Absolute Extreme Ultraviolet Photodiodes Research Articles

Preliminary design of an AXUV photodiode-based bolometer camera for MT-I spherical tokamak

The MT-I is a spherical tokamak at the Pakistan Tokamak Plasma Research Institute, and has a major radius of 15 cm and a minor radius of 9 cm. This paper presents the design and computational analysis of the Absolute eXtreme Ultra-Violet (AXUV) photodiode-based bolometer camera, which will be used for the first time in the MT-I tokamak. The bolometer camera contains a single array of 16 AXUV photodiodes placed behind an aperture to compute the line of sight (LOS) and field of view (FOV) of the system. This array is located at the midplane of the vessel, which provides a radial resolution of about 16 mm. The individual FOV of each photodiode and the collective FOV of the array are 4.1° and ±34.5°, respectively. The impurity densities and their cooling rates for elements like carbon, oxygen, and iron are used to compute the plasma emissivity distribution, the power incident on the photodiode array, and signal-to-noise ratio (SNR) analysis. The analysis reveals that the radiated power loss is about 10 % of the ohmic heating input power. The geometrical etendue falls within the range of 2.2 × 10−5 to 3.2 × 10−5 Sr cm2. The bolometer exhibits a frequency response bandwidth of 10 kHz, and its temporal resolution is around 0.1 milliseconds.

Time resolved and time integrated analysis of the AXUV photodiode diagnostics for EUV and soft X-ray emission of nanosecond-laser-produced plasma

This paper addresses the experimental results and analysis of the time-resolved and time-integrated of AXUV (absolute extreme ultraviolet) photodiode diagnostics. These results are obtained by the measurement of EUV and soft X-ray (SX) power radiation of plasma-produced under the interaction of laser pulse (intensity ∼5 × 1011 W/cm2, pulse durations in the range of 10−30 ns and wavelength 1064 nm) with one steel-316 target. The EUV and SX signals with the three specific peaks are detected by AXUV photodiode. From the analysis of time resolved of the AXUV signal, it is inferred that the first signal peak can dominantly be related to the free-free transition mechanism of radiation and also the latter related to the free-band and bond-bond transitions respectively. The energy of EUV and SX pulse emission is observed to be approximately linear, proportional to the laser pulse energy. The time integrated results show that, the average energy conversion efficiency is determined to be about 3.7 %.

Determination of the Major Impurity Radiators in the Reheat Mode Discharges in the Compact Helical System

Radiation brightness and impurity behaviors have been studied for reheat mode discharges in the Compact Helical System (CHS) by three different types of impurity diagnostics. Total radiation power measured by a pyroelectric detector significantly reduces after entering the reheat mode, whereas the line-averaged radiation brightness measured by an absolute extreme ultraviolet (AXUV) photodiode array increases especially for a center viewing chord due to the impurity accumulation in the plasma core. One possible reason for this opposite behavior between the two bolometric detectors is the reduced sensitivity of the AXUV photodiode for lower energy photons in vacuum ultraviolet (VUV) region. This speculation is supported by temporal evolutions of VUV spectra measured by a grazing incidence spectrometer. These results demonstrate that the comparison of three impurity diagnostics would be beneficial to the determination of the major impurity radiators and a comprehensive understanding of impurity behaviors in the reheat mode discharges.

Application of Tomographic Imaging to Multi-pixel Bolometric Measurements

An improved tomographic algorithm in the scheme of Tikhonov-Phillips regularisation method, has been employed for multi-pixel bolometric measurements in order to get as much information as possible while keeping the assumptions to a minimum. The effects of finite detector size have been taken into account with a full three dimensional treatment of the detector geometry. The application of tomographic imaging was implemented to a two-array AXUVD (Absolute eXtreme UltraViolet photodiode) camera on the Large Helical Device (LHD) and a two-dimensional infrared imaging bolometric (IRVB) pinhole camera on JT-60U. Pertinent examples of the results are presented both to illustrate the analysis techniques and to demonstrate the wealth of physics which can be studied.

Time resolved radiated power during tokamak disruptions and spectral averaging of AXUV photodiode response in DIII-D

Silicon absolute extreme ultraviolet (AXUV) photodiodes have been employed in a disruption radiometer diagnostic for measurement of radiant power in the DIII-D tokamak with a 170 kHz bandwidth. This is motivated by a need to improve the understanding of radiative processes in tokamak disruptions. The diagnostic described in this article has a single line of sight though the central plasma. Accounting for the photon energy dependence of the AXUV photodiode responsivity is made possible by optical filtering, with the aid of spectra from an extreme ultraviolet survey spectrometer. The appropriate effective responsivity for interpretation of the data is lower than the nominal value typically used for the detector. In the current quench phase of disruptions, it is less than half the nominal value. Comparisons with results from a foil bolometer find good agreement.

Bolometer diagnostics for one- and two-dimensional measurements of radiated power on the Large Helical Device

Bolometer diagnostics are installed on the Large Helical Device (LHD) to provide measurements of the total (broad spectrum) electromagnetic radiation emitted by the plasma. Three types of detectors are used: resistive metal film bolometers, absolute extreme ultraviolet photodiodes and infrared imaging video bolometers. Details of the installation in LHD are given for each type of detector. The detector calibration, data analysis and tomography techniques are described and compared and sample results are shown.