We present a study of the expansion of an ultracold neutral plasma (UCNP) with an initial density distribution that decays exponentially in space, created by photoionizing atoms shortly after their release from a quadrupole (or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is evident in the evolution of the plasma size and velocity, indicating that the dynamics are reasonably well described by a model of hydrodynamic expansion of a quasi-neutral plasma. However, for low plasma density and high initial electron temperature, excess ion kinetic energy in the vicinity of the central density peak suggests significant local non-neutrality at early times. Observations are compared to the well-understood self-similar expansion of a UCNP with an initial Gaussian density distribution, and a similar scaling law describes the evolution of plasma size for both cases.

中文翻译：

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密度分布呈指数衰减的超冷中性等离子体的膨胀

我们对超冷中性等离子体（UCNP）的膨胀进行了研究，其初始密度分布在空间中呈指数衰减，这是由原子从四极（或双子尖瓣）磁阱释放后不久就对其进行光电离而产生的。等离子体尺寸和速度的演变过程中明显体现出特征性的离子声时标，表明通过准中性等离子体的水动力膨胀模型可以很好地描述动力学。但是，对于较低的等离子体密度和较高的初始电子温度，中央密度峰附近的过量离子动能表明在早期存在明显的局部非中性。将观察结果与具有初始高斯密度分布的UCNP的易于理解的自相似展开进行比较，