SPARC: laser spectroscopy and laser cooling

The SPARC Working Group “Laser Spectroscopy” is involved in the preparation of the laser spectroscopy experiments at the FAIR storage rings ESR, HESR and CRYRING. Within the framework of these research projects, our activities are concentrated on the technical development of laser and detection systems, and on (the optimization of) detection and measuring methods. In the following, an overview is given of the concrete projects of the different groups and their common research activities, and how the groups shall work together. For details we like to refer to the websites of the individual collaboration partners, as listed below.

Laser spectroscopy of highly charged ions and exotic atoms is an important part of the current GSI atomic physics research program, and will be extended by the considerable advantages of FAIR. This work is on the one hand motivated by investigations of the structure and dynamics of highly charged ions, and on the other hand by tests of fundamental constants. The laser-spectroscopic investigations range from heavy ions at relativistic velocities in storage rings (ESR and later HESR), to ions almost at rest at cryogenic temperatures in traps (SPECTRAP at HITRAP). Here, the bases for later applications of this procedure, which will be to only realistic and most effective cooling method for such high energies, are studied. 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.

https://www.gsi.de/fileadmin/_migrated/pics/Laserspektroskopie_Bild_1.jpg

Optical diagnostics

Prof. Weinheimer and Volker Hannen (University of Münster) concerns itself, among other things, with the development of optical detection systems for the ESR, and of single-photon detectors for SPECTRAP. The recent development of a new mirror system at the ESR significantly improved the detection efficiency of infrared photons emitted by relativistic ion beams (0.7 c) and therefore the long seek hyperfine splitting resonance in lithium-like bismuth was observed for the first time. Presently this group is developing another detector in order to monitor XUV-photons. The first application of this detector is foreseen for the coming beryllium-like krypton experiment at the ESR, but also opens the perspective of introducing this kind of setup for future laser experiments at the HESR. Furthermore the construction of a fluorescence detection chamber for CRYRING is planned. 

Development of continuous and pulsed laser systems

This is the area in which the groups of Prof. Birkl, Prof. Nörtershäuser, Prof. Spielmann and Prof. Walther bring their expertise.

Prof. Walther’s group at the TU Darmstadt has e.g. experience in the realization of widely-tunable diode lasers and solid state lasers. They have, for example, development a fast tunable cw-laser system with a broad tuning range, which was successfully used in a laser-cooling beamtime at ESR. The development and use of this kind of lasers is already foreseen for upcoming laser experiments at FAIR. More information is given in the working package “Laser cooling”, which is interconnected with the laser spectroscopy groups at the TU Darmstadt.

The group of Prof. Birkl (TU Darmstadt) has already implemented a laser system for the excitation of the 1s-hyperfine transition (at 244 nm) in hydrogen-like bismuth (Bi82+), developing in addition a frequency stabilization system,  which is required for high-precision laser spectroscopy. The system will be used at SPECTRAP experiment. The group is presently developing the laser system for precision laser spectroscopy at ARTEMIS.

Prof. Nörtershäuser’s group (TU Darmstadt) has developed a frequency quadrupling system for a fiber laser system, used for laser cooling of Mg+ ions in SPECTRAP, which has already been demonstrated successfully in recent beamtimes. Moreover, thanks to previous BMBF funding, a pulsed dye laser system was acquired, optimized and used for measurements of the ground state hyperfine splitting in hydrogen and lithium-like bismuth at the ESR. The laser was recently upgraded and a frequency doubler was also acquired and it will be use it in the upcoming laser spectroscopy experiment on beryllium-like krypton at the ESR. 

Novel high photon-flux XUV laser sources are currently developed at the Friedrich-Schiller-University Jena and the Helmholtz Institute Jena. A collaboration of the groups of Prof. Jens Limpert and Prof. Christian Spielmann will develop powerful, compact and remote-controlled XUV laser sources based on fiber-laser technology and high harmonic generation. These sources will provide photon energies in the range of 20 to 100 eV with sufficient photon flux (up to 1013 photons/s) to effectively excite high-energy transitions in highly charged ions. In combination with the Doppler up-shift in head-on excitation at the maximum energy of the ESR (v=0.73c), for example the s1/2-p1/2 transition for Li-like Silver (Z=47) at ≈100 eV can be reached with only 40 eV photons. At HESR the same transition can be excited up to li-like Uranium. Currently, the development is focused on reducing the relative energy bandwidth of the XUV laser source to below 10-4. Within the BMBF funding period, a prototype XUV source will be created and delivered to GSI for first experiments that will be conducted at the ESR. A joint collaboration with the Atomic-Physics group at GSI and the groups of Prof. Nörtershäuser and Prof. Weinheimer will design the experimental setup, including suitable fluorescence detector, and conduct the experiments. In future, the XUV-Laser source will be employed at the HESR for ground-braking spectroscopic experiments and beam preparation.

Data Acquisition and HV Diagnostic

Prof. Nörtershäuser’s group has also developed the essential parts of a new data acquisition scheme, using single photon tagging. Here the support of the Experimental Electronic Division at GSI via the development of the VUPROM-based TDC for the photon tagging was provided. The group is also working in close collaboration with the PTB in Braunschweig and Prof. Weiheimer’s group (Münster) in order to facilitate high-voltage measurements, for example at the Electron Cooler at the ESR.  

The development of custom-made pulsed and continuous laser systems is of crucial importance for laser spectroscopy of relativistic ion beams in the ESR, HESR and CRYRING, and, in addition, for investigations of cold, trapped ions in ion traps. 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 experiments.

Laser Equipment at the ESR

Infrastructure for laser experiments at ESR

 

 

https://www.gsi.de/fileadmin/_migrated/pics/LaserEinrichtungenESR_web.jpg
Simplified schematic view of the ESR showing some parts of the setup needed for the laser beam transport.

Laser excitation of the ions can be performed either in the collinear or anticollinear configuration at any of the two straight-sections of the ring. At the end of these sections viewports are installed through which the light can be coupled into the beamline.

 

Because of safety reasons the ESR-Cave cannot be accessed when the ion beam is on, therefore the laser systems have to be placed outside the concrete walls. Our laser laboratory is located at the south-west part of the Cave. From here the laser beam is transported using either glass optical fibers or by air using high-reflectivity optical mirrors. In the latter case the laser beam is shielded by protective pipes and cages. The length transport between the laser laboratory and the interaction region at the ESR can be as long as 80 m.

 

In order to detect the fluorescence (photons) emitted by the ions an optical detection region has been implemented after the gas target. Presently two fluorescence collection systems are implemented inside the vacuum tube:

 

https://www.gsi.de/fileadmin/_migrated/pics/NachweissystemA_web.jpg
a) A mirror system consisting of ten segmented aluminium foils. The vacuum tube at this place has 3 viewports where up to 3 PMT’s (PMT: Photomultiplier) can be installed. The red arrow indicates the ion beam direction. The foils are arranged to reflect the photons emitted under an angle ≥ 20° with respect to the ion beam direction.
https://www.gsi.de/fileadmin/_migrated/pics/NachweissystemB_web.jpg
b) The second mirror system is optimized to reflect photons emitted under smaller angles. It consists of a parabolic cupper-mirror, mounted on a retractable linear feedthrough. The mirror has a slit, through which the ion beam can pass. The reflected photons pass then a viewport, a light-pipe and a glass-filter before they hit the detector. The collection efficiency is enhanced by an additional reflecting cone. The mount which holds the detector, the light-pipe and the linear feedthrough is not shown.