Far-infrared Interferometer Research Articles

Ludwig Genzel

Ludwig Genzel

Real-time fringe correction algorithm for the JET interferometer

On the JET facility, the line-integrated electron density is measured by a multichannel far-infrared (FIR) interferometer. The basic source of radiation is a deuterium cyanide laser (λ=195 μm) measuring the density along four vertical and four nearly horizontal (lateral) channels. During high-density discharges the system suffers from fringe jumps that are manually partially corrected by an off-line program. This intermittent phenomenon prevents reliable usage of the measured line-integrated density in a real-time feedback controller. To solve this problem we have developed a method to correct the fringe jumps on the lateral channels in real time. The method uses the additional methyl alcohol laser (λ=118.8 μm) interferometer present on the lateral channels to compensate the vibrations. The method has been successfully tested with off-line test data, before implementation on a VERSA Module Eurocard system for the real-time control of the JET discharges.

Studies of resonantly produced plasmas in the H-1NF heliac using a far-infrared scanning interferometer

The H-1NF heliac regularly operates using the ion cyclotron range of frequencies at 0.5 T to produce plasmas with a mixture of hydrogen and helium gases. Due to the complex three-dimensional structure of the magnetic flux surfaces, these plasmas require sophisticated diagnostic systems, with good spatial coverage, to extract meaningful physical information. This article presents a study of the dependence of the plasma density profile on resonant heating conditions and magnetic configuration, using a far-infrared scanning interferometer. Recent modifications to the system and data that illustrate the performance of the interferometer will be discussed.

Fast ELM dynamics in JT-60U

Simultaneous fast measurements of electron density and temperature, Dα emission and Mirnov probe fluctuations in JT-60U during type I ELMs have been performed. Density fluctuations, measured with two vertical chords of the far infrared interferometer (FIR) at low and high field sides, have been found instrumental in determining spatial structure of type I ELMs (fluctuations localized mainly at the low field side) and duration of its MHD phase. Two different mechanisms for the initial phase of the interaction between the ELM-induced power and particle flows into the scrape-off layer (SOL) and the divertor target have been considered. They may explain the observed difference in the behaviour of Dα emission spikes at the inner and outer targets, which is related to the value of electron collisionality at the pedestal before the ELM. In ELMs with low pedestal collisionality, sharp fronts of Dα emission spikes at the inner and outer strike points and their simultaneous occurrence, coincident with the start of large-scale oscillations observed on the FIR channels, is observed. It is concluded that in such ELMs, plasma-wall interaction is initiated by a power flow conducted to the target through electron channel. In ELMs with high pedestal collisionality (but still of type I), slower evolution of fast signals was observed, with more gradual Dα spikes and clear time delays between outer and inner Dα channels, consistent with the mechanism of convective parallel energy fluxes with the ion sound speed proposed at JET.

Prospects for Future Far-Infrared/Submillimeter Studies of the High-Redshift Universe

AbstractObservations made using COBE, SCUBA, ISO and MAMBO have provided a reasonable working knowledge of both the intensity of the submm and far-infrared background radiation and the source counts of luminous high-redshift dusty galaxies. However, because there are uncertainties in the background intensity determinations, the samples of detected galaxies are small, and most importantly, their redshift distributions are very incomplete, details of the evolution of dusty galaxies remain unresolved. The next steps forward in the field will be the launches of SIRTF and ASTRO-F, the commissioning of SOFIA and new, more capable ground-based mm/submm-wave cameras - BOLOCAM, SHARC-II and SCUBA-II - the use of ultra-long duration balloon experiments, such as BLAST, the construction of ALMA and the arrival of FIRST, and ultimately the advent of space-borne far-infrared interferometers, such as SPECS. There are also exciting prospects for direct mm/submm-wave CO-line redshift surveys using wide-band spectrographs. Using these new facilities, the number of high-redshift dusty galaxies known will be increased dramatically. Spectroscopy using SIRTF, SOFIA and FIRST will probe the astrophysical processes within these sources in detail, hopefully addressing the open question of the fraction of the counts and background radiation that is generated by the formation of high-mass stars and by active galactic nuclei (AGNs). The spatial and spectral structure of distant dusty galaxies will finally be resolved in detail using ALMA and SPECS.

