Ultracold Plasma Expansion Research Articles

Reply to "Comment on 'Ultracold plasma expansion in quadrupole magnetic field'".

In this Reply, we respond to the Comment by Schlitters etal. on our recent work [Phys. Rev. E 108, 045209 (2023)10.1103/PhysRevE.108.045209], where we present simulation results of ultracold Sr plasma expansion in a quadrupole magnetic field using a molecular-dynamics method. In the Comment, Schlitters etal. present their experimental results, some of which, from their point of view, contradict our simulation results and others that confirm the experiments of Gorman etal. [Phys. Rev. Lett. 126, 085002 (2021)10.1103/PhysRevLett.126.085002] and our results. In addition, Schlitters etal. also provide results that were not described in our work. Here we show that there is not a contradiction but a misunderstanding of our results. This is due to the fact that simulations allow the use of processing methods that are difficult to obtain in experiment. We also present different simulation results related to expansion velocity variations that reflect the experimental results of Schlitters etal. and provide an explanation for the origin of these variations.

Comment on "Ultracold plasma expansion in quadrupole magnetic field".

Bronin etal. [Phys. Rev. E 108, 045209 (2023)2470-004510.1103/PhysRevE.108.045209] recently reported molecular-dynamics simulations of ultracold neutral plasmas expanding in a quadrupole magnetic field. While the main results are in agreement with prior experimental measurements, we present data showing oscillations not captured in the simulations of Bronin etal. Plasmas formed using pulsed or continuous-wave ionization processes have similar confinement times.

Ultracold plasma expansion in quadrupole magnetic field.

We present simulation results of ultracold Sr plasma expansion in a quadrupole magnetic field by means of molecular dynamics. An analysis of plasma evolution influenced by a magnetic field is given. Plasma confinement time behavior under variation of magnetic field strength is estimated. Similarity of the time dependence of the concentration and distribution of ion velocities against the parameters of the plasma and magnetic field is established. Simulation results are in agreement with the experimental ones.

Physical processes during ultracold plasma expansion

Using the method of molecular dynamics, the expansion of a two-component, pulsed laser-produced ultracold plasma is directly calculated for various values of the number and density of particles and their electron temperatures. A new method is presented for generating and diagnosing a steady-state ultracold plasma formed under continuous wave laser irradiation. The performed calculations show the difference in the properties of an ultracold plasma obtained by pulsed and continuous wave laser irradiation.

Ion wave formation during ultracold plasma expansion.

We present the results of a direct simulation of the expansion of a two-component ultracold plasma for various numbers of particles, densities, and electron temperatures. A description of the expansion process common to all plasma parameters is given. After the escape of fast electrons from the plasma cloud, the excess positive charge is localized at the outer boundary, in a narrow layer. This layer has a characteristic front shape with a sharp drop in the charge concentration. The charged layer retains the remaining electrons during the entire expansion process. As the plasma expands, the speed of movement of the charged layer becomes constant and significantly exceeds the sonic speed of ions. In addition, the dependence of the radial velocity of ions on the radius acquires a self-similar character long before the final stage of expansion. Based on the calculation results, equations and self-similar solutions are obtained. General dependencies on plasma parameters are determined, which are compared with experimental data.

Anisotropic ultracold plasma expansion

Based on fluid model, the three-dimensional expansion of ultracold plasma is studied. The model considered both thermal pressure and ambipolar potential rising due to the local charge separation. By using isothermal assumption and quasi-neutrality condition, the time dependent asymetric expansion parametric investigation is conducted for two different ultracold plasma ion species (Sr+ and Ca+). It is found that ion temperature, in the range of ultracold limit, did not affect strongly the expanding profile but decreases the expanding density by an amount of ∼0.5% to ∼7%.The ion inertia is also found to enhance the asymetric aspect of the anisotrpic plasma expansion.

The charge imbalance in ultracold plasmas

Ultracold plasmas are regarded as quasineutral but not strictly neutral. The results of charge imbalance in the expansion of ultracold plasmas are reported. The calculations are performed by a full molecular-dynamics simulation. The details of the electron velocity distributions are calculated without the assumption of electron global thermal equilibrium and Boltzmann distribution. Spontaneous evolutions of the charge imbalance from the initial states with perfect neutrality are given in the simulations. The expansion of outer plasma slows down with the charge imbalance. The influences of plasma size and parameters on the charge imbalance are discussed. The radial profiles of electron temperature are given for the first time, and the self-similar expansion can still occur even if there is no global thermal equilibrium. The electron disorder induced heating is also found in the simulation.

Ultracold plasma expansion as a function of charge neutrality

Ultracold plasmas (UCPs) are created under conditions of near but not perfect neutrality. In the limit of zero electron temperature, electron screening results in non-neutrality manifesting itself as an interior region of the UCP with both electrons and ions and an exterior region composed primarily of ions. The interior region is the region of the most scientific interest for 2-component ultracold plasma physics. This work presents a theoretical model through which the time evolution of non-neutral UCPs is calculated. Despite Debye screening lengths much smaller than the characteristic plasma spatial size, model calculations predict that the expansion rate and the electron temperature of the UCP interior is sensitive to the neutrality of the UCP. The predicted UCP dependence on neutrality has implications for the correct measurement of several UCP properties, such as electron temperature, and a proper understanding of evaporative cooling of the electrons in the UCP.

Influence of electron evaporative cooling on ultracold plasma expansion

The expansion of ultracold neutral plasmas (UCP) is driven primarily by the thermal pressure of the electron component and is therefore sensitive to the electron temperature. For typical UCP spatial extents, evaporative cooling has a significant influence on the UCP expansion rate at lower densities (less than 108/cm3). We studied the effect of electron evaporation in this density range. Owing to the low density, the effects of three-body recombination were negligible. We modeled the expansion by taking into account the change in electron temperature owing to evaporation as well as adiabatic expansion and found good agreement with our data. We also developed a simple model for initial evaporation over a range of ultracold plasma densities, sizes, and electron temperatures to determine over what parameter range electron evaporation is expected to have a significant effect. We also report on a signal calibration technique, which relates the signal at our detector to the total number of ions and electrons in the ultracold plasma.

Recombination during expansion of ultracold plasma

Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼1010 cm−3) are investigated. The measured dependence on initial ion density is N1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.

Ultracold Plasma Expansion in a Magnetic Field

We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high-voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (<20 micros), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of the magnetic field (up to 70 G). We observe that the expansion velocity scales as B(-1/2), explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.