A tangential hard x-ray (HXR) diagnostic on the newly constructed ENN XuanLong-50 (EXL-50) spherical tokamak for fast electron emission studies is presented. The HXR detection system consists of a symmetrical CdZnTe semiconductor detector array with a spectral sensitivity range of 20–300 keV. 25 channels have been designed on the 270° horizontal vacuum port with 12 sight lines to observe the forward emission, 12 sight lines to observe the backward emission of fast electrons, and 1 for viewing the central. Currently, ten channels have been in operation in the EXL-50 experiments. The systems are designed to measure the x-ray spectra for the estimation of fast electron temperature and electron velocity distribution in the EXL-50 experiment, which will be useful for understanding the dynamics of fast electrons generated by electron cyclotron resonance heating, for plasma instability and transport studies and for the analysis of plasma heating efficiency.
Diamond based quantum sensing is a fast-emerging field with both scientific and technological importance. The nitrogen-vacancy (NV) center, a crystal defect in diamond, is a unique model system for microwave sensing application due to its excellent photo-stability, long spin coherence time in ambient conditions. In this work, we systematically optimized the measurement parameters for microwave sensing. The system noise is analyzed, and 1/f noise is suppressed by introducing a differential algorithm. The gain of avalanche photodiode and the gating window of the pulsed fluorescence is optimized to further suppress the noise floor. The decoherence of spin is characterized by varying the duration of the laser and microwave. The minimal detectable power on a standard microstrip is characterized with sampling time down to 1 ms, showing flat frequency dependence. The results have important implications toward fast measurement of broadband microwave power, especially in the field of IC testing and radar signal processing under intense electromagnetic interference.
A quasi-optical Mach-Zehnder microwave interferometer operating at 140 GHz has been developed for the ENN's spherical tokamak XuanLong-50 (EXL-50), for the purposes of line-integral electron density measurement and plasma density real-time feedback control input. The EXL-50 is designed for long pulse operation (over 5 s) and the electron density of phase I is estimated below 1019 m-3. Thus, the well-known microwave interferometer is suitable for the advantage of cost effectiveness and good stability. One of the major errors of the interferometer is vibration. To reduce it, the entire interferometer is supported by sand-filled stainless-steel columns of 0.3 m inner diameter and the vibration modes are calculated by finite elements analysis. Other sources of error, such as noise and thermal drift, are carefully handled. To reduce noise, the interferometer including cables and digitizers are carefully shielded and grounded. The phase error due to source frequency thermal drift, manifested due to uneven probe beam and reference beam path lengths, is observed in long term operation and explained by model calculation. A continuous 100 s test shows that it is reduced to about 0.04 °/s when the Gunn oscillators are temperature controlled by Peltier coolers with the industrial Proportional-Integral-Derivative control method to maintain the frequency stability. The system has been in routine operation since August 2019, with 1016 m-2 line-integral density resolution. The technical details of the interferometer and experimental results are presented.
A 15-point Thomson scattering diagnostic system is developed for ENN’s spherical torus experiment XuanLong-50 (EXL-50). A BeamTech laser with 3 J/pulse (1064 nm wavelength) at 50 Hz repetition rate is chosen for measurements during EXL-50 plasma operations. To enable measurements at low density (∼0.5 × 1018 m−3) plasma operations, the opto-mechanical subsystems are carefully designed to maximize the collection and transmission of the scattered light and to minimize the stray light level. In addition, the high bandwidth trans-impedance amplifiers and segmented high speed waveform digitizers allow for the application of muti-pulse averaging to further improve the signal-to-noise ratio. Details of the diagnostic system are described and initial experimental results are presented.
Diamond based quantum sensing is a fast-emerging field with both scientific and technological significance. The nitrogen-vacancy (NV) center, a crystal defect in diamond, has become a unique object for microwave sensing applications due to its excellent stability, long spin coherence time and optical properties at ambient condition. In this work, we use diamond NV center as atomic receiver to demodulate On-Off Keying (OOK) signal transmitted in broad frequency range (2-14GHz in a portable benchtop setup). We proposed a unique algorithm of voltage discrimination and demonstrated audio signal transceiving with fidelity above 99%. This diamond receiver is attached to the end of a tapered fiber, having all optic nature, which will find important applications in data transmission tasks under extreme conditions such as strong electromagnetic interference, high temperatures and high corrosion.
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