Electron Density Profile Measurements Research Articles

Active control of laser beam direction for LHD YAG Thomson scattering

We have developed a YAG Thomson scattering (TS) system for the measurements of electron temperature and density profiles on the large helical device (LHD). The LHD-TS has four YAG lasers, and flexible operational modes are possible by using them. For example, (1) high-energy mode: The pulse energy can be increased up to four times by firing the four lasers simultaneously. In this mode, the data quality can be improved for low-density plasmas. (2) High repetition mode: When firing the lasers at intervals of 5 ms, the lasers work as a 200 Hz laser. The laser beams are guided to the LHD by seven steering mirrors. The first mirror is real-time feedback controlled for precise beam transport. The beam pointing stability is improved successfully from 200 μrad to below 4 μrad with the feedback-control system. We describe the details of the laser system for the LHD-TS.

Microwave Reflectometry in the GAMMA 10 Device

Two types of microwave reflectometries are installed in the GAMMA 10 device for electron density profile and fluctuation measurement. Microwave is injected into the plasma in the O-inode or in the X-mode and is reflected at the cutoff layer where the electron plasma frequency corresponding to the electron density is equal to the frequency of incident wave.An ultra short pulse method uses pulse train having duration of 65 ps and repetition rale of 250 kHz generated by an impulse generator. This signal has a broad frequency spectrum in microwave region. So simple transmitting system is realized. Reflected wave is divided into five frequency channels. Their center frequencies are 7, 8, 9, 10 and 11 GHz. Time of flight of each frequency component is measured using a time to amplitude converter. Location of reflected point is calculated from the time of flight. An electron density profile is reconstructed with one-shot data.Fast frequency-modulation method is also used. The microwave source is swept from 11.5 GHz to 17.5 GHz: The signal is injected in the X-mode. Phase difference between reference wave and reflected wave is observed. Density profile is reconstructed from the phase difference data. There is low frequency fluctuation in the plasma. When sweep time is faster than 20 μ s, reliable data are obtained. This system has a rather simple receiving system.Though reflected wave has an information of fluctuation, several methods of frequency analysis are tried and distribution of fluctuation is observed simultaneously.Magnetic fluctuation is also observed by cross-polarization scattering method.

Ultrashort pulse reflectometry for electron density profile measurements on SSPX

A broadband ultrashort pulse reflectometry (USPR) diagnostic has been developed for measuring electron density profiles of the sustained spheromak physics experiment (SSPX) device. In USPR, an extremely short pulse or chirped wave form is propagated which contains a broad range of frequency components spanning the desired plasma density profile (or a significant fraction thereof). Upon reflection, each frequency component in the incident waveform reflects from a different spatial location (density layer) in the plasma, thus spreading out the reflected wave packet in time. By simultaneously collecting double-pass time delay data at many distinct frequencies, the time delay data may then be inverted to generate plasma density profiles using a single source and a single set of measurements. On SSPX, wideband mixers are utilized to up- and downconvert 6–18 GHz chirp signals to millimeter-wave frequencies (33–158 GHz) to form a 48 channel O-mode reflectometer system. In this article we describe details of the new USPR system installed on the SSPX device and provide preliminary time-of-flight results.

Novel applications of MST radars in meteor studies

MST radars have been used to detect and study meteors since the mid-1980s. By exploiting the narrow beamwidth, high-pulse repetition rate and measurement of echo phase inherent in such systems, a number of new techniques have been developed specifically for meteor studies. These include: measurements of the speed of entry of meteoroids to a precision of 0.3%; measurements of the deceleration of meteoroids during ablation; observation of the fragmentation of meteoroids. The measurement of meteor drifts and hence the study of the dynamics of the atmosphere in the ‘meteor region’ is now a routine procedure at many MST radar sites. A serendipitous adjunct to MST meteor studies is the measurement of electron density profiles in the E-region using the Faraday effect.

Comparative measurement of electron density and temperature profiles in low-temperature ECR discharges by a lithium atom beam and Thomson scattering

In this paper a method to determine spatially-resolved profiles of the electron temperature Te and density ne in an electron-cyclotron-resonance (ECR) discharge is presented. This technique is based on the observation of line emission from a neutral Li atom beam, which is injected into the plasma and excited by electron collisions. A collisional-radiative model valid for the injected Li atoms is used to predict the emission intensities as function of ne and Te for several lines theoretically. In contrast to the electron temperature regime representative for the edge of tokamak discharges (Te>5 eV), the ECR discharge offers a Te range where selected line intensity ratios strongly depend on the electron temperature. Therefore, a comparison of the measured ratios with the calculated ones yields Te profiles for the first time. The ne measurement is performed as in tokamaks by observing the attenuation of the beam due to ionization in the plasma. We present radial profiles of Te and ne for discharges in argon and xenon under different operating conditions. These results are compared with results obtained by Thomson scattering. Our measurements give evidence for a satisfying agreement between the two methods.

