The Academy of Sciences of the Czech rebublic - ASCR

Multichannel Optical System for Visible Plasma Radiation Measurement


Measured quantities:
  • hydrogen and impurity emission and its evolution during discharges
  • integral plasma radiation in the visible range both from the plasma core and from the plasma edge
  • specific spectral lines, like hydrogen or the most intensive impurity lines
Spatial resolution: ~ 1 cm
Temporal resolution: ~ 1 MHz (optical system)
Responsible person: D. Naydenkova, V. Weinzettl
Collaboration: Department of Optical Diagnostics of IPP, Turnov, Czech Republic


Diagnostic description:

The tomographic system for visible plasma radiation measurement consists of two parts (see Fig. 1), which will be installed at the same poloidal cross-section. Both parts have the same design, however, they are viewing the plasma from two different poloidal angles.



Fig. 1: Principle schema of the multichannel optical system for visible plasma radiation measurements.



The “sandwich” structure port plug, with which the optical system begins, „vacuum“ part of objective for visible plasma radiation measurements followed by the NW35 vacuum window made from Spectrosil 2000 and also contains a shutter and a cooling channel. The window is used as a part of optical system for visible light transfer to separate vacuum and atmospheric pressure parts of the system.

The objective consists of three parts. The first part, an ultra wide-angle objective achieving 110° field of view, transmits an image of the half of the plasma column into infinity. The second part is an infinite-finite distance objective that creates a real image. The third part is a system of relay lens, which re-images the light onto a linear set of optical fibres directly or through an interference filter.




 Fig. 2: The “sandwich” structure port plug (left) andtwo stage amplifier with a detector and endpiece of optical fibres (right).


Visible light detectors with spectral range of 400-1000 nm are located outside of tokamak hall due to sensitivity to X-ray radiation from the plasma. Twenty meter long optical cable are led - depending on purpose of the measurement – either to a 35-channel detector S4114-35Q by Hamamatsu, to minispectrometers HR2000+ by Ocean Optics, or to photomultipliers. An interference filter can be set as a part of optical system depending on an aim of experiments. Two stage 70-channels amplifier with amplification ratio ~ 5 x 105 was designed, manufactured and tested in IPP AS CR for output signal measurement in the range of 0-5 V (Fig. 2b). Level of noise after amplification was registered as 2 mV. The connectors between the optical cables and the detectors were designed as a part of the amplifier's box with a possibility to optimize the fibres endpiece position relatively to detectors in the arrays.

Presently, the multichannels optical system is being commissioned. In future, the parameters of the optical system will be slightly modified correspondingly to emergent physical needs.




Fig. 3: Temporally integrated spectrum for typical discharge at present stage


The existing optical system for visible plasma radiation measurements at COMPASS tokamak includes two spectrometers HR2000+ measuring in a spectral range of 247-472 nm and 457-653 nm with optical resolution 0.17 nm and 0.15 nm. It is sufficient for registration of plasma composition in different discharge regimes. Typical plasma spectrum is shown at Fig. 3. The triggering of the spectrometers allows measuring spectra with a temporal resolution limited mainly by photosensitivity of the spectrometer detector. It is ~ 7-10 ms in a range of interest. In future such measurements it will allow making rough estimation of impurities influx and hydrogen atomic recycling using electron density and electron temperature (Thomson scattering, interferometry, reflectometry) measurements.