Prof. Dr. Dietmar Block

Contact

Dietmar Block

Prof. Dr. Dietmar Block

Institute for Experimental and Applied Physics
Experimental Plasma Physics
Kiel University
Leibnizstraße 15, R. LS15-67
24118 Kiel
Tel.: +49 (0) 431 880 3862
Fax: +49 (0) 431 880 3809

Email

ORCID iD

Researchgate

 

 

 

Research Interest


I have worked on several topics of plasma physics so far. A special focus has always been on the development of advanced diagnostics to study structure formation and transport processes. This starts with impedance probes for absolute electron density measurements in the lower ionosphere, continued with spatio-temporally resolved measurements of drift wave turbulence and techniques to control and suppress turbulence and transport and has concentrated in the last years on laser manipulation of complex plasmas. Digital holography, optical tweezers, and advanced Mie scattering diagnostics are tools which are currently used in my group to investigate structure and dynamics of finite plasma crystals and binary mixtures. Please find below a short description of each research area and some related/recent publications. Thank you for your interest in my research!


Complex Plasmas - a laboratory for strong correlations

Complex plasmas are plasmas which besides electrons and ions contain highly charged micro-particles. The charging is a result of the electron and ion currents to the particle surface. As a consequence of the high charge the particles arrange in regular patterns and can form so called plasma crystals. Thus, these complex plasmas are a strongly coupled system, i.e. the thermal energy of the particles is much less than their interaction energy. My group performs experiments and MD-simulations in the field of complex plasmas since 2002. Some of my personal research highlights as well as current research topic are listed below. The papers listed right below are tutorials I have contributed to and which might help to get an overview.
 

 

Binary Mixtures

In the frame work of the DFG-Project BL555-3, we investigate the special properties of binary mixtures in complex plasmas. So far, complex plasma studies concentrated on monodisperse micro-particles which turned out to be an excellent model system for strong coupling. Systems with two different particle species (i.e. with different charge) are called binary mixtures. In complex plasmas all experiments so far show that the two particle species instantaneously demix. Our recent experiments however show that it is possible to create binary mixtures. This opens unique opportunities to study structural and dynamical phenomena in strongly coupled systems. Normal modes, waves, phase transitions, transport processes and heat conduction are just a few examples which can be addressed for the first time in binary mixture experiments. Therefore, this project aims at a systematic characterization of the structural and dynamical properties of binary mixtures in complex plasmas.

  • Frank Wieben and Dietmar Block, Photophoretic force measurement on microparticles in binary complex plasmas, Physics of Plasmas 25, 123705 (2018); https://doi.org/10.1063/1.5078561

  • Frank Wieben, Jan Schablinski, and Dietmar Block, Generation of two-dimensional binary mixtures in complex plasmas, Physics of Plasmas 24, 033707 (2017); https://doi.org/10.1063/1.4977989


Bild vom Lasersystem und der Plasmakammer

Recently, we managed to explore basic thermodynamic properties of complex plasmas and especially binary mixtures. In the Dezember 2019 issue of PRL (https://doi.org/10.1103/) we reported on successful direct measurements of entropy and its change during heating and cooling. Although entropy is one of the most fundamental quantities in thermodynamics, its direct measurement is impossible in most systems. Using two-dimensional plasma crystals, a sophisticated laser heating mechanism to heat these crystals without melting and high precision particle tracking to measure the full phase space allowed to study the change of entropy in a heating and cooling experiment. It turned out that these systems strictly follow the laws of thermodynamics even if mixtures of different particles are used. The results show on the one hand side that complex plasmas are a model system for strongly coupled systems even with respect to thermodynamic properties and on the other hand side that fundamental studies of systems with non-Maxwellian distributions should be feasible.

Diagnostics

We have worked on a number of diagnostic tools for complex plasma research:

  • Angular and polarization resolved Mie scattering
    Based on a setup developed by Asnaz et al. we studied the achievable resolution for size measurements using angular and polarization resolved Mie scattering. The basic idea is that the angular resolved scattered ligth of a particle already gives  reasonable information on its size. If this information is combined with polarization the size resolution can be increased significantly. A scetch of the angular and polarization resolved scattered light intensity is depicted below. Especially the merging and splitting of intensity maxima as a function of polarization angle provides rich information. We demonstrated that the achievable resolution of this setup is below a nanometer (see Kohlmann et al.).

    Lichtstreuung an einem Partikel (Skizze)









     
  • An optical tweezer for complex plasmas
    To manipulate position and dynamics of single particles in a dusty plasma is a sphisticated task. The concept of optical twezers, which have been invented by Ashkin et al., is not easily transfered to complex plasmas. Conventional tweezers require optices with a very high aperture. However, the experimental setup with a vacuum system, electrodes and a volume for plasma generation inhibit a straight forward application of Ashkins concept. That tweezers can nevertheless be realized to tweeze single dust particles was shown by us in 2015 (see Schablinski et al.). Our tweezer uses a moderate aperture and in addition makes use of confinement forces in the plasma itself (see sketch below).

    Forces on particles in an optical tweezer

    See F. Wieben, J. Schablinski, and D. Block,  Modification of microparticles due to intense laser manipulation.  Physics of Plasmas 26, 033701 (2019) https://aip.scitation.org/doi/10.1063/1.5090452 for some further reading and application of the tweezer setup.
     
  • Laser heating
    To manipulate the particle dynamics in 2D and 3D dust clouds is a well established technique. Our laser heating setup uses four laser beams to
    achieve a perfect Maxwellian velocity distribution. The intensities of the beams are matched to generate zero momentum transfer on average and achieve artefact free velocity spectra. Thus, this setup can be regarded as a perfect thermostat for 2D plasma clouds. A detailed description of its function is found in our paper in Physic of Plasmas. The image below shows the current setup in our experiment.

    Laser heatting setuo
     
  • Digital holography
    To measure the positions in 3D of particles in dust clouds is still a challenge. Basically to techniques have shown to be successfull: stereoscopic imaging (developed and operated by AG Melzer at University Greifswald) and digital holography. Our experiments in 2008 (see Kroll et al.) were the first to show that holographic imaging of dust clouds is possible and yields precise information on 3D positions.

 

 

 

Finite systems

One speciality of complex plasmas is that it is possible to study small bounded systems of a few particles only. Further these systems can be studied with 1D, 2D, and 3D geometry. In the frame work of the SFB TR24 Fundamentals of Complex Plasmas  we have studied in close collaboration together with the groups of Prof. Melzer and Prof. Bonitz finite 3D systems, the so called Yukawa balls. We were able to understand their fundamental construction principle als well as to study their dynamical and even thermodynamical properties. 
 

  • Structure  (shells, parabolic density profile, meta stable configurations)
  • Dynamics (Normal modes, shear stability)
  • Thermodynamics (phase transitions, melting point)