Physics Letters A 350 (2006) 129–133 www.elsevier.com/locate/pla Dependence of the cloud size on interactions for a trapped degenerate ultracold plasma S. Sevinçli, M.Ö. Oktel, B. Tanatar ∗ Bilkent University, Department of Physics, 06800 Bilkent, Ankara, Turkey Received 24 August 2005; accepted 3 October 2005 Available online 12 October 2005 Communicated by V.M. Agranovich Abstract We consider an ultracold plasma that has bosonic ions, at zero temperature. Assuming that ions are trapped by a harmonic trap, we calculate the size of the cloud for both screened and bare Coulomb interactions. Our results indicate that if clouds containing around 10 4 ions are confined with a trapping frequency of 10 kHz, stable radius is 15 μm for a fully screened two-component plasma while the radius increases to 2 mm for a one component plasma.  2005 Elsevier B.V. All rights reserved. 1. Introduction The advances in trapping and cooling methods for neutral atoms have led to remarkable experiments where quantum me- chanical nature of bosonic and fermionic particles can be stud- ied at very low temperatures. The systems where the temper- atures are low enough for quantum mechanical effects to be dominant are called ultracold systems. After the study of Bose– Einstein condensation, using neutral bosonic atoms [1], study of ultracold atoms have expanded into the investigation of fermion systems [2] and ultracold molecule production via Feshbach resonances [3]. All these experiments have the common prop- erty that the investigated particles are neutral. Another new direction in ultracold systems is the creation of an ultracold plasma [4]. An ultracold plasma is created by rapid ionization from an ultracold gas, and so far its investigation can only be done in a dynamical state. In current experiments, af- ter rapid ionization, a fraction of the electrons leave the cloud, while the remaining cloud has a central region which is a neutral two component plasma [5–12]. The electron pressure inside the plasma causes rapid expansion of the cloud, limiting the obser- vation time to hundreds of microseconds and complicating the * Corresponding author. E-mail address: tanatar@fen.bilkent.edu.tr (B. Tanatar). investigation of the properties of the ultracold plasma. Although the dynamical properties have yielded interesting phenomena, creation of a stable ultracold plasma will be interesting both from a basic science point of view and may lead to new techno- logical advances in fields ranging from precision measurements to quantum computation. For an ultracold plasma, the electron Coulomb coupling pa- rameter Γ e ≃ e 2 n 1/3 /k B T , which measures the ratio of average interaction energy between electrons to their thermal energy is close to unity [5–12]. The regime where this parameter is larger than unity is unexplored and proposes many fundamen- tal questions. Especially when quantum mechanical correla- tions between the constituent particles are taken into account, one can expect many new phenomena to emerge. As examples for the fundamental questions about this system, one can ask “What is the nature of plasma oscillations when particles are strongly interacting and quantum degenerate?” or “Can elec- trons and ions coherently combine and dissociate, leading to natural atom–plasma oscillations?”. Before attempting to tackle such questions, in this Letter, we consider a simple model of quantum degenerate plasma where the ions form a Bose con- densate, and understand the dependence of the size of such a cloud on interactions. So far, in ultracold plasma experiments, plasma is produced by rapid ionization from a cold gas trapped in a magneto-optical trap [5–12]. In the rapid ionization process, most of the energy 0375-9601/$ – see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2005.10.014