perature distribution on the divertor target
plates
4.2.1
Introduction into thermography
By thermography the surface temperature is measured. The measurement of the surface temperature of the DED divertor tiles is performed for two reasons:
a) For protecting critical components in the tokamak it is required to control their temperature and to protect them against too high temperature excursions.
b) The incoming heat flux density can be derived from the surface temperature development by solving the heat transport equation inside the divertor tiles.
In order to obtain a 2D picture of a DED-divertor target segment, an infrared camera has been acquired. Because the thermal emission of a body of T 6 600 K is predominantly in the infrared spectral region, a focal plane camera SBF-125 [54] has been used. The set up of the camera, the required calibration and results of measurements are described below.
4.2. THERMOGRAPHIC SYSTEM 81
4.2.2
The thermographic setup
The light emitted by the divertor tiles in the range from 3 to 5 micrometers was recorded. The error in the measured temperature appearing due to bremsstrahlung and photon emission by recombination or molecules in the detection wavelength range appears as an increase of the temperature below 10 K and it is negligible comparing to the typical temperatures of the graphite tiles during the discharges [43]. The measured temperatures are in the range of 300 K to 500 K.
The main component of the setup is the infrared 14-bit camera SBF-125 [54] (see figure 4.5). To image the tiles onto the infrared camera array a special optical setup was designed. The optical elements are made of CaF2 and CleartranTM ZnS.
These materials assure good spatial resolution of recorded images.
a)
b)
Figure 4.3: The transmission characteristic of the materials used to construct the optical setup: a) CaF2, b) CleartranTM ZnS; after [58]
All the materials have very good transmission in the desired wavelength range (see figure 4.3). Moreover the dependence of the transmission on the wavelength is very weak. In figure4.3the transmission characteristics of the materials is presented. The relatively high reflection coefficient of ZnS due to the high index of refraction
82 CHAPTER 4. THERMOGRAPHIC MEASUREMENTS
(n= 2.224 forλ = 4.2 µm) is compensated by an anti-reflection coating. As shown in figure 4.4 the optical setup consists of two lenses and a CaF2 vacuum window
mounted on a vessel flange. The lens parameters are shown in table 4.2
CaF window2
ZnS lens CaF field lens2
14-bit IR camera
1805 mm 893 mm
301 mm 635 mm
Figure 4.4: Optical setup of the thermographic system shown in the poloidal cross- section. The line of sight lies almost in the equatorial plane of the tokamak.
lens material radius [cm] focal length [cm] diameter [cm]
objective CleartranTM ZnS 308 25.4 7.62
field lens CaF2 140.15 35 42.5
4.2. THERMOGRAPHIC SYSTEM 83 The size of the area seen by the camera array is about 890 mm, which covers roughly five and a half rows of the divertor tiles. The size of the observed area is limited by the size of the detector (≈14 mm). The setup was optimized to reduce the errors due to aberration by the optics to a size smaller then one pixel of the camera (<30µm) [53]. The maximum spatial resolution is about 2 mm (for surfaces normal to the line of sight).
Figure 4.5: The scheme of the camera SBF-125 used in the setup.
The light goes through the optics of the camera to the focal plain array as follows: Radiation coming from the heated DED tiles first passes the antireflection- coated lens, then an antireflection-coated silicon window and into the vacuum dewar
84 CHAPTER 4. THERMOGRAPHIC MEASUREMENTS
through a chilled antireflection-coated cold filter. The light passes through the cold aperture, which prevents entry of off-axis light and subsequently into a blackened, light-tight cold shield (cooled below 100 K to eliminate infrared radiation emitted by the camera itself) where it is absorbed by the focal plane array mounted on a cold pedestal. The combination of the blackened dewar, the cold filter, the cold aperture, and the blackened, light-tight cold shield virtually eliminates spurious in- frared radiation while retaining the radiation emitted by the viewed area[54].
The detectors on the ImagIR array focal plane are photon detectors made of photovoltaic indium antimonide, InSb. The photo diode undergoes a voltage change when a photon is absorbed. Including the applied filters, the InSb array receives and detects only 3–5µm infrared light. The array has a very high photon efficiency. Typically more than 85% of the 3-5µm incident photons are absorbed and counted. Each detector registers its photon count via its voltage change. The array consists of 320×256 pixels, each of size of 30µm×30µm.
The IR-induced voltage changes on the detectors are converted via a 14-bit analog-to-digital (A/D) converter and transferred to the frame-grabber of the PC via optical fibers. The 14-bit dynamical range (from 0 to 16,383 counts) gives a possibility to measure a wide range of temperatures with high accuracy. As the temperature distribution over the DED tiles consists of hot strike zone and areas, where there is almost no heat deposited a wide dynamic range is important. The system can record the infrared images with a frequency of up to 394 Hz (sampling time 2.5 ms). The integration time is much smaller than the sampling time. The typical values of integration time are 0.191 ms or 0.073 ms. With reduced size of the array to 8×128 pixels the frequency can be increased up to 13 kHz (sampling time 76 µs).