Laser Cooling Ions in An Ultracold Neutral Plasma

该项目探索了使用激光降低由电子和带电原子组成的气体（所谓的“中性等离子体”）温度的可能性。带电的原子（离子）和电子相互施加电作用力。在普通等离子体中，离子和电子运动迅速，只有在它们快速通过彼此时才会短暂地相互推动。当等离子体温度下降时，带电粒子在彼此附近停留的时间增加，电磁力的影响变得更加明显。在足够低的温度下，气态等离子体具有致密液体的特性。有趣的是，聚变级等离子体（极热等离子体，正在研究其作为先进电力生产来源的潜力）也表现为致密液体。发生这种情况不是因为温度低，而是因为密度高。等离子体表现为致密液体的程度由等离子体密度除以温度的数学商给出。在聚变等离子体中，由于密度高，这个商很大。在本实验中，由于温度较低，该商较大。这使得所提出的实验可以在低温下使用模型系统研究融合级等离子体的物理，并对实验条件进行精确控制。由该提案资助的活动将探索超低温等离子体的低温极限。这项研究将在杨百翰大学进行，该大学赞助了全国最大的本科物理项目之一。由于将有多名本科生参与这项工作，NSF的资助将直接影响他们的科学教育和准备。在超冷中性等离子体中，强耦合参数伽玛受到无序加热过程的限制。冷离子是通过在磁光阱中光电离激光冷却的Ca原子产生的。虽然这些离子很冷，但它们具有巨大的电势能。离子在运动时被加速以使其势能最小化。钙离子的温度从几个mK增加到几个开尔文，在不到100纳秒的时间里增加了1000倍。等离子体的动力学和热力学性质与伽马成比例。不幸的是，在中性等离子体中，DIH限制γ小于2。因此，本研究的目标是利用激光冷却来降低超冷中性等离子体中的离子温度，增加伽马值。这个建议建立在Bergeson博士之前在这个领域的大量工作的基础上。这一目标的成功实现将为将来进行高伽马实验提供一个平台。离子将使用功率强大的频率加宽激光器在397nm处冷却。在冷却过渡到三维双重态D （J=3/2）能级的光学泄漏将使用850和854 nm的激光器进行堵塞。通过交替切换这些近红外激光强度，可以避免重复相干。在指定的冷却时间后，将关闭冷却光，通过扫描低强度激光在397nm吸收谱线上探测离子温度，收集激光诱导的荧光，并将测量的谱线形状拟合到Voigt谱线上。这个项目的一个挑战是利用光力来克服电子驱动的径向等离子体膨胀。模拟表明这是可能的。另一个挑战是，如何以足够快的速度散射等离子体离子中的光子，以便在改变速度的碰撞将离子动能重新分配到集合中未被捕获的部分之前，使离子减速并受到限制。数值估计表明，这在低等离子体密度下是可能的。

This project explores the possibility of using laser light to reduce the temperature of a gas composed of electrons and charged atoms (a so-called "neutral plasma"). The charged atoms (ions) and electrons exert electrical forces on each other. In ordinary plasmas, the ions and electrons move quickly and only push on each other briefly as they rapidly pass one another. When the plasma temperature falls, the amount of time charged particles spend near each other increases and the influence of the electrical force becomes more pronounced. At low enough temperatures, the gaseous plasma assumes characteristics of dense liquids. Interestingly, fusion-class plasmas (extremely hot plasmas that are being studied for their potential as an advanced source of electrical power production) also behave as dense liquids. This happens not because of low temperature, but because of high density. The degree to which a plasma behaves as a dense liquid is given by a mathematical quotient of the plasma density divided by the temperature. In fusion plasmas, this quotient is large because of high density. In the proposed experiments, this quotient is large because of low temperature. This makes it possible for the proposed experiments to study the physics of fusion-class plasmas using a model system at low temperatures with exquisite control over experimental conditions. Activities funded by this proposal will explore the limits of low temperatures in ultracold plasmas. Brigham Young University, where this research will be carried out, sponsors one of the largest undergraduate physics programs in the nation. Because several undergraduate students will be involved in this work, NSF funding will directly influence their scientific education and preparation.The strong coupling parameter, gamma, is limited in ultracold neutral plasmas by the process of disorder-induced heating (DIH). The cold ions are created by photo-ionizing laser-cooled Ca atoms in a magneto-optical trap. Although cold, these ions have an overwhelmingly large electrical potential energy. The ions are accelerated as they move to minimize their potential energy. The Ca ion temperature increases from a few mK to a few kelvin, a factor of 1000 in less than 100 ns. Kinetic and thermodynamic plasma properties scale with gamma. Unfortunately, DIH limits gamma to be less than 2 in neutral plasmas. The goal of this research, therefore, is to reduce the ion temperature in ultracold neutral plasmas using laser cooling, increasing the value of gamma. This proposal builds on extensive previous work by Dr. Bergeson in this field. Successful realization of this goal will generate a platform from which high-gamma experiments can be performed in the future. The ions will be cooled using a powerful frequency-broadened laser at 397 nm. The optical leak in the cooling transition to the 3d doublet D (J=3/2) level will be plugged using lasers at 850 and 854 nm. Repumping coherences will be avoided by alternately switching these near-infrared laser intensities. After a specified cooling time, the cooling light will be switched off and the ion temperature will be probed by scanning a low-intensity laser across the 397 nm absorption line profile, collecting the laser-induced fluorescence, and fitting the measured lineshape to a Voigt profile. One challenge in this project is using optical forces to overcome the electron-driven radial plasma expansion. Simulations suggest that this is possible. Another challenge will be to scatter photons from the plasma ions fast enough that the ions can be slowed and confined before velocity-changing collisions redistribute the ion kinetic energy to untrapped portions of the ensemble. Numerical estimates suggest that this should be possible at low plasma densities.