Star Formation in M51 Triggered by Galaxy Interaction

We have mapped the inner 360'' regions of M51 in the 158micron [CII] line at 55'' spatial resolution using the Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is peaked at the nucleus, but is detectable over the entire region mapped, which covers much of the optical disk of the galaxy. There are also two strong secondary peaks at ~43% to 70% of the nuclear value located roughly 120'' to the north-east, and south-west of the nucleus. These secondary peaks are at the same distance from the nucleus as the corotation radius of the density wave pattern. The density wave also terminates at this location, and the outlying spiral structure is attributed to material clumping due to the interaction between M51 and NGC5195. This orbit crowding results in cloud-cloud collisions, stimulating star formation, that we see as enhanced [CII] line emission. The [CII] emission at the peaks originates mainly from photodissociation regions (PDRs) formed on the surfaces of molecular clouds that are exposed to OB starlight, so that these [CII] peaks trace star formation peaks in M51. The total mass of [CII] emitting photodissociated gas is ~2.6x10^{8} M_{sun}, or about 2% of the molecular gas as estimated from its CO(1-0) line emission. At the peak [CII] positions, the PDR gas mass to total gas mass fraction is somewhat higher, 3-17%, and at the secondary peaks the mass fraction of the [CII] emitting photodissociated gas can be as high as 72% of the molecular mass.... (continued)

Sensitivity of an imaging space infrared interferometer

We study the sensitivities of space infrared interferometers. We formulate the signal-to-noise ratios of infrared images obtained by aperture synthesis in the presence of source shot noise, background shot noise, and detector read noise. We consider the case in which n beams are combined pairwise at n(n-1)/2 detectors and the case in which all the n beams are combined at a single detector. We apply the results to future missions, Terrestrial Planet Finder and Darwin. We also discuss the potential of a far-infrared interferometer for a deep galaxy survey.

Measurement of electron transport in the Madison Symmetric Torus reversed-field pinch (invited)

A recent study investigating the role of electron density fluctuations in particle transport has been conducted on the Madison Symmetric Torus reversed-field pinch. Four diagnostics enabled this experiment: a high-speed multichord far-infrared laser interferometer, a multichord Hα array, a 64-position magnetic coil array, and a Doppler spectrometer that measured impurity ion flow fluctuations. Correlation analysis is used to elucidate the relationship among density, magnetic, and impurity ion flow fluctuations. We observe that the electron density fluctuations are highly coherent with the magnetic fluctuations resulting from core-resonant resistive tearing modes. Moreover, the fluctuation-induced particle transport, obtained from the correlation between electron density and flow fluctuations, indicates that the core-resonant tearing modes do not drive significant particle transport in the plasma edge. We will address these four primary diagnostics, details of the analysis techniques, and principal results from this study.

Laser interferometric experiments for the TJ-II stellarator electron-density measurements

Laser two-color heterodyne interferometry is a proven method to measure electron density in fusion plasmas. Though only used in tokamaks with high electron densities, the idea of also using two-color laser interferometers for stellarators and small machines to replace far-infrared laser interferometers is being proposed. This will lead to low-cost, reliable, and easy to operate diagnostics for electron-density measurements. In this article, we present the interferometric experiments we have used for calibration of a two-color laser heterodyne system for electron-density measurements in the TJ-II stellarator. These experiments have been based on the use of a novel interferometric scheme: the heterodyne/homodyne interferometer. Finally, we describe the interferometer we have installed in the TJ-II stellarator and present the first results on mechanical-vibration subtraction and electron-density measurements with a two-color laser interferometer in a stellarator (TJ-II, Madrid, Spain).

New type of digital phase linearizer for real-time interferometric measurement

A new type of digital phase linearizer has been developed for real-time electron density measurement using a multichannel far-infrared (FIR) interferometer. The phase linearizer can measure phase shifts up to 640 fringes. The size of internal memory is 16 bits-2 Mwords and the sampling frequency of the waveform data is up to 1 MHz. The internal clock for the counter is selectable to 100 or 200 MHz and the frequencies of the input signals, i.e., the reference signal and the probe one, are around 1 MHz. In the performance test the resolution of the phase components is achieved at 1/100 fringes, which corresponds to the line integrated density of 9.0×1016 m−2. For real-time measurements the phase linearizer has digital-to-analog converter (DAC) output. The resolution of DAC output is 12 bits and the amplitude is ± 5 V. The density feedback system on the large helical device has been utilized.

