Helical System Research Articles

Multilayer mirror based soft x-ray spectrometer as a high temperature plasma diagnostic

A fully calibrated multilayer mirror (MLM) based soft x-ray spectrometer has been developed and installed on the Compact Helical System (CHS). This MLM based spectrometer is a new approach to devising a soft x-ray spectrometer with medium time and energy resolutions. The spectrometer mainly consists of a MLM as the dispersive element and a 20-channel (p-i-n) pin diode array for the detection of the soft x-rays. Both the MLM and the pin diodes have been calibrated at the KEK (High Energy Accelerator Research Organization) Photon Factory for an energy range of 300–1200 eV. The reflectivity of the mirror was found to increase from 3% (photon energy of 335 eV) to 25% (photon energy of 1050 eV). The pin diodes were found to have almost 100% efficiency for creating electron-hole pairs in the calibrated energy range. The spectrometer was installed on the CHS with the aim of fast electron temperature measurement and study of fast magneto-hydrodynamic (MHD) events occurring in plasmas. The experiments show that in the energy range of measurement, the electron “temperature” determined from the slope of the soft x-ray spectrum is much lower than that measured by the Thomson scattering diagnostic. Analysis showed that the soft x-ray spectrum is highly contaminated by impurity emission. Therefore it may be possible to measure electron temperature with this diagnostic if we choose another energy range where we can measure the continuous spectrum. The present time resolution of the system is of the order ∼0.1 ms, which made it possible to study the behavior of the plasma during fast MHD events. Modulations in the soft x-ray intensity were observed during MHD events in CHS plasmas. Analysis of these shows that these may be due to modulations in the temperature or the impurity concentration.

Experimental test of the radial force balance equation in the compact helical system

Experimental test of the radial force balance equation was done in the Compact Helical System Heliotron/Torsatron [S. Okamura et al. Nucl. Fusion 39, 1337 (1999)]. A radial electric field is measured with a heavy ion beam probe, while plasma rotation and drift velocity of fully ionized carbon are measured with charge exchange spectroscopy. The two measurements agree with each other to within 10% of the radial electric field in a wide range of electron densities of 0.3–2.0×1019 m−3.

ECE-imaging work on GAMMA 10 and LHD

Millimeter-wave two-dimensional (2-D) imaging array in the frequency range of 70 GHz has been developed for the application to the GAMMA 10 tandem mirror at the University of Tsukuba. It is installed in the plug cell, and successfully measures time evolutions of three-dimensional (3-D, radial and axial) profiles of line density and the second-harmonic electron cyclotron emission (ECE). The ECE imaging is also being applied to large helical device (LHD) at the National Institute for Fusion Science. In order to cover the frequency range of ECE on LHD, the novel detector using monolithic microwave integrated circuit (MMIC) technology is designed and fabricated. The detector consists of the integration of a bow-tie antenna, a down-converting mixer using a Schottky barrier diode, and hetero-junction bipolar transistor (HBT) amplifiers on a GaAs substrate. The response of the detector is compared with that of the previous hybrid-type detector. The heterodyne response up to 10 GHz was confirmed at 70–140 GHz for the monolithic-type detector. The imaging system using the monolithic detector has been applied to the measurement of ECE from GAMMA 10 and compact helical system (CHS) plasmas.

Alfvén eigenmodes excitation by external loop antennas in the Compact Helical System heliotron/torsatron

Four loop antennas are installed in the Compact Helical System (CHS) heliotron/torsatron in order to externally excite Alfvén eigenmodes (AEs). These antennas are used in two different operation methods: one is as a set of four external loop antennas, and the other two pairs of electrodes inserted near the plasma boundary. For these two operations, magnetic perturbations in the frequency range of 10–250 kHz are generated in the plasma. The plasma response for the applied magnetic perturbations is measured by magnetic probe arrays. Information of AEs can be derived from the analysis of the response. In the excitation experiment of AEs using the “electrode method,” magnetic fluctuations are resonantly enhanced in a certain electron density during the afterglow of a neutral beam heated plasma, where no energetic ions exist. The fluctuations have the toroidal mode number n=1 and the frequency lies just above the lower bound of the toroidicity-induced Alfvén eigenmode (TAE) gap in the plasma core region. The excited fluctuations are thought to be related to TAEs.

