Department of Electrical Engineering, Tohoku University, Sendai 980-8579, Japan.
Space Plasma, Power and Propulsion Laboratory, Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
Phys Rev Lett. 2018 Jan 26;120(4):045001. doi: 10.1103/PhysRevLett.120.045001.
A specially constructed experiment shows the near perfect adiabatic expansion of an ideal electron gas resulting in a polytropic index greater than 1.4, approaching the adiabatic value of 5/3, when removing electric fields from the system, while the polytropic index close to unity is observed when the electrons are trapped by the electric fields. The measurements were made on collisionless electrons in an argon plasma expanding in a magnetic nozzle. The collision lengths of all electron collision processes are greater than the scale length of the expansion, meaning the system cannot be in thermodynamic equilibrium, yet thermodynamic concepts can be used, with caution, in explaining the results. In particular, a Lorentz force, created by inhomogeneities in the radial plasma density, does work on the expanding magnetic field, reducing the internal energy of the electron gas that behaves as an adiabatically expanding ideal gas.
一项专门设计的实验表明，当从系统中移除电场时，理想电子气体会近乎完美地绝热膨胀，导致多方指数大于 1.4，接近绝热值 5/3，而当电子被电场捕获时，多方指数接近 1。这些测量是在磁场喷嘴中膨胀的氩等离子体中的无碰撞电子上进行的。所有电子碰撞过程的碰撞长度都大于膨胀的标度长度，这意味着系统不能处于热力学平衡状态，但可以谨慎地使用热力学概念来解释结果。特别是，由径向等离子体密度不均匀产生的洛伦兹力对膨胀的磁场做功，减少了作为绝热膨胀理想气体的电子气体的内能。
