity experiments on the integer quantum Hall effect at the limits of universality

The localization/delocalization transitions in the Landau levels (LL) related to the integer quantum Hall effect (QHE) are examined by studying the DC and high-frequency (35 GHz) conductivity σxx of a two-dimensional electron system (2DES) in AlGaAs/GaAs in the temperature range 0.3–4 K. Because of the observed asymmetric shape of σxx(B) between its QHE minima the peak widths are determined separately for the low and high magnetic field side. In the DC data the temperature dependence of the low-field width follows a power law with exponents close to those expected from scaling theory, the high-field width shows far higher exponents. In the high-frequency data only at the lowest temperatures (hf≫kT) the width becomes temperature independent according to the dynamical scaling theory.

High-Field-Side Pellet Injection Technology

High-speed injection of pellets, composed of frozen hydrogen isotopes and multimillimeter in size, is commonly used for core fueling of magnetically confined plasmas for controlled thermonuclear fusion research. Straight guide tubes have typically been used to transport/deliver pellets from the acceleration device to the outside, or magnetic low-field side, of the torus/plasma (distance of −5 to 10 m for most installations). Recently, alternative pellet injection schemes have been used in plasma fueling experiments, including inside launch from the magnetic high-field side on ASDEX-U and top launch (vertically downward) on Tore Supra and DIII-D. These schemes require the use of curved guide tubes in which the pellets are subjected to stresses from centrifugal and impact forces. Thus, with curved guide tubes the speed at which intact pellets can be delivered reliably to the plasma is limited. In impact experiments on flat plates, it was found that deuterium (D2) pellets can survive single collisions at normal velocities in the range 20 to 35 m/s. Several series of tests with various curved guide tube configurations have been carried out, showing that intact pellets can be reliably delivered at speeds of several hundreds of meters per second. The experimental data are summarized and discussed. Also, a model is under development at Tore Supra for predicting these phenomena, and preliminary comparisons with the data are discussed.

Kinetic model of Alfvén wave light-ion gyroresonance heating

The absorption of Alfvén waves by gyroresonant interaction with the light-ions in an ionospheric oxygen-hydrogen plasma with a parallel magnetic field gradient is reconsidered, taking into account simultaneously the plasma inhomogeneity and the finite temperature of the resonant ions (Ti≠0). A kinetic full wave equation is derived, that is valid in the vicinity where the wave frequency matches the local proton gyrofrequency. It is analytically solved for the case of a Lorentzian distribution function. The energy transmission, reflection and absorption coefficients, for waves incident from the high magnetic field side onto the gyroresonant interaction region, are found to be the same as for the cold plasma case (Ti=0). Conversely, the absorption of waves incident from the low magnetic field side is found to be enhanced and strongly depends on the ionic temperature, whereas their transmission to the high magnetic field side occurs still in the same proportion as for the cold plasma case. Elaborating local dispersion curves and evaluating the full wave solutions enable us to interpret qualitatively these results and to extrapolate them for the case of a Maxwellian distribution function. For realistic ionospheric plasma conditions a sensible increase of the gyroresonant absorption of the waves is thus found.

High-Efficiency Plasma Refuelling by Pellet Injection from the Magnetic High-Field Side into ASDEX Upgrade

High-efficiency refuelling of ELMy H-mode tokamak discharges with solid deuterium pellets injected from the magnetic high-field side is demonstrated. Compared to standard low-field side injection, the fuelling efficiency was enhanced by a factor of 4, the pellet penetration more than 2 times. This experimental result can be qualitatively explained by the magnetic force pushing a diamagnetic plasma cloud towards lower magnetic field, causing rapid particle loss for shallow low-field side injection, but enhancing fuelling efficiency and pellet penetration for high-field side injection.

Exchange coupling energy determined by ferromagnetic resonance in 80NiFe/Cu multilayer films

Ferromagnetic resonance (FMR) experiments were carried out for [ 80NiFe(60 A ̊ )/Cu (d Cu A ̊ )] N , ( d Cu = 10–12, N = 2–5) multilayer films fabricated by a sputtering method. The main resonance peak had a broad shoulder located on the higher magnetic field side. Namely, the exchange energy between the magnetic layers was antiferromagnetic. In order to estimate the exchange energy, the FMR absorption spectrum was calculated using the Landau-Lifshitz equation of motion. The calculation model was improved so that we were able to analyze the spectrum for a multilayer with N magnetic layers. The exchange energy obtained from FMR analysis was compared with that estimated from the saturation magnetic field in the magnetoresistance (MR) curve. The positions and the intensity ratio of the resonance peaks were dependent on merely the exchange energy but not on the number of layers. The MR ratio was roughly proportional to the exchange energy.