A system for Alfvén-eigenmode spectroscopy using electrodes inserted into the compact helical system heliotron/torsatron plasma

A system for Alfvén-eigenmode spectroscopy in the compact helical system heliotron/torsatron is presented. In this system, four electrodes inserted near the last closed flux surface are employed as the exciter of Alfvén eigenmodes (AEs). A set of magnetic probes is employed as the detectors of excited magnetic fluctuations. An alternating current path along the confinement magnetic field line is generated in the plasma edge between two electrodes which intersect with a specified magnetic field line. Another two electrodes also produce another current path. The polarity between these two current paths is adjusted to specify the toroidal mode number of the excited field perturbations. The frequency of the alternating current is in the range of 10–250 kHz. The possibility of the excitation of AEs by this system is discussed.

An application of electrically cooled Si detector to fast neutral measurement on CHS

A new type of fast neutral particle analyzer using Si diode cooled by Peltier module has been designed and developed. It is known that leak current in a solid-state detector is reduced by its use in low temperature. It is shown in this article that Peltier effect provides a very handy way of cooling the detector as long as a temperature around 0 °C is aimed at. The detector was easily cooled down to −1.6 °C and the performance of the detector was studied by the use of x/γ rays from Am241. The leak current was remarkably reduced as the detector is cooled improving energy resolution from 9.2 keV at room temperature to 4.3 keV at −1.6 °C. In addition, the lower detectable energy limit was expanded from 25 keV to 14 keV. A Si detector is also sensitive to visible light and x ray. Therefore, the detector of our “reference design” is furnished with an evaporated 1000-Å-thick aluminum layer to reduce the visible light and with a lead collimator to reduce the x ray. The detector was finally installed on the Compact Helical System device for a test of overall performance and an energy spectrum from a diagnostic neutral beam was successfully measured. This test was conducted without an Al layer and this result suggests a possibility of eliminating it.

Soft x-ray detector array for the study of magnetohydrodynamics instabilities in the compact helical system heliotron/torsatron

A 20 channel PIN-photodiode array is installed on the compact helical system (CHS) heliotron/torsatron to measure the radial profiles of soft x-ray (SXR) emission and fluctuations. In neutral beam injection (NBI) heated plasma on CHS, sawtooth oscillations are often observed in the SXR signals. The sawtooth has a characteristic feature of the off-axis sawtooth crash that takes place first near the 1/q=1/2 rational surface located at the normalized minor radius ρ≈0.4−0.6, where q is the safety factor. In most of cases high frequency precursor and low frequency postcursor oscillations having m/n=2/1 mode structure (m, n: the poloidal and toroidal mode numbers) appear before and after the crash. The total plasma beta derived from the measurement of SXR emission-peak position is slightly but clearly decreased by the sawtooth crash. This reduction suggests rapid transport of energetic beam ions from the core region toward the plasma edge region.

Development of fast response calorimeter for neutral beam shine-through measurement on CHS

A fast response calorimeter has been developed for the neutral-beam (NB) shine-through measurement. This calorimeter has the following advantages. (1) Temporal variation of the heat load onto the calorimeter can be measured. (2) Measurement under the relatively high heat flux environment is possible. (3) The calorimetric measurement under continuous and steady-state heat load environment is also possible. The verification of the measurement principle was done using an NB-injection system on the compact helical system (CHS) at the National Institute for Fusion Science (NIFS). The measured NB power densities are compared to the power densities being evaluated by the CHS-NB profile database. It was experimentally confirmed that the time constant of the measurement is about 7 ms.

Study of structural effects of Langmuir probe array on edge fluctuation measurement in the compact helical system heliotron/torsatron

A new type of Langmuir probe array is installed on the compact helical system heliotron/torsatron for the study of edge plasma turbulence. The probe array consists of four sets of the standard triple probes which are composed by four electrodes. Each triple probe set is slightly displaced both in the toroidal and poloidal directions to minimize the shadow effects among these probe sets. In order to investigate the structural effects of Langmuir probe arrays, the results obtained by this new probe array are compared with the results previously obtained by the old probe array where the triple probe sets were displaced only in the poloidal direction from each other. Stationary plasma parameters near the plasma edge such as electron temperature, density, and plasma potential were consistently obtained by two probe arrays. However, the new probe array displaced toroidally is more reliable to measure the particle flux induced by fluctuations where accurate measurement of the poloidal electric field fluctuations is required, compared with the old probe array.

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411]. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied.

