Speaker
Description
In experiments involving beam-plasma interactions, direct imaging techniques can serve as powerful tools for in situ visualization and analysis of plasma processes and parameters. An important example of direct imaging technique is shadowgraphy, which is based on optical modulation by plasma inhomogeneities. Major limitations of existing shadowgraphic techniques, however, are their complexity and poor sensitivity to small plasma density fluctuations. Dark-field shadowgraphy, a novel design based on schlieren principles, promises to deliver orders of magnitude higher signal-to-noise ratio with less stringent requirements on probe laser parameters.
To demonstrate the use of dark-field shadowgraphy in probing beam plasma interactions, we present preliminary results from two experimental campaigns at the FACET-II, SLAC. The first experiment (E340) aims at studying the transition between beam-driven plasma waves (PWFA) and a wakeless regime characterized by breakdown of periodic wave structures and formation of ion channel in a narrow plasma column. Implementing dark shadowgraphy, we observed a clear distinction between image patterns from the PWFA and wakeless regime with good agreement to expected plasma wave periods, numerical simulations, and other experimental measurements. The second experiment (E305) aims at characterizing filamentation instabilities formed by an ultra-relativistic electron beam interacting with a plasma background, the same physical process powering important astrophysical phenomena such as gamma ray bursts. First dark shadowgraphy images from the experiment indicated sensitivity to instability-forming conditions, promising use of the diagnostics in laboratory astrophysics platforms for probing beam plasma instabilities.
| Working group | WG3 |
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