Development of a combined interferometer using millimeter wave solid state source and a far infrared laser on ENN's XuanLong-50 (EXL-50)

The sources used in MFCI are a HCN laser and two solid-state diode sources. HCN laser is a gas laser with a resonant cavity length of 3 m, the operating substance is a mixture of nitrogen, methane and hydrogen gas with a ratio of N₂:CH₄:H₂ = 1:1:5 [8]. HCN laser is electrically excited continuous wave laser, the output power is more than 10 mW at a wavelength of 337 μm for the EH₁₁ mode [9]. The solid-state diode source output power is above 10 mW in 340 GHz band, original input frequency is in Lo₁ = 10.6 GHz band, 32 times frequency doubling, oscillator frequency within 12 MHz is continuously adjustable, frequency modulation step is better than 32 ± 3 kHz, differential frequency drift between two solid-state sources is less than ± 100 kHz. The HCN laser interferometer and SSI use AlGaN/GaN-HEMT detectors with response frequencies of 100 kHz and 1 MHz, respectively. The NEP of the detector is 10 pW Hz^−1/2 and the sensitivity is 250 mV mW⁻¹.

The overall layout of the combined interferometer is mainly divided into five parts: HCN laser system in the laser room, waveguide transmission system, optical path system of HCN interferometer under the device, SSI system under the device and the corner cube reflector fixed in the top window as shown in figure 2. To save space directly below the EXL-50 and to facilitate maintenance, the HCN laser system was placed against the wall on the negative level of the laboratory hall and a laser room was built for it. The laser room can provide an electromagnetic shielding, dust-free, vibration reduction and constant temperature operating environment for HCN laser system, the distance between the HCN room and the table below EXL-50 is about 6.5 m. A dual waveguide is used to transmit the beam from the laser room to below the EXL-50. Due to the limited space below the device, a double-layer optical platform was designed to mount the optical path system of MFCI. The design needs to consider the possibility of expanding the number of probe channels for SSI in the future, and the SSI has a longer wavelength than the HCN interferometer, and the close proximity to the window is more conducive to the optical path calculation and reduces the laser power loss, so the SSI system is installed on the upper layer of the optical platform, and the HCN laser interferometer tabletop optical path is installed on the lower layer.

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Figures 3(a) and (b) show the layout of the window above the device, the HCN laser interferometer occupies three windows in the central direction of the device, among which the outermost window is shared with the SSI. The SSI has four vertical measurements and occupies four windows on the outside of the device. In the MFCI system, the corner cube reflectors are the main component that determines the propagation path and size of gaussian beam [10]. The three windows used in the HCN laser interferometer are arranged linearly with a window spacing of 125 mm, and the three outermost windows used in the SSI are distributed in the shape of a square triangle with a spacing of 125 mm between each two windows. The installation structure of the corner cube reflector is shown in figures 3(c) and (d), and the corner cube reflector is installed in the stainless steel base, which is directly mounted on the window flange through four stainless steel columns. The diameter of the window is 70 mm, the beam waist of HCN laser interferometer and solid source interferometer at the corner cube reflector is 23.8 mm and 46.5 mm respectively, ensuring the processing accuracy and avoiding the loss of laser power, the diameter of the corner cube reflector is set to 60 mm. The material of the corner reflector is aluminum gold plating, and the preparation accuracy is better than 50 arc-seconds, since the corner reflector is mounted on the outside of the window, there is no need to consider the effect of high temperature and plasma environment on the corner cube reflector. The window temperature of EXL-50 can reach 200 °C during operation, so the quartz crystal is chosen instead of TPX lens for the window material, and the transmission rate of quartz crystal for both wavelengths is over 90% after precise thickness calculation for 337 and 882 μm laser.

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The optical layout of the MFCI system was designed with ZEMAX software [11] using Gaussian optics, and the beam profile diameter (D) and the beam waist diameter (d) of the main position in the optical path is given in figures 4 and 5. Figure 4(a) shows the optical path design of HCN laser table, where the HCN laser and other optical devices are fixed on an optical table, and the laser output from the HCN laser is divided into a probe beam and a reference beam, and the reference beam needs to be phase-modulated by a rotating grating, then part of the two beams are combined on the table into the reference-detector, and the other part enters two dielectric waveguides (Acrylic tubes of 80 mm inner diameter) with a length of 5 m, and is transmitted by the waveguides to the bottom of EXL-50. The material of rotating grating is brass. Compared with aluminum grating, the hardness of brass grating is higher, and the grating grooves are less likely to be damaged. Compared with titanium grating, brass grating has better reflectivity and less power loss to far-infrared laser. The rotating grating is 8 mm in thick and 122 mm in diameter, and is directly connected to the output shaft of the motor, the number of grooves is G = 1825, and the blaze angle is 54°. The motor speed is set as 54.8 rev s⁻¹, which will produce a beat frequency signal of 100 kHz [12].