Experimental evidence for finite-width edge channels in integer and fractional quantum Hall effects.

We report our transport study of a narrow and low-density two-dimensional electron system (2DES) defined by a pair of split gates, in the integer (IQHE) and fractional quantum Hall effect (FQHE) regimes. The gate-bias-dependent breakdown on the high and low magnetic field side of the IQHE and FQHE diagonal resistance minima is studied. In the IQHE case, edge states of width comparable to the magnetic length, in the single-electron Landau-level picture, cannot explain the observed breakdown data and much wider edge channels (wide band of edge states) are needed. These wide edge channels are interpreted as the compressible regions when the 2DES near the sample boundary phase separates into compressible and incompressible regions. The same breakdown behavior is observed in the fractional quantum Hall state and is interpreted as a similar phase separation in the FQHE regime.

Microwave reflectometry with the extraordinary mode on tokamaks: Determination of the electron density profile of Petula-B

The use of an ordinary mode reflectometer has already proved to be a good means for determining the density profile of a plasma, but the fact that this mode reflects where its frequency equals the local plasma frequency restricts its use to the region of the density gradient. The extraordinary mode reflectometry investigated here permits the observation of the scrape-off layer as well as the near-axis region and in certain conditions also the high magnetic field side of the density profile. It is also shown that this mode requires a range of frequency sweep for measuring a given density profile smaller than the ordinary mode. On the Petula-B tokamak, the density profile has been measured in 200 μs, its evolution when applying the lower hybrid current drive could be also followed. Since the measurement is particularly sensitive near the reflection point of the wave, it could be used to determine precisely the position of magnetic islands and the fluctuation amplitudes. In addition, the access and place occupied in the vacuum chamber being very small, this diagnostic should be compatible with reactor conditions.

Main origin of asymmetric lineshape of the dangling bond ESR spectrum in hydrogenated amorphous Si films

It has been well known that the dangling bond ESR spectrum of hydrogenated amorphous silicon films deposited on quartz substrates has an asymmetric lineshape which is nearly Gaussian for the high magnetic field side, while nearly Lorentzian for the low magnetic field side. A new spectrum with g = 2.0010 .0005 and the linewidth Δ H = 3 G is resolved in this Gaussian line by adjusting the amplitude of magnetic field modulation and is identified as the Si-E′ center. The presence of this center in the near-surface of the quartz substrates is mainly responsible for the asymmetric lineshape.

Reduction of order in the geometric optics of plasmas

Small amplitude waves in plasmas are usually described by large systems of linearized Maxwell and kinetic equations. A procedure of elimination of components and reduction of the order of the system is discussed within the geometric optics approximation. In some cases characterized by Hermitian generalized dielectric tensors ℰ describing the unreduced problem, the successive reduction procedure yields, at each step, energy conserving reduced systems that preserve the general form and first-order nature of the equations. The number of equations in the final reduced system is equal to the number of degenerate vanishing eigenvalues of ℰ. The theory is applied in the case of transverse waves, propagating along the magnetic field in plasmas with plane parallel stratification. Both cold streaming plasma and kinetic problems are considered by using the same-order reduction procedure. The cold plasma case, at low densities, becomes doubly degenerate at cyclotron resonance, reflecting mode coupling between the vacuum electromagnetic and electron cyclotron modes. Mode conversion coefficients found from the solution of the reduced system of two first-order differential equations, characterizing this case, are in an excellent agreement with the results of the numerical solutions of the full unreduced system of equations. The kinetic case is approached by viewing the plasma as consisting of many beamlets each governed by the cold fluid approximation. The problem represents a multiply degenerate situation as many beamlets are in resonance at a time. Renormalized perturbation analysis of the partially reduced system in the low-density case predicts results similar to those found in the cold plasma, with possible broadening of the resonance region. At large densities, for propagation from the lower magnetic field side, the wave is reflected and the cyclotron resonance is inaccessible. In contrast, for propagation from the high magnetic field side, the electromagnetic energy is transferred to the electrons via the mechanism of Landau cyclotron damping.

Hot-Electron Magnetophonon Effect in n-InSb at 77 K Investigated by Magnetic Field Modulation Technique

Second derivative of the transverse and longitudinal magnetoconductance is carried out by using a method of pulse electric field application combined with the magnetic field modulation technique. Resonant conductance minima shift to higher magnetic field side with increasing the electric field in the transverse configuration, while in the longitudinal configuration the ordinary maxima do not shift to higher magnetic field side but new conductance maxima are observed.