There is an increasing need for techniques allowing a whole Terahertz spectrum (or a electric field time-evolution) to be recorded at each shot of a pulsed source. We review here the principles and performances of the recently introduced DEOS single-shot Time-Domain Spectroscopy method [1]. A key point of DEOS is a novel conceptual approach of a classic electro-optic detection method, that uses chirped laser pulse probes. This novel point of view led to a new type of design that allows a numerical reconstruction of the input THz signal, from a single-shot measurement, with unprecedented bandwidth and time-resolution. We present here the theoretical framework, experimental tests, as well as numerical investigations aiming at exploring the bandwidth and resolution limits of DEOS.
[1] Roussel et al., Light Science & Applications 11, 14 (2022) https://doi.org/10.1038/s41377-021-00696-2
KEYWORDS: Sensors, Field programmable gate arrays, Free electron lasers, Data processing, Electronics, Silicon, Synchrotrons, Analog electronics, Data acquisition, Diagnostics
KALYPSO is a novel detector operating at line rates above 10 Mfps. The detector board holds a silicon or InGaAs linear array sensor with spectral sensitivity ranging from 400 nm to 2600 nm. The sensor is connected to a cutting-edge, custom designed, ASIC readout chip, which is responsible for the remarkable frame rate. The FPGA readout architecture enables continuous data acquisition and processing in real time. This detector is currently employed in many synchrotron facilities for beam diagnostics and for the characterization of self-built Ytterbium-doped fiber laser emitting around 1050 nm with a bandwidth of 40 nm.
The Accelerator for the European X-Ray Free Electron Laser delivers femtosecond electron bunches at an energy of currently 14GeV at a repetition rate of up to 4.5MHz in bursts of up to 2700 pulses every 100ms to distribute them between different undulator beamlines. The emitted femtosecond x-ray laser pulses at wavelengths between 0.05nm and 6nm can serve up to three experiments in parallel.
To measure the longitudinal bunch profile of the electron bunches, three detection systems based on electro-optical spectral decoding have been installed and are currently being commissioned. The systems are capable of recording individual longitudinal bunch profiles of all bunches in a burst with sub-ps resolution at a bunch repetition rate of 1.1 MHz, sampling the electron Coulomb field with laser pulses at 1030nm. A short detector latency of about 10µs also gives the prerequisites to establish a fast intra-burst feedback to stabilize the bunch profile. Bunch lengths and arrival times of entire bunch trains with single-bunch resolution have been measured as well as jitter and drifts for consecutive bunch trains.
For comparison of detection techniques at one position, the laser signal is split and measured with a time-stretch setup in parallel.
The required high peak current in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. A novel in-vacuum polychromator (CRISP4) has been developed for measuring coherent radiation in the THz and infrared range. The polychromator is equipped with five consecutive dispersion gratings and 120 parallel readout channels. It can be operated either in short (5-44 μm) or in long wavelength mode (45-430 μm). Fast parallel readout permits the monitoring of coherent radiation from single electron bunches. Due to the large wavelength range covered and the absolute calibration of the device, Kramers-Kronig based phase retrieval allows to online reconstruct a longitudinal bunch profile from the measured coherent radiation spectrum. The device is used as a bunch length monitoring and tuning tool during routine operation at the Free-electron Laser in Hamburg (FLASH). Comparative measurements with the transverse deflecting structure show excellent agreement of both methods.
A high peak current, low emittance, short pulse electron beam can produce intense, laser-like radiation in a single pass through a long periodic magnetic structure. The construction of such free-electron lasers (FELs) based on self-amplified spontaneous emission (SASE) has become feasible by recent advances in accelerator technologies. Since SASE FELs do not require any optical components they are promising sources for the generation of intense, sub- picosecond laser pulses which are continuously tunable over a wide wavelength range in the vacuum ultraviolet (VUV) and X-ray region. In the first phase of the VUV-FEL (phase I) at the TESLA Test Facility at DESY, SASE was achieved for the first time in the VUV at wavelengths between 80 and 180 nm. The concept of the VUV FEL at DESY and first experimental results are presented. The second phase of the TESLA Test Facility (phase II), which includes an increase of the electron beam energy to 1 GeV, aims at the construction of a SASE FEL operating in the soft X-ray region. An overview of the current status and the activities toward a soft X-ray FEL user facility is given.
Conference Committee Involvement (1)
Advances in X-ray Free-Electron Lasers II: Instrumentation
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