RF-Carpets for Exotic Nuclei: Design, Simulation and Measurements

  Timo Dickel [1,2]  ,   Samuel Ayet San Andrés [1,2]  ,  Daler Amanbayev [1]  ,  Hodaya Dafna [3]  ,  Boaz Kaizer [3]  ,  Israel Mardor [3,4]  ,  Ivan Miskun [1]  ,  Amichay Perry[3]  ,  Wolfgang R. Plaß [1,2]  ,  Christoph Scheidenberger [1,2]  ,  and FRS Ion Catcher Collaboration   
Justus-Liebig-University Giessen, Giessen, Germany
GSI Helmholtzcenter for heavy ion research, Darmstadt, Germany
Soreq Nuclear Research Center, Yavne, Israel
Tel Aviv University, Tel Aviv, Israel

At the upcoming accelerator facilities such as FAIR and SARAF-II, high precision experiments will be performed with exotic short-lived nuclei, including mass measurements. In order to do experiments with highest accuracy the exotic nuclei have to be slowed down to almost at rest. This is achieved by stopping and thermalizing the ions in a cryogenic gas-filled stopping cells. To extract the ions from the gas, an RF carpet is used to provide a repelling force and DC field to guide the ions to the extraction nozzle.

These cryogenic gas-filled stopping cell have an active volume of 50 to 200 l filled with helium, and are operated at temperatures of about -200°C and pressures up to 300 mbar. In the system currently operational at the FRS Ion Catcher experiment a PCB based RF carpet has a diameter of 250 mm and concentric electrodes with a density of 4 electrodes per mm. An RF frequency of 6.5 MHz and RF amplitudes up to 130 Vpp are used. Along the stopping volume DC fields are up to 50V/cm.

With the help of simulations the electronics, geometry, RF frequency and amplitude of the RF carpet have been optimized for fast transport along the RF carpet without ion losses, while minimizing the power input to the cryogenic system. The electronics of the RF Carpet were designed for the desired operation frequency, trying to dissipate as less heat as possible. Simulations of ion motion in the stopping cell, the electronic design of the RF Carpet and measurements are is good agreement. High accuracy mass measurements (<ppm) with the MR-TOF-MS coupled to the cryogenic stopping cell of the very short lived 213Rn (19.5 ms half-life) have been performed and show the unique capabilities of these devices.

Currently an improved driving electronics for the next generation RF carpet is in development.