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Relativistic electron beams and radiation sources using high power laser produced plasmas

Final Report Summary - REBRADLAP (Relativistic electron beams and radiation sources using high power laser produced plasmas)

Report on Marie Curie Incoming International Fellowship (2012-2013)

REBRADLAP
Visit of Professor Karl Krushelnick (University of Michigan USA) to
Laboratoire d’Optique Appliquée (Palaiseau France)


From September 2012 to August 2013, Professor Karl Krushelnick visited the research group of Prof. Victor Malka of Laboratoire d’Optique Appliquée (ENSTA) and Ecole Polytechnique in Palaiseau France. The visit was funded by a Marie Curie Incoming International Fellowship grant (REBRADLAP). The visit was very successful resulting in the publication of several scientific papers in collaboration with the group of Prof. Malka, with several others in preparation.
Prof. Krushelnick also gave invited lectures in Europe during this period as well as teaching at a Winter School in Les Houches (Laboratory Astrophysics). He also gave a laboratory-wide colloquium at the Rutherford Appleton Laboratory in the UK in May 2013 and participated in an experiment at the Central Laser Facility at the Rutherford Appleton Lab during January 2013.
As a result of the visit of Professor Krushelnick further collaborative experiments are planned for the upcoming year (2014) at the Laboratoire d’Optique Appliquée as well as at the Center for Ultrafast Optical Science at the University of Michigan.

Short Summary of Research Results
1. Magnetic fields in Laser Wakefield Accelerators
2. Investigation of the use of microundulators for laser driven sources of coherent radiation
1. Magnetic fields in Laser Wakefield Accelerators
The dynamics of magnetic field formation have been observed over a picosecond time scale. PIC (Particles-In-Cell) simulations agree well with the experimental data obtained[1], which shows a field that spreads over a diameter of around 250 µm with a maximum strength of approximately 3 MG. The temporal and spatial scales from the simulations are also in good qualitative agreement with the experimental results. It is clear from the simulations that the growth of the magnetic field at the ionization front of the plasma occurs simultaneously with the current of electrons streaming out of the laser wakefield region and crossing the edge of the ionization front. This population of electrons is produced by the “bowshock” of lower energy electrons (which can be 10’s to 100’s of keV) as the non-linear wakefield is formed which immediately precedes transverse wavebreaking and electron injection into the plasma wave. The temporal duration of these fields can be longer than the laser pulse since the “return current” generated in this way is composed of relatively low energy electrons which travel more slowly than the relativistic electron beam in the forward direction.
During laser wakefield acceleration experiments the formation of an extended region of magnetic field at the ionization boundary of the plasma has been revealed. These fields are indicative of the dynamics of return current generation due to the acceleration of an electron beam by the generation of highly nonlinear, 3D relativistic plasma wave structures. These magnetic fields have dynamics on sub-picosecond timescales and it is possible to use measurements of the evolution of these fields to quantify mechanisms involved in plasma wave generation and electron trapping in plasma driver electron accelerators. In particular, we may identify the point at which plasma waves transition from a primarily longitudinal plasma wave to a plasma “bubble” as a precursor to electron self-trapping and acceleration.
1) B. Walton, A. E. Dangor, S. P. D. Mangles, Z. Najmudin, K. Krushelnick A. G. R. Thomas, S. Fritzler, and V. Malka, “Measurements of magnetic field generation at ionization fronts in high intensity laser produced plasmas” New Journal of Physics 15, 025034 (2013).

2. Investigation of the use of micro-undulators for laser driven sources of coherent radiation
As part of a collaboration with LOA, the University of Michigan and the University of Florida (USA) we have investigated the use of extremely small laser-machined permanent magnetic undulators to provide a extremely compact well controlled radiation source from the laser generated electron beams [2].
The magnetic material is NdFeB, the period of the laser machined undulator is 400 microns and the width of the undulator is 200 microns. The input aperture could be varied from 200 µm to 400 or 600 µm however increase of the gap led to corresponding decrease in the magnetic field and a reduction in the K parameter of the undulator. The length of the undulators constructed initially was 50 periods.
The small value of the K parameter in general leads to difficulty for the application of this undulator in a Free Electron Laser using the laser wakefield electron source. Calculations showed such an undulator would require about 1000 periods for “good” bunching of the electron beam which is necessary for gain in an FEL. This would require an undulator structure of 40 cm in length – although this is still potentially feasible but would require improevments in the undulator machining techniques.
Initial experiments with the 50 period micro-undulator showed that the electron beam from the laser wakefield accelerator could be aligned to pass cleanly through the undulator – however undulator radiation from the interaction of the beam with the B-field could not distinguished from the bremsstrahlung produced from the beam as the outer parts of the beam collided with the undulator structure. This was mainly due to shot-to-shot variability of the electron beam pointing from the laser wakefield accelerator.
Further work to measure the x-rays resulting from the undulator and to reduce the noise by reducing the divergence of the electron beam as well as reducing the pointing instability of the electron beam is ongoing.

2) K. Krushelnick et al., “Investigations of micro-undulators as compact sources of radiation” (in preparation for submission to Physical Review – Special Topics in Accelerators and Beams, 2013).