Effect of length of outer electrode on plasma characteristics in coaxial gun
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摘要: 同轴枪放电装置能够产生高速度(~100 km/s)、高电子密度(~1016 cm–3)以及高能量密度(~1 MJ/m2)的稠密等离子体, 因而在聚变能、天体物理及航空航天等领域得到了广泛关注. 通过光电信号的测量以及对输运过程中等离子体时空演化过程的观察, 本文主要对比分析了不同长度外电极下的同轴枪放电等离子体特性. 外电极长度的增加, 带来了喷射等离子体电子密度、发光强度的降低以及轴向速度、准直性与输运距离的显著提高, 而由箍缩效应所形成的等离子体柱在放电过程中对中心电极的延长作用则是引起长短外电极同轴枪中等离子体参数差异的主要原因. 延长的中心电极一方面与长外电极在轴向长度上得以匹配, 使等离子体在枪内能够获得更长的加速时间, 进而提高其喷射速度; 另一方面则会造成带电粒子的大量损耗以及更高的碰撞复合损失, 导致等离子体电子密度与发光强度的降低. 等离子体的轴向动能直接影响着其喷出后的传播距离, 而喷口处等离子体的扩散角则主要受电子密度与径向洛伦兹力的约束, 二者共同决定了等离子体的准直性及输运衰减特性.Abstract: The dense plasma produced by a coaxial gun possesses an extremely high velocity (~100 km/s), electron density (~1016 cm–3) and energy density (~1 MJ/m2), which has great potential applications in fusion energy, astrophysics and aerospace physics. Through the measurements of electrical and optical signals, as well as the temporal and spatial evolution of the ejected plasma, the plasma characteristics of two different outer electrodes in length are investigated. As the outer electrode is lengthened, the axial velocity, the collimation and the propagation distance of plasma are all enhanced while the electron density and the optical intensity decrease, this can be ascribed to the extension of plasma column formed by Z-pinch on the central electrode during the discharge. When moving across the end of the inner electrode, the plasma sheet can be stretched into a bow shape due to the Coulomb and Lorentz force. With the appearance of axial current, part of the plasma sheet near the head of the inner electrode converges toward the center, and then generates a plasma column with much higher electron density and temperature. On the one hand, the extending of the plasma column can match the outer electrode in length and therefore the plasma column gains longer accelerating time in the coaxial gun resulting in the growing of ejected velocity. On the other hand, it also brings higher losses of the charged particles and recombination rates between the plasma and the wall of electrodes, resulting in the decrease of electron density and optical intensity. Moreover, the axial kinetic energy, the electron density and the radial Lorentz force of ejected plasma are jointly responsible for the collimation and the attenuation characteristics in its propagation. As the axial velocity and electron density increase, the axial kinetic energy of ejected plasma increases, which induces a longer propagating distance. In contrast, with the electron density and radial Lorentz force growing, the density gradient and thermal expansion of ejected plasma are enhanced correspondingly, leading the energy density to decrease and finally the propagating distance to shorten. In conclusion, a high collimation plasma jet trends to generate in a high axial velocity, electron density and with a relatively long outer electrode.
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Key words:
- coaxial gun /
- dense plasma /
- z-pinch /
- collimation /
- plasma column
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图 2 不同长度外电极同轴枪在充电电压为5 kV, 气压为10 Pa放电条件下的电压、电流以及光电流波形 (a) 短外电极; (b) 长外电极
Figure 2. Typical electrical and optical signals of discharge in a coaxial gun with (a) short and (b) long external electrode. V = 5 kV and P = 10 Pa.
图 3 气压10 Pa时, 同轴枪放电等离子体速度随充电电压的变化
Figure 3. The variation of plasma velocity with the charging voltage of the coaxial gun at 10 Pa.
图 4 高速相机拍摄的同轴枪放电等离子体图像, 充电电压5 kV, 气压10 Pa, 曝光时间为5 μs (a) 短外电极; (b) 长外电极
Figure 4. High-speed camera photographs of discharge in a coaxial gun with (a) short and (b) long outer electrode. V = 5 kV, P = 10 Pa, the exposure time is 5 μs.
图 5 短外电极同轴枪放电中的等离子体片发展过程
Figure 5. Development of plasma sheet during discharge in a coaxial gun with short outer electrode.
图 6 长外电极同轴枪放电中的等离子体片发展过程
Figure 6. Development of plasma sheet during discharge in a coaxial gun with long outer electrode.
图 7 同轴枪在气压为10 Pa的放电条件下, 电子密度随充电电压的变化
Figure 7. The variation of electron density with the charging voltage of the coaxial gun at 10 Pa.
图 8 数码相机拍摄的不同外电极长度条件下的放电照片 (a) 短外电极; (b) 长外电极. 气压为10 Pa, 充电电压为7 kV, 曝光时间为1 s
Figure 8. Digital camera photographs of discharge in a coaxial gun with (a) short and (b) long outer electrode. P = 10 Pa, V = 7 kV, the exposure time is 1 s.
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