Energy and spatial resolved measurement of soft x-ray emission with photon counting x-ray charge coupled device camera in compact helical system

Two-dimensional profiles (32×16 spatial channels) of energy spectra of x-ray emission are measured with an energy resolution of 0.16 keV using the photon counting soft x-ray charge coupled device (CCD) camera by optimizing the intensity of x ray with attenuation Be filter in the compact helical system. The energy spectra measured with the CCD camera agrees with that estimated from electron temperature and density measured with YAG Thomson scattering.

Energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System and Large Helical Device

Recent results of energetic ion driven MHD instabilities observed in theheliotron/torsatron devices Compact Helical System (CHS) and Large HelicalDevice (LHD) are presented. Alfvén eigenmodes (AEs)and fishbone-like burst modes (FBs) destabilized by energetic ions were observed inNBI heated plasmas of CHS. The AEs are toroidicity induced Alfvén eigenmodes (TAEs)and global Alfvén eigenmodes (GAEs), where the identified toroidal mode numbersare n = 1 and 2 for TAEs and n = 0 for GAEs. The frequencies of the FBs are less than,at most, half of the minimum TAE gap frequency and do not exhibit the obvious densitydependence related to Alfvén velocity. The modes have characteristic features of theenergetic particle modes or the resonant TAEs excited by circulating energetic beamions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as inFBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBsand about 25% in TAEs. In several shots, the power spectrum of the TAEs is split intomultiple peaks having the same toroidal mode number through non-linear evolution ofTAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal modenumber. This research has been extended to LHD plasmas heated by neutral hydrogen beamswith about 130 keV energy. Similar to CHS, TAEs and FBs were observed in relatively lowdensity plasmas at low toroidal magnetic field (Bt = 1.5 T).

Large magnetoresistance in SrFeO 2.95

A large magnetoresistance effect in a Fe-based perovskite has been observed. A typical magnetoresistance (MR) is 15.5% for SrFeO 2.95 at 5 K, and also a high field induced MR is noted at a temperature well above the antiferromagnetic transition temperature. Magnetization and transport measurements show that two transitions occur at temperatures around 107 and 65 K, respectively. The upper transition appears to correspond to the antiferromagnetic transition. The second transition, at low temperature, shows a resistivity minimum which coincides with the weak anomaly in the χ( T) curve. The upturn in ρ below 65 K arises from a T 1/2 dependence of the conductivity. This upturn in ρ at low temperatures is a manifestation of electron–electron interaction and quantum interference effects in electronic conduction, such as weak localization in this helical magnetic system.

Overview of long pulse operation in the Large Helical Device

The Large Helical Device is the world's largestheliotron type helical system, with the plasma confining magnetic fieldbeing generated by only external superconducting coils. One of the mainobjectives of the LHD project is to sustain high temperatureplasmas for a long time in steady state. The plasma vacuum vesseland the divertor are water cooled, and a heat load of 3 MW can beremoved continuously. The NBI, ECH and ICRF heating systems, diagnostic instruments and data acquisition system are designed for long pulse operation. The presentstatus of these systems and the recent experimental results of long pulse operation are reviewed. A steady state discharge with NBI was obtained for 35 s. The ECH discharge duration was extended to 120 s with a duty factor of 95%. Plasma sustainment by ICRF alone was achieved for 2 s. The performance of these long pulse operations is summarized.

Helical focus wave modes and helical nondiffracting beams

A number of years ago Durnin and colleagues considered the possibility of nondiffracting beams, beams defined to be nonsingular free space exact solutions of the scalar wave equation having the same intensity distribution in every plane normal to the axis of propagation [J. Durnin, J. Opt. Soc. Am. A 4, 651–655 (1987)]. Assuming harmonic time dependence and cylindrical coordinates, i.e., a Fourier decomposition of the time and space coordinates, the Bessel function mode solutions of the scalar Helmholtz equation were pointed out as a class of diffraction-free solutions. Similarly, using a nonorthogonal helical coordinate system, helical mode solutions of the scalar Helmholtz equation that are nondiffracting in the same sense as the Bessel beams can be determined [P. L. Overfelt, Phys. Rev. A 46, 3516–3522 (1992)]. In this paper, when a bidirectional plane wave decomposition [I. M. Besieris et al., J. Math. Phys. 30, 1254–1269 (1989)] is assumed, this same helical coordinate system can be used to obtain helical focus wave mode (FWM) solutions to the scalar wave equation. The fundamental Gaussian FWM can be decomposed into two oppositely polarized helical FWMs. Many relationships between the helical and cylindrical FWMs and their beam solution counterparts will be shown.

