大功率准分子激光器气体流场数值分析

Numerical Analysis of Gas Flow Field of High-Power Excimer Laser

准分子激光器作为放电激励的气体激光器,其激光介质气体在主放电电极间辉光放电产生激光的同时,也在放电区域产生气态和固态的放电产物以及大量的热。通常在激光腔内放置横流风机,以维持放电区域气体的流动和更新,从而保证激光器以重复频率工作时,每一次放电不受前一次放电产物和热量的影响。对于大功率(>300 W)准分子激光器,其放电区域体积大,且具有更高的放电电压和放电重复频率,因此对主放电电极间的气体流场具有更高要求。本课题组采用二维模型,针对一套自行设计的大功率准分子激光器放电腔结构和气体循环系统进行了稳态流场数值分析。数值分析结果显示,当风机转速为3500 r/min时,放电区域气体的平均流速高于32.76 m/s,且气体流速均匀。这一结果从理论上证明了该循环系统可以满足大功率准分子激光器的工作要求。

Objective The dielectric gas in excimer lasers generates light via glow discharge between the main discharge electrodes, which produces gaseous and solid discharge products and a large amount of heat in the discharge area. Generally, a crossflow fan is used in the laser chamber to maintain and renew the gas flow in the discharge area, thus ensuring that each discharge is not affected by the products and heat from the previous when the laser operates repeatedly. For highpower(> 300 W) excimer lasers, their discharge area is large;moreover, the high-power excimer lasers have higher discharge voltage and frequency than medium-or low-power excimer lasers, so there are higher requirements for the gas flow field between the main discharge electrodes. This article studies a 600 W high-power excimer laser with a laser discharge voltage of up to 30 k Vand a discharge repetition rate of up to 600 Hz. Its discharge area generates more discharge products, and has a heat-generation power of close to 50 k Wat full power output, so a higher gas-renewal rate is required between the discharge electrodes. Therefore, when developing this high-power excimer laser, the structural design of the discharge chamber and gas-circulation system within it are critical, and flow-field analysis is required to provide a theoretical basis for the design.Methods A multiple-reference-frame model is used to simulate the steady-state flow field in the laser. In the model, the rotation of the crossflow fan provides all the kinetic energy of the flow field. The flow field simulation adopts the standard k-ε turbulence model. To improve the reliability of the simulation results, we also selected the renormalization group(RNG) and Realizable k-ε turbulence models for simulation calculations and compared the results.Results and Discussions Figure 2 shows the numerical calculation results of the flow field in the laser chamber. Figures 2(a) and(b) show the global flow velocity distribution and absolute pressure distribution, which provide a reference for the access position of the gas-purification system. Figures 2(c) and(d) show the flow velocity distribution and flow velocity vector diagram in the laser discharge area. The simulation results show that the gas-renewal rate in the discharge area meets the requirements. The uniformity of the flow field in discharge area is ideal. Figure 3 shows the simulation results using the RNG and Realizable k-ε turbulence models. It can be seen by comparing the simulation results that the flow velocity distribution trends in the laser chamber calculated by the standard, RNG, and Realizable k-ε turbulence models are roughly the same. Figure 4 is a comparison diagram of the distribution of flow velocity along the central axis of the electrodes calculated using three different models. The calculated variation trend in the flow velocity on the central axis of the electrodes is nearly the same for all three models. The simulation results show that when the fan speed is 3500 r/min, the average gas velocity in the discharge area is higher than the target value of 32.76 m/s, and the gas velocity is uniform. It is proven theoretically that this set of self-designed gas-circulation system can meet the working requirements of the high-power excimer laser. The average flow velocity of the working gas between the electrodes of the excimer laser tends to be correlated linearly and positively with the fan speed, and the torsional moment of the fan tends to be linearly and positively correlated with the square of the fan speed. This result provides a reference for the power and speed control of the crossflow fan drive motor when the laser operates at different frequencies. Furthermore, the torsional moment of the fan is basically linear with respect to the air pressure in the chamber. This result provides insight into the selection of the structural materials of the cross-flow fan, the requirements of the overall assembly rigidity of the fan, and performance requirements of the magnetic coupler driving the fan.Conclusions In this paper, a two-dimensional numerical study of the flow field is carried out for a self-designed highspeed gas-circulation system in the chamber of a high-power excimer laser. The important parameters of high-power excimer laser design, such as the renewal rate of the gas in the laser discharge area, uniformity of the gas flow, pressure distribution in the chamber, and torsional moment of the fan, are discussed. The results provide a reference for the access position of the gas-purification system on the chamber, power and speed control of the motor at different operating frequencies, selection of structural materials for the crossflow fan, rigidity requirements of the overall fan assembly, and performance requirements of the magnetic coupler that drives the fan. The simulation results show that when the fan speed is 3500 r/min, the average gas velocity in the discharge area is higher than the target value of 32.76 m/s, and the gas velocity is uniform. This set of self-designed gas-circulation systems is theoretically proven to be able to meet the working requirements of the high-power excimer laser. It should be noted that we set the cavity temperature to a fixed value and ignore the effect of cavity temperature variation on flow filed. So, our simulation has certain difference with the actual results.