Research
Laser Cooling of Highly Relativistic Ion Beams

Laser Cooling of Highly Relativistic Ion Beams

Laser cooling in accelerators is becoming more relevant considering the upcoming FAIR facility at GSI, where the SIS100 synchrotron will serve as the core accelerator. The ion beams will be accelerated to velocities close to the speed of light. For many of the following experiments, cold ion beams, in the sense of a narrow longitudinal momentum distribution, are highly advantageous. Laser cooling is a promising approach to achieve low momentum spreads without compromising the relativistic velocities of the ions.

Schematic principle of laser cooling with three lasers.
Schematic principle of laser cooling with three lasers.

Laser cooling of highly relativistic ion beams relies on the momentum transfer through resonant absorption of photons. By detuning the laser frequency, only the fastest ions within the beam interact resonantly with the photons and therefore get slowed down. Given the initially broad momentum distribution of the ion beams, three laser systems will be employed: one continuous wave laser and two pulsed lasers. By detuning them slightly from one another, the effective cooling of the entire beam is possible.

The cw and long pulsed laser systems are developed in our group, while the short pulsed laser is developed at Helmholtz-Zentrum Dresden-Rossendorf. All are based on the principle of two second-harmonic-generation steps to reach the final wavelength of 257nm.

Second enhancement cavity with eliptical focusing.
Second enhancement cavity with eliptical focusing.

The cw laser systems consists of an external cavity diode laser (ECDL) emitting a wavelength of 1029nm. The ECDL is stabilized to allow a modehop free detuning of 25GHz. The output of the ECDL is amplified in an Yb-doped fiber. For frequency conversion, an LBO crystal is placed within a Pound-Drever-Hall (PDH) stabilized enhancement cavity to generate the second harmonic at 514.5 nm with high efficiency. Employing a BBO crystal in an elliptical enhancement cavity, the light is then frequency doubled to the wavelength of 257.25 nm. The elliptical focusing prevents the typical degradation of the BBO crystal, enabling more output power in the UV spectral range.

Reference

Jens Gumm, Denise Schwarz, Thomas Walther, High Power UV Lasers Employing Elliptically Focusing Enhancement Cavities, Review of Scientific Instruments, vol. 96, issue 3 (2025) (opens in new tab)

Schematic setup oft the cw laser system
Schematic setup oft the cw laser system
Setup of the pulsed laser system.
Setup of the pulsed laser system.

The long pulsed laser system is based on a distributed feedback (DFB) diode laser emitting a wavelength of 1029nm. The infrared light is amplified in an YB-doped fiber and then cut in to picosecond pulses. The pulses are Fourier-transform-limited with durations between 46 and 734ps with repetition rates between 1 and 10MHz. the pulses are then further amplified in four stages of Yb-doped fiber amplifiers.

The second harmonic is generated in a single-pass configuration using an LBO crystal, resulting in a wavelength of 514.5nm. Using a BBO crystal, the light is again frequency doubled to 257.25nm. The infrared seed laser can be tuned mode-hop-free over a range of 3 nm, corresponding to a tuning range of approximately 0.75 nm in the UV.

Schematic setup of the pulsed laser system

In the past years we were able to perform laser cooling experiments at the ESR at GSI with both of our laser systems. In these experiments ions (C3+) with relativistic velocities could successfully be cooled, leading to narrow momentum distributions. The pulsed systems have a broad frequency spectrum, allowing them to interact with a wide range of ion velocities at once. The cw laser achieved cooling of the ions by detuning it over the whole ion momentum distribution, achieving a momentum spread lower than with the electron cooler at the ESR. In those experiments we gained important input in which way our laser systems further need to be improved to ensure their hands-free operation.