A two-stage light gas gun is used to accelerate the sphere 6061 aluminum projectiles to a speed range of 2.3-6.3 km/s in an almost vacuum environment. Spectrometer and pyrometer are adopted to measure the time-frequency characteristics of the collision-generated spectra when 6061 aluminum projectiles impact on 6061 aluminum targets with thicknesses varying from 1 mm to 26 mm. Based on the thermal ionization, theories of the single and the secondary collision are established. Experimental spectra show that threshold velocities of evaporation and plasma phase transition are 2.45 km/s and 4.7 km/s when spherical aluminum alloy projectile impacts on aluminum alloy target. Theoretical calculation demonstrates that neither the single impact nor the secondary impact of debris or secondary compression in the oblique direction can explain the ionization at low-speed impact. The threshold velocity of ionization, however, calculated by the combined secondary collision both in the oblique and horizontal direction during the impact of spherical projectile colliding on the target is most reasonable to explain the plasma produced by relatively low-speed hypervelocity impact.

两级轻气枪用于在几乎真空的环境中将球体6061铝弹加速到2.3-6.3公里/秒的速度范围。采用光谱仪和高温计测量6061铝弹撞击厚度为1~26mm的6061铝靶时碰撞产生光谱的时频特性。在热电离的基础上，建立了单次碰撞和二次碰撞的理论。实验光谱表明，球形铝合金弹丸撞击铝合金靶时蒸发和等离子体相变的阈值速度分别为2.45km/s和4.7km/s。理论计算表明，无论是单次撞击还是碎片二次撞击或斜向二次压缩都不能解释低速撞击时的电离。然而，球体弹丸撞击目标时，通过斜向和水平方向的组合二次碰撞计算的电离阈速度最能解释相对低速超高速撞击产生的等离子体。