Terahertz Spectroscopy Detection Applied in Tokamak Device

A Terahertz or THz wave usually refers to an electromagnetic wave with a frequency between 0.1 THz and 10 THz, between the microwave and the infrared. The terahertz time-domain spectroscopy system is a coherent detection technology that can simultaneously obtain the amplitude information and phase information of the terahertz pulse. By performing Fourier transform on the time waveform, the optical parameters such as absorption coefficient and refractive index of the sample can be directly obtained.

In a nuclear fusion tokamak device, the interaction of the plasma with the wall material will produce dust. The cause of dust is very complex, involving a variety of plasma processes, while dust particles hitting or depositing on the first wall may change the thermal conductivity and other properties of the wall material. Terahertz time-domain spectroscopy has high detection sensitivity and has been widely used in the analysis of material properties.

The dust components in the tokamak device are mainly tungsten, tantalum, molybdenum, carbon, boron, oxygen, antimony, chromium, manganese, etc. Theoretical simulations show that the absorption spectrum of these components deposited on the first wall of tungsten in the terahertz band can be Feature recognition, the position of the terahertz frequency domain characteristic line of different components combined with the tungsten surface is different, and the same component changes with the thickness of the film deposited on the tungsten, although the terahertz wave is reflected back into the first wall to The characteristic line position of the Hertzian frequency domain spectrum is unchanged, but the line intensity will change accordingly. According to this feature, the analogy and molecular dynamics methods can be combined to determine the peak position of several characteristic lines deposited on the tungsten surface by different components. The experimental results are compared with theoretical calculations to determine the composition. Its work includes the following steps:

Step 1: When the Tokamak magnetic confinement fusion device is working, the terahertz wave is perpendicularly incident on the area to be inspected, that is, the first wall of the tokamak tungsten. The probe measures and records the reflection from the first wall of the tokamak tungsten. Terahertz time domain spectrum

Step 2: analyzing and processing the terahertz time domain spectrum, and performing a Fourier transform on the terahertz time domain spectrum in the effective frequency domain to obtain a terahertz frequency domain spectrum;

Step 3: Computer theory compares the terahertz frequency domain spectrum of different dust components deposited on the first wall of tungsten, and records the terahertz characteristic line position of different dust components; for the same dust component, simulate different thicknesses and calibration thickness (such as The relative intensity ratio of the terahertz characteristic line at 1 μm), the relationship d between the thickness d of the deposited layer of each dust component and the relative intensity ratio R=f(d) is obtained, and integrated into a computer database;

Step 4: Select the characteristic line of the terahertz frequency domain spectrum obtained in step 2, and compare the position of the terahertz characteristic line with the position of the terahertz characteristic line of different dust in the database to determine the dust component;

Step 5: After determining the dust component, compare the terahertz characteristic line of the dust component with the terahertz characteristic line of the calibration thickness of the database to obtain a relative intensity ratio, and then compare the thickness-to-strength ratio of the deposited layer in the database. The functional relationship is compared to determine the thickness of the deposited layer;

Step 6: Output the composition and thickness information of the tokamak tungsten first wall deposit layer and store it in a computer system to complete the detection of the dust composition and thickness of the first wall of the tokamak tungsten.

Using the reflection terahertz time-domain spectroscopy technology combined with the computer database, it provides the conditions for the rapid and synchronous detection of the composition and thickness of the dust deposit layer on the first wall of the magnetic confinement fusion tokamak device, and the first to the Tokamak in the window of the magnetic restraint device. The wall detection area emits a terahertz wave, and receives the reflected time domain spectrum, converts it into a frequency domain spectrum, and finally compares it with a computer database to determine the dust composition and the thickness of the deposited layer, thereby achieving synchronization line and no contact and no damage detection.

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