GENERATION OF LARGE AREA INDUCTION THERMAL PLASMA BY SUPERIMPOSING 50-KHz OSCILLATING FIELD

功率在百千瓦以上的电感耦合射频热等离子体，正逐渐成为加工新型功能材料和破坏环境消耗物质的高温、高反应性的重要热源。虽然等离子体的体积被限制在直径50到60毫米的兆赫量级的发生器的应用频率，大面积等离子体强烈要求更高的处理速率作为一个未来的技术。本课题尝试在小面积直流等离子体上叠加低频（50kHz）磁场来扩展高温等离子体场。由于这种低频场的穿透深度长达几百毫米，因此可以产生直径更宽的等离子体。基于动态等离子体模型的数值模拟表明，在760 torr和76 torr的压力下，激发等离子体膨胀和维持直径为100 mm的等离子体的最小功率分别约为90和60kW。计算还表明，达到新的稳定状态所需的时间约为几十ms，在此之后，即使没有直流电源输入，广域等离子体也能在50 khz的场下保持稳定。实验采用频率为50 khz，最大功率为300 kw的电源进行。利用单色仪观测到794和801 nm的两个ArI光谱强度，定量分析了瞬态现象。通过在60 torr软真空条件下进行的几次实验，发现功率为1 kW的小尺寸直流等离子体在55 ~ 60 kW的50 khz功率指示水平下膨胀，并在膨胀开始20 ms后达到了新的稳定状态。在直径超过10 cm的稳定模式下，成功地产生了相当均匀的8000 ~ 10000 K等离子体温度分布。

The inductively-coupled radio frequency thermal plasma produced with a power over a hundred kW is now becoming gradually an important source with high-temperature and high-reactivity either for processing of new functional materials or for destruction of circumstance depleting substance. Although the volume of the plasma is restricted to 50 to 60 mm in diameter by the applied frequency of the generator of MHz order, a large area plasma is required strongly for higher rate of processing as a future technology. In this project, an attempt is made to expand the high temperature plasma field by superimposing a low-frequency (50kHz) magnetic field on a small area dc plasma. Because the penetration depth of such a low frequency field is as long as a few hundreds mm, a more wider plasma in diameter can be produced.Numerical simulations based on the dynamic plasma model showed that the minimum power to motivate the plasma expansion and sustain a wider plasma of 100 mm in diameter was found to be around 90 and 60kW for the pressure of 760 and 76 torr, respectively. The calculation also showed that the time necessary for reaching a new steady state is around a few tens of ms and after this the wide area plasma is kept stationary by 50-kHz field even without dc power input. Experiments were carried out with using a power supply of 50-kHz frequency and 300-kW maximum power. Two ArI spectral intensities of 794 and 801 nm were observed by using a monochromator to analyze the transient phenomena quantitatively. Through several shots of experiments carried out under 60 torr soft vacuum condition, the small size dc plasma with 1-kW power was found to expand at the 50-kHz power indicator level of 55 to 60 kW,as expected and a new steady state of the plasma was achieved 20 ms after the initiation of the expansion. A quite uniform profile of the plasma temperature of 8,000 to 10,000 K was produced successfully in steady mode over 10 cm diameter.