Measurement of electron density profile in the lower ionosphere during nighttime by rocket observations of MF and VLF radio waves

Measurements are made on the magnetic field strength of MF and VLF radio stations by rocket observations during nighttime, along with measurements made on the Langmuir probe (dc probe) current and Doppler observations of MF. In the E layer, we can observe the standing wave of the VLF magnetic field strength, and also the ordinary and extraordinary components of MF. In order to make the theoretical values by the full-wave calculation coincide with these observed values with reference to the dc probe currents, we adjust the electron density and the distribution of the electron collision frequency with altitude. By these methods, the electron density of the E layer is estimated from the bottom to the top, and we find the existence of multiple weak Es layers. By MF Doppler observation, we successfully estimate the electron density of the upper E layer, and can estimate the effective electron collision frequency near 110 km in altitude from the strength of the upward component of the extraordinary wave. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 83(2): 20–28, 2000

Measurement of electron density profile in the lower ionosphere during nighttime by rocket observations of MF and VLF radio waves

Measurements are made on the magnetic field strength of MF and VLF radio stations by rocket observations during nighttime, along with measurements made on the Langmuir probe (dc probe) current and Doppler observations of MF. In the E layer, we can observe the standing wave of the VLF magnetic field strength, and also the ordinary and extraordinary components of MF. In order to make the theoretical values by the full-wave calculation coincide with these observed values with reference to the dc probe currents, we adjust the electron density and the distribution of the electron collision frequency with altitude. By these methods, the electron density of the E layer is estimated from the bottom to the top, and we find the existence of multiple weak Es layers. By MF Doppler observation, we successfully estimate the electron density of the upper E layer, and can estimate the effective electron collision frequency near 110 km in altitude from the strength of the upward component of the extraordinary wave. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 83(2): 20–28, 2000

Optical mode structure of the plasma waveguide

The quasibound modes of an evolving plasma waveguide were investigated by using variably delayed end-injected and side-injected probe pulses. The use of these different coupling geometries allowed the probing of the waveguide's optical modes during two temporal regimes: early-time plasma channel development, characterized by leaky optical confinement, and later channel hydrodynamic expansion characterized by stronger confinement. The wave equation was solved to determine the available quasiguided optical modes and their confinement for experimentally measured electron density profiles. The guided intensity patterns and spectra measured at the waveguide exit were successfully explained in terms of these mode solutions. The spectrum of broadband end-coupled probe pulses was found to be unaffected by the guiding process, mainly because those modes which survived to the waveguide exit were well-bound, and for strongly bound fields, the transverse mode profiles are wavelength independent. By contrast, side coupling to the quasibound modes of the plasma waveguide was seen to be highly mode and frequency selective.

Installation of the advanced heavy ion beam probing diagnostic on the TJ-II stellarator

An advanced heavy ion beam diagnostic has been developed for the TJ-II stellarator based on the simultaneous utilisation of two different detection systems for the secondary ions: a multiple cell array detector and a 30° Proca-Green electrostatic energy analyser. This innovative design aims at enlarging the HIBD capabilities to allow the instanteneous measurements of electron density and plasma potential profiles together with their respective fluctuations.

Preliminary electron density profile and fluctuation measurements on GAMMA 10 using ultrashort-pulse reflectometry

The use of ultrashort pulses (full width at half-maximum=1–100 ps) as a source of broadband, time-resolved radiation has high potential for application in microwave diagnostics for fusion plasmas. Here we report on the ultrashort-pulse reflectometry system developed for electron density profile and fluctuation measurements on the GAMMA 10 tandem mirror. The four-channel system (selectable frequencies of 8–17 GHz at 1 GHz intervals) measures the double-pass time-of-flight from the vacuum window to the cutoff layer. The hardware has an uncertainty of ±40 ps and can be operated at a repetition rate of 400 kHz. The system is operational, however, data from plasma experiments is unavailable at the moment. Results from a single-channel O-mode system show time-of-flight measurements consistent with values calculated from profiles obtained by a scannable single-channel interferometer. Fluctuation measurements are also consistent with results from a Fraunhofer diffraction diagnostic. These results suggest that with further refinements, ultrashort- pulse reflectometry will yield a simple yet reliable diagnostic for fusion devices.

