Recent progress in magnetically confined plasma research has brought the goal of controlled thermonuclear fusion within reach. Simultaneously, the increased size, temperature, and density of these devices has resulted in rapid changes in diagnostic techniques. In this article, recent developments in instrumentation are reviewed together with trends for the future. The topics discussed include far‐infrared laser interferometry and polarimetry, ruby laser television Thomson scattering, excimer and Nd‐glass laser scattering, ion temperature determination via large‐α scattering with FIR and CO2 lasers, collective scattering, Schottky diode mixer technology, synchrotron radiation diagnostics and imaging, ion beam probes, x‐ray diagnostics and imaging, neutron diagnostics, resonance fluorescence scattering, ultraviolet diagnostics, and internal magnetic field measurement.

Instrumentation for magnetically confined fusion plasma diagnostics

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

The characteristics of low frequency waves in the presence of × rotation of a tandem mirror plasma are investigated using the Fraunhofer diffraction method. The observed dispersion relations are in good agreement with those of drift waves including a Doppler shift due to the × rotation velocity. The effect of the radial electricfield on the drift waves is studied quantitatively by applying a bias voltage to the end plates of the tandem mirror. The fluctuation level is observed to depend on the radial electric field Er. The fluctuation has a maximum value when Er 0 and decreases with increasing Er, regardless of its sign. The radial confinement time estimated from the particle balance equation decreases as the fluctuation level increases. The dependence of the fluctuation level agrees with that evaluated from the quasi-linear theory of drift wave turbulence.

Control of the radial electric field and of turbulent fluctuations in a tandem mirror plasma

The {bold k}-{omega} spectra of low-frequency waves which exist in a tandem-mirror plasma are observed by using the Fraunhofer-diffraction method. The observed dispersion relations are in good agreement with those of drift waves including a Doppler shift due to {bold E}{times}{bold B} rotation velocity. The fluctuation level is observed to depend sensitively on the radial profile of a plasma potential. It has a maximum value when a slightly negative electric field is formed, and decreases with increase in an electric field regardless of its sign.

Ambipolar potential effect on a drift-wave mode in a tandem-mirror plasma

Oscillations induced on the profile of a Gaussian CW CO2 laser beam by being transmitted through the TOSCA Tokamak plasma are confined within envelopes whose shapes are correctly predicted by theory, permitting plasma fluctuation intensity and wavelength to be deduced as functions of frequency by a curve fitting procedure. The analysis shows spontaneous density fluctuations integrated across the TOSCA minor diameter to be mainly transverse to the magnetic field, with intensity maxima between 40 and 100 kHz and ne( nu ) varies as nu -2 towards higher frequencies. At 200 kHz n3( nu )/n3 is about 104 times thermal, and the total fluctuation level n3/ne is a few percent. Coherent oscillations near 100 kHz, associated with rotating MHD structure, are occasionally observed. Wavelengths are found to lie between 1 and 6 cm, and correspond to kappa perpendicular to rho i in the range 0.2-2.0, pi i being the ion gyroradius for the prevailing plasma conditions.

http://iopscience.iop.org/article/10.1088/0032-1028/25/6/003/pdf

Measurement of long wavelength turbulence in a Tokamak by extreme far forward scattering

The transmission of a gaussian beam of electromagnetic radiation through a refracting medium such as plasma containing within it a localized sinusoidal disturbance, is described entirely in terms of refraction and diffraction. The lowest order terms of an expansion for the intensity profile of a beam in the front focal plane of a lens situated a focal length beyond a beam waist in the medium are identified with well-known optical effects usually treated independently, including shadowgraph, scattering, and schlieren. The latter is shown to be indistinguishable from spontaneous heterodyning of the scattered radiation with the unperturbed beam, and is sensitive to frequency, wavelength, and the location of the disturbance in the medium. An experiment using a HeNe laser with monochromatic ultrasound in atmospheric air has verified the predictions of the model over a wide range of conditions. As a plasma diagnostic, the method may offer a unique means of measuring otherwise inaccessible very long wavelength instabilities such as trapped ion modes in large Tokamaks like JET.

https://iopscience.iop.org/article/10.1088/0032-1028/24/7/009/pdf

Fourier optics approach to far forward scattering and related refractive index phenomena in laboratory plasmas

The possibilities to use periodically pulsed lasers for plasma scattering diagnostics are discussed. An experiment with successful application of a periodically pulsed frequency-doubled Nd:YAG laser is described and results are given. Application of the method to monitor continuously, with millisecond time resolution, parameters of Tokamak type plasmas, is considered.

On scattering diagnostics with periodically pulsed lasers to follow the continuous evolution of time dependent plasma parameters

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Fourier optics approach to far forward scattering and related refractive index phenomena in laboratory plasmas

Measurement of long wavelength turbulence in a Tokamak by extreme far forward scattering

On scattering diagnostics with periodically pulsed lasers to follow the continuous evolution of time dependent plasma parameters