Page 3 - Development of a Zeff diagnostic using visible and near-infrared bremsstrahlung light for the C-2W field-reversed configuration plasma

10D130-3
Nations et al.
Rev. Sci. Instrum. 89, 10D130 (2018)
FIG.2. Multi-purposeopticalmountforanintegrateddiagnosticsystem.
easy access to the center of the assembly for mounting fibers. Lastly, the optical mount utilizes laser-cut alignment keys to enable rapid and reliable in situ reconfiguration of the viewing geometry.
B. Bandpass filters
Light exiting the long optical fibers is collimated, trans- mitted through bandpass filters, and refocused onto individual detectors. Here, for the purpose of measuring bremsstrahlung radiation, bandpass filters are selected where the transmis- sion region is matched with regions of the plasma spectra that are “practically” free of emission lines (see Fig. 3). In the present analysis, bandpass filters are centered at 523.5 nm (∼1.8 nm FWHM, Ø3/8′′, Omega Optics) and 1000 nm (∼10 nm FWHM, Ø1/2′′, Edmund Optics) for the VIS and NIR systems, respectively. The filters sit between a collimat- ing lens and a focusing lens to ensure an angle-of-incidence of ∼0◦ and complete collection of filtered light by the detectors.
C. Detectors
For the expected range of plasma conditions in C-2W (ne ∼ 1–3 × 1013 cm−3 and T e up to ∼1000 eV), bremsstrahlung radiation levels are quite small; the theoretical power of
FIG. 3. Normalized filter transmission curves and measured spectrum near (a) 523 nm and (b) 1000 nm.
filtered bremsstrahlung expected to reach each detector is of only a few nanowatts. To resolve these weak signals, photo- multiplier tubes (PMT) and silicon avalanche photodiodes (APD) are employed for the VIS and NIR diagnostic systems, respectively.
The VIS system employs 15 PMT detectors (Hamamatsu R928, 13% quantum efficiency and 55 mA/W cathode radiant sensitivity near 523 nm), powered by a high-voltage power supply (1100 V). Signals are amplified using transimpedance amplifiers developed in-house with variable gain and offset.
C-2W’s NIR system utilizes 15 Si APD modules (Laser Components A-CUBE-S1500-01) that have an intrin- sic responsivity of ∼45 A/W at 1000 nm (gain M = 100), a Ø1.5 mm detector chip, a system bandwidth of 1 MHz, and active temperature compensation electronics. Further, APD gain and temperature can actively be controlled by means of two potentiometers located in the backside of the module.
Arrays of visible and near-infrared light emitting diodes
(LED) are coupled with glass diffusers and used as pulsed
sources (5 ms pulse width) with near-Lambertian surfaces
for detector calibration. A detailed quantitative description
of the calibration process has been presented in previous
15,16
work.
IV. RESULTS AND DISCUSSION
A. Line-integrated bremsstrahlung intensity
The separatrix radius (rs) is approximately equal to the excluded flux radius (r ∆Φ )17,18 which is measured by mag-
19
Figure 4(a) shows the evolution of r∆Φ
netic flux probes.
for a relatively short lived (∼5 ms), well-centered plasma
FIG. 4. (a) Measured bremsstrahlung intensity for VIS (top) and NIR (bot- tom) systems as a function of time and chord impact parameter for plasma shot #104693. The dashed lines represent the time evolution of the separatrix radius. (b) Line-integrated electron temperature from far-infrared interferometry diagnostic.