A high etendue spectrometer suitable for core charge eXchange recombination spectroscopy on ITER.

A feasibility study for the use of core charge exchange recombination spectroscopy on ITER has shown that accurate measurements on the helium ash require a spectrometer with a high etendue of 1mm(2)sr to comply with the measurement requirements [S. Tugarinov et al., Rev. Sci. Instrum. 74, 2075 (2003)]. To this purpose such an instrument has been developed consisting of three separate wavelength channels (to measure simultaneously He/Be, C/Ne, and H/D/T together with the Doppler shifted direct emission of the diagnostic neutral beam, the beam emission (BES) signal), combining high dispersion (0.02 nm/pixel), sufficient resolution (0.2 nm), high efficiency (55%), and extended wavelength range (14 nm) at high etendue. The combined measurement of the BES along the same sightline within a third wavelength range provides the possibility for in situ calibration of the charge eXchange recombination spectroscopy signals. In addition, the option is included to use the same instrument for measurements of the fast fluctuations of the beam emission intensity up to 2 MHz, with the aim to study MHD activity.

Using Cramer-Rao theory as spectrometer design tool aimed at quantitative complex-spectrum analysis.

In this paper the use of the Cramer-Rao lower bound (CRLB) to aid in the design of diagnostic systems dealing with complex spectra is discussed. Specifically it is discussed how a priori knowledge, used to improve the estimation, can be included in the CRLB analysis. This provides improved predictions for the spectrometer characteristics that will lead to an optimal system.

Spot-size reduction in terahertz apertureless near-field imaging.

We show measurements and calculations of the terahertz (THz) near field of a metal tip with a specially formed, semicircular apex that allows us to identify the separate contributions of the tip apex and shaft to the measured signal. We find that when the tip-crystal distance is not modulated the measured near-field signal is overwhelmed by contributions from the tip shaft, resulting in a relatively large THz spot size. When the tip-crystal distance is modulated, with subsequent lock-in detection at the modulation frequency, only the near-field distribution of the semicircular apex is observed, resulting in a much smaller THz spot size and thus improved spatial resolution.

Towards terahertz near-field microscopy.

We have detected sub-wavelength spot sizes in the near-field of a metal tip, which is illuminated with terahertz (THz) pulses. The THz near-field is detected by using electro-optic detection in a (100) oriented GaP crystal. Contrary to conventional electro-optic detection, which uses (110) oriented detection crystals, (100) crystals only allow the detection of THz light, polarized perpendicular to our crystal surface. This component is strongly localized near the apex of the tip, which has sub-wavelength dimensions. The detection process is blind to the incident THz radiation, which is polarized parallel to the crystal surface. As a result, a sub-wavelength THz spot size with an intensity full-width half maximum (FWHM) diameter of lambda/200 is observed.

Measurement of THz Spot Sizes with a λ/200 Diameter in the Near-Field of a Metal Tip.

We report on a method to obtain asub-wavelength resolution in terahertz time-domain imaging. In our method,a sharp copper tip is used to locallydistort and concentrate the THz electricfield. The distorted electric field, presentmainly in the near field of the tip, iselectro-optically measured in an (100)oriented GaP crystal. By raster scanning the tipalong the surface of the crystal we find asmallest THz spot size of 10 µm forfrequencies from 0.1 to 2.5 THz. For ourpeak frequency of 0.15 THz this corresponds to aresolution of λ/200. Our setup has thepotential to reach a resolution down to afew µm, and is a promising candidate tostudy single, living cells in the THzfrequency range.