Experimental study of a high intensity radio-frequency cooler

Within the framework of the DESIR/SPIRAL-2 project, a radio-frequency quadrupole cooler named SHIRaC has been studied. SHIRaC is a key device of SPIRAL-2, designed to enhance the beam quality required by DESIR. The preliminary study and development of this device has been carried out at Laboratoire de Physique Corpusculaire de CAEN (LPC Caen), France. The goal of this paper is to present the experimental studies conducted on a SHIRaC prototype. The main peculiarity of this cooler is its efficient handling and cooling of ion beams with currents going up as high as $1\mu \mathrm{A}$ which has never before been achieved in any of the previous coolers. Much effort has been made lately into these studies for development of appropriate optics, vacuum and rf systems which allow cooling of beams of large emittance ($\sim 80\pi \mathrm{mm}\mathrm{mrad}$) and high current. The dependencies of SHIRaC’s transmission and the cooled beam parameters in terms of geometrical transverse emittance and the longitudinal energy spread have also been discussed. Investigation of beam purity at optimum cooling condition has also been done. Results from the experiments indicate that an emittance reduction of less than $2.5\pi \mathrm{mm}\mathrm{mrad}$ and a longitudinal energy spread reduction of less than 4 eV are obtained with more than 70% of ion transmission. The emittance is at expected values whereas the energy spread is not.

Figure 1

Schematic of the SHIRaC optics system: Design of the three parts of SHIRaC.

Figure 2

3D layout of the differential pumping vacuum system of SHIRaC.

Figure 3

Design of the rf electronic system (top) and the real rf system of SHIRaC (bottom).

Figure 4

The voltages delivery scheme to the quadrupole electrodes of the rf supply and the power supplies for the dc axial guiding voltage.

Figure 5

Plot of the rf voltage amplitudes as a function of the input voltages for various frequencies set in the function generator and for a configuration of eight turns per hollow coil.

Figure 6

3D schematic of the planned test setup for the SHIRaC prototype.

Figure 7

Space charge effect on the ion transmission: variation of the transmission versus beam current for 2.5 Pa buffer gas pressure.

Figure 8

Stability diagram at $1\mu \mathrm{A}$ beam current: variation of the transmission as a function of the Mathieu parameter q for various buffer gas pressure.

Figure 9

An overview of the MCP detector (top) and cooled ions TOF spectrum for $1\mu \mathrm{A}$ of beam current under the optimum cooling conditions (bottom).

Figure 10

Measurements of geometric transverse emittances versus Mathieu parameter $q$ (i.e., rf voltage) for various beam currents.

Figure 11

Measurements of the longitudinal energy spread with various buffer gas pressure $P_{\mathrm{RFQ}}$ and for beam current of 50 nA: variation of the transmission as a function of the retarding dc voltage (top) and variation of its differential versus the retarding dc voltage (bottom).

Figure 12

Space charge effect on the longitudinal energy spread: variation of the longitudinal energy spread with the beam current.

Figure 13

The rf voltage amplitude effect on the longitudinal energy spread: variation of both the energy spread and its corresponding transmission as a function of the Mathieu parameter $q$.