Laser wakefield betatron x-ray probe for femtosecond time-resolved measurements of warm dense matter

At the ALLS laser facility¹ we have explored and optimized the wakefield acceleration of electrons to the GeV energy range and now are beginning to explore the application of the Betatron x-rays generated in such a wakefield as an ultrashort probe pulse for the study of transient heating of warm dense matter². Such broadband Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure as material transforms from solid state to Warm Dense Matter state. This can be done by observing the K-shell related absorption lines as a function of time which are directly related to the ionization states of the matter. The X-ray probe system consists of a Kirkpatrick-Baez Microscope to focus the Betatron emission to a small probe spot on the sample being measured. A separate femtosecond duration heater beam is used to heat the same spot in thin foil sample creating a warm dense matter state of known absorbed energy density. An X-ray focal spot size of 50 um was employed to probe a 50nm thick aluminum foil target uniformly heated over a region of 250um diameter. The transmitted X-ray spectrum in the region of the aluminum K-edge absorption lines (around 1.5keV photon energy) was spectrally resolved using a flat Potassium Acid Phthalate Bragg crystal spectrometer.This allowed us to measure the absorption lines in the region of the aluminum K-edge whose exact wavelength is a function of the ionization state of aluminum.The overall spectral resolution was 1.5 V, with a detection bandwidth of 24 eV in the geometry employed.However, due to the broadband synchrotron-like nature of the Betatron emission the overall photon efficiency was on the order of 10´5. Approximately 10 photons were detected by the X-ray CCD per laser shot within the 24 eV observation bandwidth.Thus a spectral measurement requires hundreds of shots.Details of the system setup and initial measuremen- results using this Betatron X-ray probe to measure the ionization states of the wann dense aluminum will be presented.