Properties of the Photodissociated Gas in NGC 6334

To investigate the properties of photodissociated gas in strong radiation fields, the massive star formation region NGC 6334 has been mapped with the Far-Infrared Imaging Fabry-Perot Interferometer (FIFI) instrument in the far-infrared fine-structure transitions [C II] 158 μm, [O I] 146 μm, and [O I] 63 μm. Bright [C II] 158 μm emission is found throughout the cloud, whereas the [O I] 146 μm emission is associated only with the star-forming ridge. Comparison of the relative intensities of the lines with single-component photodissociation region models suggests densities of nH ~ 104 cm-3. The models imply unphysically large radiation fields for three sources, particularly for NGC 6334A, which is probably caused by self-absorption in the [O I] 63 μm line. An alternative method for estimating physical conditions, based on the correlation between [C II] 158 μm and CO line intensities, is explored. This method implies hydrogen column densities of NH 1022-23 cm-2, which agree well with those from molecular excitation models. The relative distribution of the [C II] 158 μm and [O I] 146 μm emission supports clumpy photodissociation region models that suggest the [C II] 158 μm emission arises from interclump gas and thus should be more extended than the [O I] 146 μm emission that arises from the denser clumps. The spatial coincidence of [C II] 158 μm emission peaks with minima in the molecular gas emission indicates that at least some of the molecular holes contain photodissociated gas. The relative pressures of the ionized, photodissociated, and molecular gas are compared: the photodissociated and molecular gas are in approximate pressure equilibrium, but the ionized gas is overpressurized by at least an order of magnitude at the continuum sources.

Plasma density behavior in the Hefei tokamak-7

The density profiles were measured in the Hefei tokamak-7 (HT-7) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1997), p. 61] ohmic discharges by means of a new multichannel far-infrared (FIR) laser interferometer. The progress on the extension of the HT-7 ohmic discharge operation region was introduced. The experiment results at the density limit, the multifaceted asymmetric radiation from the edge (MARFE) phenomena, the rf (radio frequency) boronization experiments, and the fueling efficiency studies were reported. The plasma physics in the molecular beam injection (MBI), the pellet injection (PI), and the gas puffing (GP) fueling experiments was studied and discussed.

First results from the far-infrared polarimeter system on the Madison Symmetric Torus reversed field pinch

The far-infrared laser interferometer on the Madison Symmetric Torus has recently been upgraded to allow simultaneous interferometry and polarimetry measurements. Centered around a heterodyne laser system that operates at λ≃432 μm, the diagnostic has a frequency response of 1 kHz for the polarimeter and up to 1 MHz for the interferometer. The distribution of laser power through the 11 chord system is managed with wire mesh beamsplitters. Concurrent with the system upgrade to polarimetry, a detailed study of the effect of the wire mesh beamsplitters on beam polarization was conducted. The study shows that the mesh characteristics must be included to extract the polarimetry phase measurement from the raw data. The modifications to the data analysis as well as preliminary five chord polarimetry profile data are presented.

Interferometric measurement of high-frequency density fluctuations in Madison symmetric torus

As a consequence of recent improvements to the far-infrared (FIR) interferometer time response, from bandwidth of 10 kHz up to 1 MHz, high-frequency density fluctuations can now be resolved on the Madison symmetric torus (MST) reversed-field pinch. The phase measurement allows absolute calibration of the fluctuation amplitude while eleven chords generate information on the spatial distribution. Density fluctuations up to 200 kHz are observed. A unique feature of the MST interferometer is that six chords are toroidally displaced 5° from the remaining five. Hence, besides information on the poloidal structure, i.e., m=1, toroidal information is also available. By computing the cross power, coherence, and cross phase between two toroidally displaced chords, one can determine the toroidal rotation speed, dispersion, and correlation length.

Far infrared laser interferometer system on the Large Helical Device

A multichannel far-infrared laser interferometer system has been developed for the measurement of the spatial and temporal behaviors of the electron density on the Large Helical Device (LHD) at the National Institute for Fusion Science. Of several candidates for high power laser sources a 119 μm CH3OH laser was taken as a probing light. The optical configuration is of the Michelson interferometer type with 13 channels. The optical system of the interferometer is mounted on a massive stainless frame floated on three vibration isolating mounts. The resolution of the fringe counters is 1/100 fringes, and it corresponds to a line averaged density of 5.6×1016 m−3 at the central chord. Preliminary results have been obtained in the initial experiments on the LHD.

