## 1 Introduction  
The LIPS-300 is a high-power high-thrust gridded ion thruster which has been designed for the new generation of large-scale truss-type satellite platforms in China [1]. Ion optics is a crucial component of the gridded ion thruster. LIPS-300 ion optics is a three-grid structure which is composed of three dashed molybdenum grids. From the inside to the outside, it is composed of the screen grid, the accelerator grid, and the decelerator grid. The grids are fixed to the main ring through mounting rings with about 1 mm gaps between each of them. Due to the plasma-thermal radiation and plasma deposition in the discharge chamber, a radial temperature field with a high center temperature and low edge temperature will be formed on the ion optics. The temperature field is symmetrical along the radial direction of the grids and approximates to a quadratic curve distribution [2]. The temperature of the screen grid center can reach 400-500 \({}^{\circ}\)C and the temperature of the edge is 100-300 \({}^{\circ}\)C lower than the center [3]. During operation, plasma will rapidly heat the grids and cause thermal expansion. The bending stress and tensile stress generated in the grids lead to the deformation of grids. The first-order thermal deformation is an expansion outward along the spherical radius of the grids, and the maximum thermal deformation occurs at the center of the dished grids [4]. The center temperature of the screen grid is around 50-100 \({}^{\circ}\)C higher than the accelerator grid center temperature [3]. Therefore, the screen grid thermal deformation is greater than the accelerator grid deformation, which causes a decrease in the grid gap. The reduction of the grid gap will affect the divergence angle of the plasma beam and the magnitude of thrust, increase the threshold value of electron backstreaming, and aggravate grid erosion [5; 6; 7]. More seriously, it will increase the probability of the short circuit of grids, then causing the thruster to shut down [8; 9]. Therefore, it is necessary to accurately measure the amount of grid thermal deformation and the change of grid gap, and provide support for the control of the thermal deformation of the ion optics and the optimization of the thruster performance. There have been few reports on grid thermal deformation and grid gap measurement. Only National Aeronautics and Space Administration (NASA) conducted related measurement experiments in the 1990s. [PERSON] [10] used a small high-precision stepper motor to control probes to measure the grid thermal deformation by contact. The measurement accuracy was up to 25 \(\upmu\)m. [PERSON] [11] placed a mirror in front of the thruster side and measured the thermal deformation of the T5 ion thruster grids by taking a reflection image of the mirror with a telemicroscope. [PERSON] [12] designed a kind of optical system based on an He-Ne laser, and measured the thermal deformation of 900-series ion optics. However, the above methods all had defects. [PERSON]'s method was a contact measurement that could not be measured in the plasma beam flow, and [PERSON]'s method calculated the gap through the change of angle so the precision was low. [PERSON]'s optical measurement method was very demanding on the lens and sensitive to environmental vibration. Based on the improvement of the above problems, [PERSON] [13] measured the thermal deformation of the grid using a telemicroscope and bolted probes. This measurement method could operate under the condition of the ion thruster with the plasma beam, and the grids' thermal deformation and hot gap of the NASA Solar Electric Propulsion Technology Readiness (NSTAR) thruster 30 cm titanium ion optics were measured successfully. [PERSON] [14] measured the grid thermal deformation of the molybdenum ion optics of NSTAR thruster by using the measurement method of [PERSON], and the results of measurement verified the effectiveness of the [PERSON] method.