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The optical design of the HCN laser interferometer below EXL-50 is shown in figure 4(b), and all the optics are mounted on a 2.5 m × 1.8 m non-magnetic optical table. The probe beam is transmitted over long distances through a telescope system consisting of convex and concave mirrors, and is divided into three beams of perpendicularly incident detection laser into EXL-50 by the 45-degree angle beam splitter directly below the window, and then reflected by the corner cube reflector in the upper window and returned the same way, and the returned beam is reflected by the beam splitter above the 45-degree angle beam splitter into the optical path of the table. The reference beam is divided into three beams by beam splitters, which are combined with the probe beams returned from the corner reflector to form the beat frequency signal and enter the probe-detectors. The SSI system is shown in figure 5, the whole system table is located 60 cm above the HCN laser interferometer table, supported by multiple supporting legs standing on the HCN interferometer table, and the height of the supporting legs can be finely adjusted to facilitate the level adjustment of the SSI table. The SSI table is a 20 mm thick optical aluminum plate with grooves cut at some positions to avoid blocking the probe beam of the HCN laser interferometer. The optical path design of the SSI is similar to that of the HCN laser interferometer, the difference is that the probe beam and the reference beam of the SSI are generated by two solid-state sources respectively, and the frequency difference between the two solid-state sources is 1 MHz without a rotating grating for phase modulation.

When the propagation direction of the incident beam in the plasma has an angle with the plasma electron density gradient, the incident beam will be deflected due to refraction effect, and finally the outgoing beam will deviate from the incident beam to some extent. For the axisymmetric parabolic plasma profile [13], if the plasma density distribution is

$$\begin{eqnarray}&&{n}_{{\rm{e}}}\left(b\right)={n}_{{\rm{e}}0}\left[1-{\left(\displaystyle \frac{b}{a}\right)}^{2}\right],\end{eqnarray} \tag{ 1 }$$

where b is the beam collision parameter, ${n}_{{\rm{e}}0}$ is the density of the plasma center, the deviation angle of probe beam caused by refraction effect is

$$\begin{eqnarray}&&\theta \left(b\right)={\sin }^{-1}\displaystyle \frac{{V}_{0}{\left[{\left(\displaystyle \frac{b}{a}\right)}^{2}-{\left(\displaystyle \frac{b}{a}\right)}^{4}\right]}^{1/2}}{{\left[{V}_{0}{\left(\displaystyle \frac{b}{a}\right)}^{2}+{\left(\displaystyle \frac{1-{V}_{0}}{2}\right)}^{2}\right]}^{1/2}}.\end{eqnarray} \tag{ 2 }$$

In equation (2), ${V}_{0}=\tfrac{{n}_{{\rm{e}}0}{e}^{2}}{{\varepsilon }_{0}{m}_{{\rm{e}}}{\omega }^{2}},$ b/a is the normalized radius ρ, the high density ($1\times {10}^{19}\,{{\rm{m}}}^{-3}$) of EXL-50 is substituted into ${n}_{{\rm{e}}0},$ for HCN laser interferometer and SSI, the radial distribution of refraction angle θ is shown in figure 6.

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The distance between the diagnostic window and the plasma center is 1.5 m, the offset (${\rm{\Delta }}$) of refraction effect in the upper window was estimated based on the position of HCN laser interferometer and SSI, ${{\rm{\Delta }}}_{{\rm{HCN}}}$ is about 0.3 mm and ${{\rm{\Delta }}}_{{\rm{SSI}}}$ is about 7 mm. Due to the use of a corner cone, the offset of the lower window caused by refraction effect is twice that of the upper window, so the offset of the lower window is ${{\rm{\Delta }}}_{{\rm{HCN}}}=0.6\,{\rm{mm}}$ and ${{\rm{\Delta }}}_{{\rm{SSI}}}=14\,{\rm{mm}}.$ Thus, it can be seen that refraction effect has little influence on HCN interferometer. Under the condition of high-density discharge, the refraction effect will cause the SSI with wavelength of 882 mm to lose some power at the corner cube reflector and the lower window. However, judging from the window diameter and beam diameter, the maximum power loss of beam will not exceed 10%, and for the detector, the focusing mirror in front of the detector causes the offset of the beam from the refraction effect to be small at the detector position, so the effect of refraction on SSI in MFCI system is acceptable.

This compensation interferometer mainly compensates the vibration of the corner cube reflector on the top of the device. Since the corner cube reflector is directly fixed on the EXL-50, when the EXL-50 is discharging (the phase of density climbing and falling), the corner cube reflector will produce several tens microns vibration along with the device.

The compensation interferometer can be developed on the shared channel of HCN laser interferometer and SSI shown in figure 5. The installation position of the beam splitter and the schematic diagram of the compensation interferometer are shown in figure 7. S1 and S2 are nickel mesh beam splitters on HCN laser interferometer, S3 and S4 are nickel mesh beam splitters on SSI. Considering the power distribution of the entire optical path system, S1 and S2 use a nickel mesh of 200 Lines per Inch (LPI), S3 and S4 use a nickel mesh of 50 LPI. Through tests, the transmittance of 200 LPI nickel mesh to 337 μm laser is about 60%, the transmittance of 50 LPI nickel mesh to 882 μm laser is about 60%, and the transmittance of 50 LPI nickel mesh to 337 μm laser is more than 80%. The HCN laser loses some power when it passes through the 50 LPI nickel mesh, however, this design has the possibility to make two interferometer systems measure simultaneously and thus perform vibration compensation.

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