Cii] Emission from NGC 4038/9 (the “Antennae”)

We present observations of NGC 4038/9 in the [C II] 158 μm fine-structure line taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). A fully sampled map of the galaxy pair (without the tidal tails) at 55'' resolution has been obtained. The [C II] emission line is detected from the entire galaxy pair and peaks at the interaction zone. The total [C II] luminosity of the Antennae is L[C II] = 3.7 × 108 L☉, which is about 1% of the far-infrared luminosity observed with IRAS. The main part of the [C II] emission is probably produced by photodissociation regions (PDRs), and a minor fraction may be emitted from H II regions. A small part of the [C II] emission comes from a standard cold neutral medium (CNM); however, for high temperatures (T ~ 100 K) and high densities (nH ~ 200 cm-1) of the CNM, up to about one third of the observed [C II] emission may originate from CNM. From PDR models, we derive densities on the order of ~105 cm-3 and far-UV (FUV) intensities of 460χ0, 500χ0, and 240χ0 for the PDRs in the interaction zone, NGC 4038, and NGC 4039, respectively. However, PDRs with densities on the order of ~102 cm-3 and FUV intensities on the order of ~100χ0 could also explain the observed [C II] emission. The minimum masses in the [C II]-emitting regions in the interaction zone and the nuclei is a few ×107 M☉. A comparison with single-dish CO observations of the Antennae shows a [C II] to CO intensity ratio at the interaction zone that is a factor of 2.6 lower than usually observed in starburst galaxies, but still a factor of about 1.3 to 1.4 higher than at the nuclei of NGC 4038/9. Therefore, no global starburst is taking place in the Antennae. [C II] emission arising partly from confined starburst regions and partly from surrounding quiescent clouds could explain the observed [C II] radiation at the interaction zone and the nuclei. Accordingly, there are small confined regions with high star formation activity in the interaction zone and with a lower star formation activity in the nuclei. This supports the high density and high FUV intensity of the PDRs in the interaction zone and the nuclei.

Poloidal magnetic field measurements on TEXT-Upgrade

A polarimetry capability has been added to the UCLA multichord far-infrared heterodyne interferometer system on TEXT-Upgrade. The polarimeter utilizes a phase technique in order to measure the Faraday rotation angle of the incident laser beam upon transmission through the plasma. From these data one derives information on the poloidal magnetic field, current density and safety factor profiles. The response of these profiles to electron cyclotron resonance heating and fast current ramps is investigated.

First results from the three-view far-infrared interferometer for the H1 heliac

A three-view, 40 beam far-infrared (λ = 433 μm) scanning interferometer has recently been installed and operated on the H1 heliac. The optical system is described and first plasma results are presented.

Fast tokamak plasma flux and electron density reconstruction technique

Density profiles in TEXT-U are obtained using a vertical viewing far-infrared (FIR) interferometer. To obtain the local (inverted) density, we have developed a simple analytic model of the plasma equilibrium configuration which is faster than EFIT (a flux surface reconstruction program) and can be easily computed between discharges. This analytic solution of the Grad–Shafranov equation is valid as long as the pressure p is a function of poloidal flux ψ, i.e., p=p(ψ). The procedure incorporates both magnetic and FIR density data to solve the Grad–Shafranov equation, and provides a density profile which is self-consistent with the reconstructed equilibrium flux surfaces. Examples are presented.

Liquid-helium cooled scanning far-infrared Fabry-Perot interferometer for astronomical observations with a balloon-borne telescope

We have developed a new liquid-helium cooled far-infrared Fabry-Perot interferometer (SFP) with high spectral resolution which will be incorporated and flown with our balloon-borne far-infrared 60 cm φ telescope FIT-FIT. This telescope has been built and already flown with other radiometric equipment in collaboration with the Geneva Observatory. The new spectrometer is designed to measure astronomical emission lines between 140 and 220 μm wavelength. Of special interest are the [NII]-line at 205 μm as well as the [CII]-line at 158 μm. In order to achieve high sensitivity we had to minimize the thermal radiation from the interferometer and its optics. For this reason all optics, including the scanning Fabry-Perot interferometer SFP, are liquid-helium cooled. This paper describes special new features of the design, operation, and test of its performance at liquid-helium temperature with optically pumped far-infrared gas lasers.