Shaukat Khan

Generation of Ultrashort and Coherent Synchrotron Radiation Pulses at DELTA

Pump-probe experiments to study ultrafast dynamic phenomena such as electron transfer, lattice vibrations, phase transitions, chemical reactions, or spin dynamics require two short radiation pulses as well as good control of the time delay between them. The first pulse to excite (“pump”) the sample under study is usually a femtosecond laser pulse in the near-visible regime. For the second pulse to analyze (“probe”) the state of the sample as a function of the delay, however, light with shorter and tunable wavelength would be desirable. Conventional synchrotron light sources produce pulses with a typical duration of 30–100 ps (FWHM), given by the electron bunch length in a storage ring, which is not well suited for ultrafast studies. The bunch length can be reduced to a few picoseconds in the so-called low-α mode (e.g., [1]) by lowering the momentum compaction factor α of the storage ring. Radiation pulses in the femtosecond range, however, are obtained more easily by extracting synchrotron light from a small fraction of the electron distribution, rather than the whole bunch, which can be achieved with the laser-based methods described below.

Coherent Harmonic Generation at DELTA: A New Facility for Ultrashort Pulses in the VUV and THz Regime

Synchrotron radiation sources producing high-brilliance radiation with tunable wavelength in the VUV to X-ray regime are ideally suited to study the structure of matter on the atomic scale. With a pulse duration of typically 30-100 ps (FWHM), however, they cannot resolve dynamic phenomena such as lattice vibrations, phase transitions, chemical reactions, or fast magnetization changes. Mode-locked lasers, on the other hand, produce intense pulses with a duration below 100 fs, but only at near-visible wavelengths. In the quest for short pulse duration and short wavelength, good progress has been made. High harmonic generation (HHG) or X-rays from a laser-generated plasma have become standard techniques in many laser laboratories. Linac-based free-electron lasers (FELs) have reached the Ångström wavelength regime, a pulse duration of a few femtoseconds, and unprecedented peak brilliance [1]. As for conventional synchrotron light sources, pulses of a few picoseconds can be produced by lowering the momentum compaction factor and thus reducing the bunch length (see e.g. [2]). Even shorter pulses are obtained by combining electron bunches with femtosecond laser pulses.

Measurement of the Laser-Induced Energy Modulation Amplitude at the Short-Pulse Facility at DELTA

The short-pulse facility at the synchrotron light source DELTA operated by the TU Dortmund University employs coherent harmonic generation (CHG) to provide ultrashort pulses in the vacuum ultraviolet and terahertz regime. Here, a laser-electron interaction results in a modulation of the electron energy which is transformed into a density modulation by a magnetic chicane. Measurements of the energy modulation amplitude with different techniques including an RF phase modulation are presented. A combination of the results allow to estimate the energy spread of the electron beam.

Investigation of Beam Instabilities at DELTA using Bunch-by-bunch Feedback Systems

At the 1.5-GeV electron storage ring DELTA operated by the TU Dortmund University as a synchrotron radiation user facility, bunch-by-bunch feedback systems are in use for electron beam diagnostics and for the suppression of multibunch instabilities. An automatic readout of bunch position data allows a real-time modal analysis during machine operation. An excitation of particular multibunch modes enables the determination of growth and damping times for all modes independently. Further investigations of beam stability and natural damping times of all modes even below the instability threshold have been performed. In addition, first bunch-by-bunch data taken from the booster synchrotron are shown.