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5. What's New at HRIBF - Laser Ion Source Development
(Yuan Liu, Spokesperson)

We have recently made good progress in realizing a laser ion source to extend the RIB beam capability at HRIBF. New measurements of resonant laser ionization on several elements of interest have been recently conducted at HRIBF with a new commercial Ti:Sapphire laser and two additional Ti:Sapphire lasers from the University of Mainz. The commercial Ti:Sapphire laser was a prototype of the Ti:Sapphire laser that is being developed by Photonics Industries International, Inc, in Bohemia, New York. It will be the first commercial laser that meets the specific requirements for a resonant ionization laser ion source (RILIS) system. The Ti:Sapphire laser was procured in 2006 and the final product is expected to be delivered in 2007.

In collaboration with the research group led by Klaus Wendt of the University of Mainz, the Photonics Ti:Sapphire laser was successfully incorporated with two Ti:Sapphire lasers provided by the Mainz group and a standard hot-cavity surface ionization source developed at HRIBF. A 100-W Nd:YAG laser at 532 nm, also newly purchased by HRIBF, provided the required pump for the three Ti:Sapphire lasers. Shown in Fig.5-1 is a photo of the RILIS laser system and associated optical setup in operation at the HRIBF off-line Ion Source Test Facility 2 (ISTF-2).

Figure 5-1: The RILIS laser system used for the measurements at HRIBF.

Three photon ionizations of Sn, Ni, Mn, Fe and Al have been obtained with the new laser system. This work is the first time Mn and Fe ion beams are produced in a hot-cavity LIS using Ti:Sapphire lasers. More than 1.2 μA of Sn and Mn, and more than 500 nA of Ni laser ions were observed. The ionization schemes used for Mn and Fe are illustrated in Fig. 5-2. The final ionization step is of crucial importance for both the efficiency and selectivity of the RILIS. Resonant ionisation via autoionizing or Rydberg states can significantly increase the ionization efficiency and reduce the laser power needed for saturation of the transitions. The Photonics Ti:Sapphire laser is unique in (1) it uses only two mirror sets to cover the fundamental output range of 700-960 nm, and (2) it provides continuous wavelength tuning over the full wavelength range. Therefore, it is ideally suitable for RILIS applications with considerably simplified operation. Accordingly, spectroscopic studies of high lying Rydberg levels and autoionizing states in Ni, Mn, Fe, and Al were conducted using the Photonics Ti:Sapphire laser for the last ionization step. Both Rydberg states and autoionization states were successfully observed in Ni, Mn and Fe. The spectrum of the Rydberg levels in Mn is shown in Fig. 5-3. Analysis of the observed Rydberg and autoionization states is in progress.

This work is an important step in developing a state-of-the-art RILIS system based on all-solid-state Ti:Sapphire lasers for the HRIBF research program.

Figure 5-2: Three-photon excitation and ionization schemes for Mn and Fe.

Figure 5-3: Measured Rydberg series in Mn versus the wavelength of the third excitation step.

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