Particle and energy transport in tokamaks and other toroidal confinement devices is dominated by turbulence generated by flows and gradients. In order to understand and control of this transport, diagnostic instrumentation was developed to study the structure and magnitude of microturbulent processes and to identify the origins of plasma loss. This review will cover the primary instruments that have been developed to measure fluctuating quantities associated with transport: density, δn, temperature, δT, potential, δφ, and magnetic field, δB, and their correlations. The methods discussed are Langmuir probes, heavy ion beam probes, collective and phase scintillation scattering, beam emission and ordinary spectroscopy, reflectometry and enhanced scattering, electron cyclotron emission, and several magnetic methods. The emphasis here will be on techniques applicable to microturbulence whose scale length is greater than the ion cyclotron radius and much less than the minor radius. Limitations and strengths of each method will be described and compared. Techniques will be discussed for estimating fluctuation intensities and wave number spectra or, equivalently, multipoint correlations in radial, poloidal, and toroidal directions. Large scale or magnetohydrodynamic-like plasma oscillations are typically studied with tomographic techniques or external probes and are reviewed elsewhere.

Diagnostic instrumentation for microturbulence in tokamaks

A novel imaging diagnostic has recently become operational on the DIII‐D tokamak for the study of density fluctuations at the outer edge of the plasma The phase contrast imaging approach overcomes the limitations of conventional scattering techniques in the spectral range of interest for transport‐related phenomena, by allowing detection of long‐wavelength modes (up to 76 cm) with excellent spatial resolution (5 mm) in the radial direction Additional motivation for the diagnostic is provided by wave‐plasma interactions during heating and current drive experiments in the ion cyclotron range of frequencies Density perturbations of 4×107 cm−3 with a 1‐MHz bandwidth can be resolved The diagnostic employs a 76‐cm‐diam CO2 laser beam launched vertically across the plasma edge An image of the plasma is then created on a 16‐element detector array: the detector signals are directly proportional to the density fluctuations integrated along each chord Wavelengths and correlation lengths can be inferred from the spatial mapping The phase contrast method and its application to DIII‐D are described and tests and first plasma data are presented

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A phase contrast interferometer on DIII‐D

Imaging techniques offer attractive alternatives to small-angle Thomson scattering for scale lengths of density fluctuations causing diffraction in the Raman-Nath regime. This is the case for fluctuations with wavelengths above about 3 mm for Tokamak sized plasmas, when a CO2 laser probe beam is used. Four methods ('phase scintillation', phase contrast, wavefront shearing and interferometry) are compared, on the basis of their transfer properties. They offer new means of studying density fluctuations of magnitude up to the dimensions of the plasma, such as those associated with turbulence, magnetic islands, convective cells or driven waves. The long wavelength limitations are discussed in detail and are related to those encountered for far-field techniques. An experimental comparison of 'phase scintillation' and phase contrast is given, and the criteria relative to the depth of field of imaging instruments in this context are considered.

/pdf/imaging-methods-for-the-observation-of-plasma-density-2ahomvzf97.pdf

Imaging Methods for the Observation of Plasma-Density Fluctuations

Phase Contrast Imaging (PCI) is a new diagnostic that was built for the Alcator C-Mod tokamak. It measures line-integrated (along 12 vertical chords) plasma density perturbations with good temporal (2-500 kHz) and wavenumber (0.5-12 /cm) resolution. The Quasi-Coherent (QC) fluctuation mode was studied using the PCI and other diagnostics. The mode was found to cause fluctuation of density, electric and magnetic field in the plasma edge with typical frequency of 100 kHz and typical poloidal wavenumber of about 5/cm. The mode was found to be responsible for confinement properties of the “Enhanced D-alpha H-mode” (a particularly favorable regime of tokamak operation). Through numerical modeling, the physical origin of the fluctuations was tentatively identified as “resistive X-point” mode (a kind of resistive ballooning mode strongly affected by the X-point configuration of magnetic field lines). The PCI system has been upgraded to detect waves in the ion cyclotron range of frequencies (ICRF, 40-80 MHz) by means of optical heterodyning – a technique based on modulation of the diagnostic laser beam near the wave frequency. The upgraded system was then used to study propagation of the Fast Magnetosonic Waves. These waves, which have never been measured in detail in past experiments, are being used to heat the tokamak plasma at the megawatt power level. The measured results were compared to the simple cold-plasma dispersion relation and to predictions of the full-wave 3D numerical modeling. Thesis Supervisor: Miklos Porkolab Title: Professor of Physics

Phase Contrast Imaging on the Alcator C-Mod tokamak

The laser phase contrast method is suitable for measuring long wavelength fluctuations propagating perpendicular to the laser beam axis. The obtainable data, which have so far been integrated along line-of-sights, are improved to be spatially resolved along the beam axis by selecting a particular direction of the sheared magnetic field of a toroidal confinement system utilizing a slit attached to a phase plate. First, the spatial resolution was formulated, and its validity was examined using an ultrasonic wave which simulated density fluctuations in plasmas. Then, the method was applied to plasma discharges in the Heliotron E device, and wavenumber (K)-frequency (f=Ω/2π) spectra, S(K, f), shows that the spatial regions were surely resolved.

Improvement of the laser phase contrast method for measuring the spatial distribution of electron density fluctuations in heliotron E

The plasma electron density fluctuation spectrum can be obtained from the measurement of the phase fluctuation spectrum of a probing electromagnetic wave measured some distance from the plasma. If the mean square fluctuation of the probing beam is much less than unity and the observer-plasma distance is well within the Fresnel distance of the irregularities, a direct relationship exists between the density and phase spectra. As an example of this procedure, an experiment is described which uses a probing beam wavelength of 2 mm to measure the density irregularities. It is then proposed that a suitable wavelength for the present large toroidal plasma devices is in the 10 mu m region.

The measurement of large-scale density fluctuations in toroidal plasmas from the 'phase scintillation' of a probing electromagnetic wave

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The measurement of large-scale density fluctuations in toroidal plasmas from the 'phase scintillation' of a probing electromagnetic wave