Megavoltage computed tomography for tomotherapeutic verification

A helical tomotherapy system is being developed at the University of Wisconsin—Madison as a means of improving Intensity Modulated Radiotherapy (IMRT) delivery. The goal of this thesis was to develop megavoltage computed tomography (MVCT) capabilities to image the patient in the treatment position, before, during, or after IMRT treatment. This MVCT system uses a 738‐channel xenon detector and is similar to helical kilovoltage CT (kVCT) except that it utilizes the treatment linac as the x‐ray source. Due to the high (4 MV) photon energy, MVCT contrast is inferior to kVCT, but should be adequate to verify patient positioning for radiotherapy. Techniques were developed to preprocess transmission data and to correct for artifacts. Image quality was compared for several different reconstruction algorithms. MVCT scans on a tomotherapy research system demonstrated relative contrasts below 1% for 25 mm objects using a 7 cGy dose to the center of the phantom. High‐contrast resolutions are 1.25 mm for air cavities. Through calibration of this system with known materials, the reconstructed values have been demonstrated to be linear with electron density, which benefits subsequent dose calculations. MVCT using transmission data collected during IMRT treatment delivery was also investigated. By overcoming the problems of incomplete data availability and difficulty normalizing modulated beams, treatment data was incorporated into the reconstruction process. This has demonstrated improved image contrast while reducing the additional imaging dose, and also presents the opportunity to collect all of the data necessary for image reconstruction during the treatment delivery.

Thermal transport barrier in heliotron-type devices (Large Helical Device and Compact Helical System)

In the discharges of the Large Helical Device [O. Motojima et al., Proceedings of the 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437], a significant enhancement of the energy confinement has been achieved with an edge thermal transport barrier, which exhibits a sharp gradient at the edge. Key features associated with the barrier are quite different from those seen in tokamaks (i) almost no change in particle (including impurity) transport, (ii) a gradual formation of the barrier, (iii) a very high ratio of the edge temperature to the average temperature, (iv) no edge relaxation phenomenon. In the electron cyclotron heating (ECH) heated discharges in the Compact Helical System [K. Matsuoka et al., in Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, France, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411], the internal electron transport barrier has been observed, which enhances the central electron temperature significantly. High shear of the radial electric field appears to suppress the turbulence in the core region and enhance the electron confinement there.

Transport barrier formation and bifurcation patterns ofpotential profiles in the compact helical systemheliotron/torsatron

In the compact helical system, several bifurcation patterns havebeen observed in potential profiles ofelectron-cyclotron-heated plasmas. In a bifurcation pattern witha strong shear of radial electric field, an internal transportbarrier is confirmed to be established for electron thermalenergy. Probabilistic treatment with realization of the patternor internal transport barrier is discussed.

Influence of sequence-dependent cytosine protonation and methylation on DNA triplex stability.

To investigate cytosine protonation and its influence on the sequence-dependent thermal stability of DNA triplexes in detail, we have employed homo- and heteronuclear NMR experiments on specifically (15)N-labeled oligodeoxynucleotides that were designed to fold into intramolecular triple helices of the pyrimidine motif under appropriate conditions. These experiments reveal that cytosines in central positions of the triplex are significantly protonated even at neutral pH. However, semiprotonation points for individual cytosine bases as determined from pH-dependent measurements show considerable differences depending on their position. Thus, protonation is disfavored for adjacent cytosines or for cytosines at the triplex termini, resulting in a smaller contribution to the overall free energy of the triple helical system. In contrast, protonation of the base upon substitution of 5-methylcytosine for cytosine in the triplex third strand is only affected to a minor extent, and triplex stabilization by the methyl substituent is shown to primarily arise from stacking energies and/or hydrophobic effects.

Multilayer mirror soft x-ray spectrometer for fast electron temperature measurement on the compact helical system

A multilayer mirror (MLM) soft x-ray spectrometer has been installed on the Compact Helical System. The x-ray energy spectrum from ECH and neutral beam injected heated low beta plasmas was measured as a function of radiation energy. Modulations of the x-ray intensity associated with magnetohydrodynamic instability were observed. The electron temperature measurement was also investigated using the MLM x-ray spectrometer. The data suggest the possibility of a fluctuation measurement of the electron temperature Te with fast time resolution using the MLM spectrometer.