Optimized instrumentation for edge Te and ne measurements on COMPASS-D tokamak from He I line intensity ratios

Instrumentation has been developed for the measurement of electron temperature and density profiles at the boundary of COMPASS-D tokamak plasmas from He I line intensities. The technique is applicable to measurements of ne and Te over the relevant ranges of 1017–1020 m−3 and 10–500 eV. Spectra from eight viewing chords viewing a thermal helium jet are recorded on a CCD sensor with ≈1 ms time resolution. A 0.6 m Czerny-Turner spectrometer, corrected for astigmatism using an internal, cylindrical lens optimizes throughput by availing of a 110×110 mm2, 300 l/mm grating. ne and Te profile measurements from L-mode (low) and quiescent H-mode (high) confinement plasmas are presented.

The Wigner distribution as a tool for time-frequency analysis of fusion plasma signals: application to broadband reflectometry data

The Wigner distribution is introduced as an appropriate tool for processing data from fusion plasma diagnostics whose signals have a time varying frequency spectrum, and is thus presented as a particularly suited form for the time-frequency analysis of transients and other non-stationary phenomena. Its effectiveness is illustrated byapplying it to the problem of electron density profile measurement through broadband microwave reflectometry in fusiondevices, its advantages being demonstrated via a detailed comparison with the well known short time Fourier transformspectrogram. In particular, the Wigner distribution is used in a novel application as a means not only to retrievefrom reflectometry data the instantaneous frequency needed for profile inversion, but also to provide an accuraterepresentation of reflectometry signals in the time-frequency plane. Further, in a careful discussion stressing itsbenefits relative to more standard approaches based on the quadrature signal, the analyticsignal is proposed as being appropriate to routinely extract from reflectometry data anunambiguously defined set of instantaneous amplitude, phase and frequency. The problemsassociated with the fact that digitized reflectometry data give rise to discrete time signals areproperly handled, and the different estimates for the instantaneous frequency obtained from the Wignerdistribution and the spectrogram by calculating their first frequency moments and mean frequencies,as well as by locating their peaks, are also compared.

Fast pulsed radar reflectometry for the Textor Tokamak

Progress in fusion research shows an increasing demand for diagnostics with high temporal and spatial resolution in order to study small and fast phenomena in fusion plasmas. Therefore, a new ten-channel pulsed radar reflectometer is under development for the TEXTOR Tokamak, to measure electron density profiles in the range of 0.4–4×1019 m−3 at a high repetition rate of 2 MHz. An additional feature of this diagnostic is the possibility to perform correlation measurements at 10 MHz repetition rate to investigate density fluctuations. The reflectometer will scan the plasma with 1-ns-long radar pulses in the 18–57 GHz range. In spite of the long flight time of the pulses of about 100 ns, the above mentioned high repetition rates could be achieved by time multiplexing the generation and reception of the radar pulses. Temporal selection of the received pulses is performed by fast switching of the local oscillators inputs of the heterodyne receiver, to generate time windows in which the reception of the reflected pulses is expected. An embedded VME controller will manage the system and store the data with a speed of 20 Msamples/s, up to a maximum of 64 Mbyte data per plasma shot. In order to facilitate the handling of such a huge amount of data, an advanced data reduction scheme is being developed. Remote operation with a fast data link from FZ Jülich (Germany) to FOM Nieuwegein (The Netherlands) will be possible.

Integrated reflectometer-interferometer system for plasma density profile measurement

Experimental results are described from an integrated interferometer-reflectometer system implemented on TEXT-Upgrade. In plasma regions where the systems have spatial overlap, excellent agreement is observed between the independently measured electron density profiles. Combined interferometer-reflectometer data are used to obtain the optimal total density profile. By removing chords from the interferometer, one can examine how resolution is compromised as the channel number decreases and spacing varies. Depending on the expected profile shapes (i.e. flat or peaked), one can design an integrated interferometer-reflectometer system employing a minimum number of chords while still providing crucial profile information.

Frequency Selective Tunnel Coupling to the Plasma Fiber

The optical resonances of plasma fibers are observed by monitoring the spectrum and spatial profile of their quasibound modes, which are excited through side or tunnel coupling. The features of the coupling compare favorably to calculations based on the measured electron-density profiles. This is the first direct observation of optical electromagnetic resonances of a plasma structure.

