Kurt Hirsch

The LIDAR Thomson scattering diagnostic on JET (invited)

By combining the time‐of‐flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3‐J pulse energy, 300‐ps pulse duration, 0.5‐Hz repetition rate) together with a 700‐MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial...

First results from the LIDAR Thomson Scattering System on JET

The LIDAR Thomson Scattering System on the JET tokamak is described. Backscattering geometry and the time of flight method are employed to measure profiles along the major radius in the equatorial plane. First electron temperature profiles with a spatial resolution of better than 15 cm obtained with this new diagnostic technique are presented.

High resolution emission and absorption spectroscopy for erosion product analysis in boundary plasmas

The highly resolved spectra of the silicon multiplet around 251 nm are studied for plasmas in front of a C/C-SiC target interacting with a nearly cylindrically symmetric low temperature plasma jet. The erosion rates from this target are deduced from the silicon density distribution, which is determined from the line intensity ratios and details of the line profiles. Under the conditions of the plasma parameters in this plasma-target interaction experiment these spectral line parameters depend on the optical depths of the emitting and absorbing silicon atoms along the line of sight. The spectral line central branching ratios of lines with quite different products of absorption oscillator strength and statistical weight are most sensitive to the optical depth. In the case of high plasma jet currents the density of the eroded silicon is found to reach values of up to 5×1018 m−3 in agreement with gravimetric measurements.

Spectroscopic investigations of pulsed microwave generated Ar, N2 and silane plasmas

Abstract The Plasmodul (Proc. ISPC 15, Orleans, V (2001) 1853), an array of four parallel coaxial microwave guides is used as source for (square wave) pulsed plasmas in argon, nitrogen and silane at pressures of 5–50 Pa. The plasma is homogeneously extended over an area of 12 by 12 cm 2 . The temporal and radial behaviour of the electron temperature T e (r, t), the electron density n e (r, t) and the densities of several molecules, atoms and ions are investigated with different diagnostic tools. The data are compared and discussed for the three different plasma gases.

A laser diode as a light source for calibrating the time base of a streak camera

The dc operation of some laser diodes produces high‐frequency amplitude modulation of the laser output in the GHz frequency range. This phenomenon provides a simple and inexpensive method for checking the sweep linearity of a fast‐gated streak camera.

A high‐resolution Lidar–Thomson scattering diagnostic for JET

A LIDAR–Thomson scattering system with better than 5 cm spatial resolution to resolve edge electron temperature and density gradients in H‐mode operation in JET is needed. A dynamic range in measurable electron temperature of 0.1–1.5 keV and a minimum detectable electron density of 1019/m3 are required. To achieve these levels of performance, it is planned to use a modified section of the existing JET LIDAR–Thomson scattering system. A new detection system, consisting of a streak camera and an intensifier unit, will be used. Spectral dispersion is to be performed by a three‐channel edge filter spectrometer. Recording and digitization of the scattered signal will be performed by a CCD camera read out by a personal computer. The system is described and results of experiments to determine the minimum visible light levels using a commercial streak camera, intensifier unit, and CCD camera and a pulsed laser diode light source are briefly presented. In addition, simulations of expected signal‐to‐noise ratio per...

A new method for estimating and identifying systematic error in Thomson scattering diagnostics

A method has been developed to identify and estimate systematic error in the calibration of Thomson scattering systems. The basis of the technique is an iterative recalculation of electron temperature using successively more self‐consistent calibrations. A computational method was developed to calculate from a large dataset a calibration correction factor for each channel. The correction factor was then applied and the fitting routine rerun on the whole dataset with the new calibration factors. The resulting frequency distribution of chi‐squared values was close to the theoretical distribution while the calculated electron temperatures were changed by only 1 to 2%. The technique succeeded in identifying and quantifying errors in a dataset known to be of poor quality.

High-power ruby and alexandrite lasers for LIDAR-Thomson scattering diagnostics

By combining the time-of-flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density can be measured with a single set of detectors for all spatial points. The technique was demonstrated for the first time on the JET tokamak and has been in routine operation since July 1987. Originally a ruby laser (3 J pulse energy, 300 Ps pulse duration, 0.5 Hz repetition rate) was used together with a 700 MHz bandwidth detection and registration system which yielded a spatial resolution of about 12 cm. A large filter spectrometer with 6 spectral channels covering the wavelength range 400 - 800 nm gives a dynamic range for the temperature measurements of 0.2-20 keV. The original system is described, examples of measurements are given and compared with the results of other diagnostics. The system is being upgraded to make measurements at 10 Hz and a major component of the new system is an Alexandrite laser (1-2 J pulse energy, 350+1-50 ps pulse duration, 10 Hz repetition rate) which is currently being constructed. The new laser and other technological improvements being incorporated into the upgraded diagnostic will also be described.