supervisor : Caroline Champenois
mail :
tel : +33 413946413
lab : Physique des interaction ioniques et moléculaires, Marseille, campus de saint-Jérôme.
web page of the CIML group : []
Fundings : grant secured

Abstract : The proposed PhD project aims at measuring several self-diffusion properties within a strongly correlated non-neutral plasma, simulated by a laser-cooled ion cloud in a linear radio-frequency trap. The experimental protocol to design will take advantage of the engineering of different dark states built by laser-atom coupling, to control the relevant time scales of the internal state dynamics and of the pressure force imposed by Doppler laser cooling. These measurements are very important to benchmark potential models and test plasma theories out of the conventional plasma regimes [3,4].
Context and Challenges : The support of these experiments are trapped ion clouds of few thousands ions as they offer a versatile and highly controllable platform which makes possible original experimental approaches. For our concern, this cloud of atomic ions stored in a radio-frequency trap is a practical realisation of a finite size One Component Plasma (OCP) where the role of the neutralising particles is played by the confining potential [1]. The OCP is a reference model in the study of strongly coupled Coulomb systems. Its figure of merit is the plasma parameter , which is the ratio between the Coulomb potential energy between closest neighbours and the thermal kinetic energy of the ions. With the control on Doppler laser-cooling and on the steepness of the trapping potential, the plasma parameter of a trapped-ion based finite OCP can be experimentally tuned over several orders of magnitude spanning gas ( lower than 0.1) liquid ( of the order of 1 to 100) and crystal phases of the plasma ( larger than 200).  We work with calcium ions Ca+, laser-cooled to the mK range, in a linear quadrupolar radio-frequency trap. As they reach temperatures lower than 1 K, trapped ions bunch in the trapping potential well and arrange in a stationary structure that minimise the total potential energy (trapping+Coulomb repulsion), to form what is called a Coulomb crystal. An example of these structures is shown on the figure obtained by the image of their induced fluorescence on an intensified CCD camera. The picture is formed by several hundreds of ions, laser cooled to less than 100 mK. When an extra laser is sent on the atomic sample. It is responsible for the excitation of a large part of the ions in a long lived dark state. The dark and bright ions split spatially because of the radiation pressure induced by the laser-cooling process which acts only on the bright ions.

The spatial segregation induced by the state-selective radiation pressure offers a unique tool to measure self-diffusion coefficient within a strongly correlated non-neutral plasma [2]. Our set-up, where the relevant time scales and the strength of the pressure force can be tuned by numerous atom-laser interaction parameters is one of the few systems where diffusion coefficient in a strongly correlated OCP at equilibrium can be measured. Furthermore, by dressing trapped atoms with three coherent photons [7], we are able to engineer a new eigenstate of the {atom+photons} system and the high spectroscopic control that can be reached on this dark state will allow to explore different regimes of diffusion.
Program : The first part of this project will be to identify the experimental parameters that will give access to relevant measurements of diffusion coefficients. Then, the compatibility of these parameters with Doppler laser cooling of the large ion cloud will have to be validated before the measurement period can start. It will be a real asset to carry out these measurements within the liquid and the crystal phase of the ion cloud as the diffusion at the microscopic scale is expected to be different.

To perform these measurements the candidate will use and maintain an already existing experimental setup, involving diodes and solid state lasers and a linear radio-frequency trap. Typical cold ion laser technique, such as photoionisation and Doppler cooling will also rapidly be part of the candidate experimental routine. The engineering of a three-photon dark state and the frequency stabilisation of all the involved lasers are reached through an optical frequency comb, stabilised by a local ultra-stable laser and absolute frequency measurements are allowed through our connection to the REFIMEVE network. The main experimental data will come from a detailed analysis of the cloud pictures, from which information concerning the self-diffusion within the cloud can be extracted, thanks to the support of a model that is presently under construction and validation, in collaboration with statistical physics theoreticians. We have also built a molecular dynamics simulation of the experiment that can be used to complement the experimental studies and the analytic model to reach an insight at the microscopic level. This code will be a relevant tool available to guide the future experiments toward a relevant set of parameters [5,6].
The acquired skills concern ion trapping in radio-frequency traps, high resolution frequency control of laser systems, coherent and non-coherent atom-laser interactions, image processing, and modelling for diffusion in a charged finite system.
Environment : PIIM brings together physicists and physical chemists who study dilute media - such as gases, plasmas and beams of ions, atoms and/or molecules - as well as their interactions with matter and light. The CIML group who hosts this project has a strong expertise in radio-frequency ion trapping and optical frequency metrology.
Application : We are looking for candidate motivated for an experimental project with a good background in atomic physics and optics. Some basic coding and data processing skills will be appreciated for this application.

References :

[1] C. Champenois, Tutorial: About the dynamics and thermodynamics of trapped ions,
J. Phys. B: At. Mol. Opt. Phys. 42 154002 (2009)
[2] T. S. Strickler, et al. Experimental Measurement of Self-Diffusion in a Strongly Coupled Plasma PRX 6, 021021 (2016)

[3] B. Scheiner and Scott D. Baalrud,  Testing thermal conductivity models with equilibrium molecular dynamics simulations of the one-component plasma, Phys. Rev. E 100, 043206 (2019)
[4] S. Balruud and J. Daligault, Mean force kinetic theory: A convergent kinetic theory for weakly and strongly coupled plasmas. Phys. Plasmas, 082106 (2019)
[5] M. Marciante, C. Champenois, A. Calisti, J. Pedregosa-Guttierez and M. Knoop, Ion dynamics in a linear radio-frequency trap with a single cooling laser, Phys. Rev. A. 82 (2010) 033406.
[6] A.Poindron, J. Pedregosa-Gutierrez, C. Jouvet, M. Knoop and C. Champenois, Non-destructive detection of large molecules without mass limitation J. Chem. Phys. 154, 184203 (2021);
[7] M. Collombon, et al. Experimental Demonstration of Three-Photon Coherent Population Trapping in an Ion Cloud, Phys. Rev. Applied 12, 034035 (2019) , hal-02064988 ,

Modifié 1 fois. Dernière modification le 04/05/23 12:43 par Caroline Champenois.