SPARC Laser cooling

Introduction

This working group prepares for laser cooling experiments at the FAIR storage rings and synchrotrons. These experiments require continuous technical advancement, both in the area of laser development, as well as in detection and analysis methods, in order to master the challenges at the new experimental facilities.

An important development for FAIR is laser cooling of relativistic ions at the experimental storage ring (ESR). Here, the fundamentals for later applications of this procedure to highly relativistic ion beams at the SIS100/300 are being made. Laser cooling is the only cooling method in this energy range that will be available. All these experiments require continuous technical advancement, both in the area of laser development, as well as in detection and analysis methods, in order to master the challenges at the new experimental facilities. Laser cooling of relativistic heavy ion beams has already been demonstrated successfully at GSI in 2004 and 2006. This work has been coordinated by the experts Prof. Schramm (TU-Dresden) and Dr. Bussmann (HZDR), who will - based on their vast experience - also lead these experiments in the future. Within GSI, laser cooling is being driven by Prof. Thomas Kühl und Dr. Danyal Winters, who are also both involved in laser spectroscopy.

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Schottky spectrum of a stored and bunched C3+ ion beam in the ESR at GSI Darmstadt. When the laser scans (scan time = 10 s) the number of synchrotron sidebands (vertical lines) is reduced, as indicated by the slanted green lines, giving a clear sign of laser cooling. The cooling time was set by the scanning time of the laser and was much shorter than the beam life time of about 400s.
Optical diagnostics

The group of Prof. Birkl (TU-Darmstadt) concerns itself, among other things, with the use of fluorescence detectors for optical diagnostics. This is absolutely necessary, since in previous beamtimes it was shown that the evaluation of Schottky spectra offers only limited access to the dynamics of laser-cooled ion beams. Therefore, the construction of a dedicated optical detection chamber is intended, which is optimized first for the interests of laser cooling, but will also be laid out to serve other experiments. Here, synergies arise with the group of Prof. Weinheimer (University of Münster) concerning the development of optical detection systems for the ESR, and of single-photon detectors.

Development of continuous and pulsed laser systems

The group of Prof. Walther at the TU Darmstadt has e.g. experience in the realisation of widely-tuneable diode lasers and solid state lasers. They have already demonstrated that during the previous BMBF funding period, by the development of a fast tuneable cw-laser system with a broad tuning range, as was required for laser cooling (2012). This fast tuneability will, in the future, allow for short cooling times at the ESR, and for the development of new optical ion beam diagnostics. In cooperation with the group of Prof. Schramm (TU-Dresden), the next step is to develop a complementary, pulsed, bandwidth-limited, picosecond laser system, which pulse duration is tuned such to perfectly match the initial momentum spread of the ion beam. This will make it possible to simultaneously cool all the „hot“ ions in the beam, and thus conserve the high phase-space density of the cooled ion beam – a procedure that will be indispensable for future applications of laser cooling at FAIR.

While the group at the TU-Darmstadt will create the picosecond frontend, the group of Prof. Schramm at the HZDR has expertise in the development of short-pulse high-power lasers, which is required for the construction of an associated amplifier system. The techniques and procedures required for this project will also interesting for the development of other new laser systems for spectroscopy at FAIR.

The development of custom-made pulsed and continuous laser systems is of crucial importance for laser cooling and spectroscopy of relativistic ion beams in the ESR (HESR, SIS100/300). In the framework of this BMBF research co-operation, the above mentioned groups will interlace themselves further, and will use their expertise to contribute to the different laser spectroscopy and laser cooling experiments.

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