Observations of virtual cathode formation and electron density profile measurements in an intense ion beam diode

A recently developed high pulse rate intense ion beam diode has been exploited to measure the relative density profile of electrons in the ion diode’s acceleration gap as a function of time. This diode was magnetically insulated and it produced 100 keV Ar+ beams at up to 4 A/cm2 using an active anode plasma ion source. The diode also had a backfill of 10−2 Torr of He that was collisionally excited by energetic electrons in the acceleration gap. When the excited He atoms radiatively decayed, the emitted light was measured and used to deduce the relative electron density profile with space and time resolution. The profile was peaked toward the center of the gap and dropped off significantly toward both the anode and the cathode. The absence of electron density near the physical cathode is evidence for magnetic field profile modification from the diamagnetic drift of the electrons in the diode. With a macroscopic electric field of order 100 kV/cm, the electron current takes many tens of nanoseconds, from the moment the cathode begins to emit, to reach the Child–Langmuir current. The extent to which this limit is exceeded depends strongly on the profile of the insulating magnetic field and appears to be dominated by the dynamics of the electron flow in the diode gap. In contrast, the delay between when electrons are first observed in the gap spectroscopically and when the ion beam is first formed is independent of the magnetic field profile. This delay is about the time that it takes an argon ion to transit the acceleration gap, suggesting that ion inertia is the rate limiting factor.

Finding better B 0 and B 1 parameters for the IRI F2-profile function

Deficiencies in the IRI profile specifications for the bottomside F2 layer required a review of the IRI profile parameters B 0 and B 1. Using the available database of measured electron density profiles from several Digisonde stations, the IRI profile model is assessed. The paper describes a new technique to calculate the B 0 and B 1 parameters that “best” reproduce the measured profiles. The use of the Digisonde's Chebyshev polynomial presentation of the profiles offers a unique advantage for calculating the best B 0 and B 1.

Gravity waves propagation in the thermosphere observed from electron density profile and total electron content measurements

Ionospheric observations with five minute intervals between ionograms were made during a campaign from 19th to 23rd June 1996 at the Rome station (41.8N, 12.5E). The data obtained from ionospheric vertical sounding have been analysed together with the Total Electron Content (TEC) data obtained by the GPS receiver measurements. Both the apparatus were installed in the same station. Short periodicity phenomena occurring in the considered period were observed and interpreted as resulting from the propagation of AGWs in the thermosphere. TEC and electron density were then analysed during AGWs activity.

Imaging of the outer valence orbitals of CO by electron momentum spectroscopy — Comparison with high level MRSD-CI and DFT calculations

A newly constructed energy dispersive multichannel electron momentum spectrometer has been used to image the electron density of the outer valence orbitals of CO with high precision. Binding energy spectra are obtained at a coincidence energy resolution of 1.2 eV fwhm. The measured electron density profiles in momentum space for the outer valence orbitals of CO are compared with cross sections calculated using SCF wavefunctions with basis sets of varying complexity up to near-Hartree-Fock limit in quality. The effects of correlation and electronic relaxation on the calculated momentum profiles are investigated using large MRSD-CI calculations of the full ion-neutral overlap distributions, as well as large basis set DFT calculations with local and non-local (gradient corrected) functionals.

Density profile consistency, particle pinch, and cold pulse propagation in DIII-D

It has been recently proposed that, for highly turbulent discharges, there exists a consistent density profile for the trapped electrons in a high aspect ratio circular cross-section tokamak, which has a radial variation proportional to 1/q(r), where q is the usual safety factor. It is shown here that this result can be extended to include passing electrons and noncircular cross-section moderate aspect ratio tokamaks. This new prediction for the density profile is compared to the time evolution of the measured electron density profile in low confinement mode (L-mode) shots in the tokamak known as DIII-D [L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I. p. 159], where the q profile is changed in time during the discharge. Once an expression for the consistent density profile is known, it is trivial to obtain an expression for Vp/D, where Vp is the particle pinch velocity and D is the particle diffusion coefficient. This expression is compared with the value of Vp/D, which is obtained from an analysis, utilizing the ONETWO transport code [R. Dominguez and R. Waltz, Nucl. Fusion 27, 65 (1987)], of certain high confinement mode (H-mode) DIII-D discharges, which are free from edge localized modes. The dependence of density on q can be extended to a dependence of temperature on q through the adiabatic relation. The dependence of temperature on q can then predict one type of cold pulse propagation phenomena. By way of introduction, a simple analogy with a dynamic incompressible fluid system is made.