1. In-Situ Measurements of Laser-Driven Relativistic Electron Currents in Underdense Plasma M.C. Kaluza, H.-P. Schlenvoigt, B. Beleites, F. Ronneberger, and H. Schwoerer Institute for Optics und Quantum Electronics, University of Jena, Germany S.P.D. Mangles, A.G.R. Thomas, Z. Najmudin, C.D. Murphy, A.E. Dangor, and K. Krushelnick Imperial College London, United Kingdom 385. Wilhelm and Else Heraeus Seminar, Bad Honnef, 13 th February 2007

2. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 2 Outline Motivation Faraday-rotation technique Experimental setup Experimental results from Faraday-rotation Discussion and outlook

3. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 3 Motivation Use of laser-generated mono-energetic electron beams for various applications, e.g. generation of ultra-short X-ray pulses, requires controllable electron beam generation. Direct insight into acceleration dynamics desirable. Non-invasive diagnostic high spatial and temporal resolution!  Faraday-rotation technique

4. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 4 Faraday-Rotation Technique  Rotation of probe polarization:  Measure  rot  and n e all over the plasma to get B-field distribution! Probing of magnetic fields in plasma with linearly-polarized pulse: If  magnetically induced difference of  for the two circularly-polarized probe components

5. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 5 Experimental Setup J ETI laser parameters: E laser = 800 mJ,  laser = 80 fs, f/6 OAP, I laser  2…3  10 18 W/cm 2 Probe pulse:  probe  100 fs, probe = 800 nm

6. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 6 Faraday-Rotation Measurements Two polarograms from two (almost) crossed polarizers: polarogram 1 Deduce rotation angle  rot from pixel-by-pixel division of polarogram intensities: polarogram 2 560 µm 340 µm

7. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 7 Faraday-Rotation Measurements Really due to a rotation of the plane of polarization? 50 µm main pulse

8. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 8 Faraday-Rotation Measurements Reversal of intensity ratio by changing polarizer angles:  Intensity changes indeed due to rotation of probe polarization! How large are rotation angle  rot and magnetic field B(r) ?

9. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 9 Faraday-Rotation Measurements Deduction of rotation angle  rot :

10. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 10 Faraday-Rotation Measurements Deduction of rotation angle  rot : suppression necessary beyond plasma boundaries

11. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 11 Faraday-Rotation Measurements Measurement of electron density n e :

12. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 12 Faraday-Rotation Measurements Measurement of electron density n e :

13. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 13 Faraday-Rotation Measurements Measurement of electron density n e :

14. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 14 Faraday-Rotation Measurements Calculation of magnetic field distribution: modified Abel-inversion of  rot (assuming cyl. symmetry)  B. Walton, Ph.D. thesis, IC London (2004)

15. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 15 Faraday-Rotation Measurements Calculation of magnetic field distribution: modified Abel-inversion of  rot (assuming cyl. symmetry)  B. Walton, Ph.D. thesis, IC London (2004)

16. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 16 Faraday-Rotation Measurements Measured B-field with peak of ~ 0.5 MG is eqivalent to ~ 2.8 kA, ~ 8 µm radius, ~ 100 fs duration (limited by temporal resolution) ~ 280 pC electron bunch Calculation of magnetic field distribution: modified Abel-inversion of  rot (assuming cyl. symmetry)  B. Walton, Ph.D. thesis, IC London (2004)

17. Malte Kaluza Measurement of Relativistic Electron Currents Heraeus-Seminar 13 th February 2007 17 Summary Measurement of magnetic field distributions from laser-driven electron currents in underdense plasma using Faraday-rotation Direct observation of highly localized electron current: I bunch ~ 2.8 kA, r bunch ~ 8 µm,  bunch ~ 100 fs, Q bunch ~ 280 pC Experimental data currently analyzed: - temporal evolution of electron current, - dependence on electron density For more information see poster by H.-P. Schlenvoigt