Development of a radio-frequency quadrupole cooler for high beam currents

The SHIRaC prototype is a recently developed radio-frequency quadrupole (RFQ) beam cooler with an improved optics design to deliver the required beam quality to a high resolution separator (HRS). For an isobaric separation of isotopes, the HRS demands beams with emittance not exceeding $3\pi \mathrm{mm}\mathrm{mrad}$ and longitudinal energy spread $\sim 1\mathrm{eV}$. Simulation studies showed a significant contribution of the buffer gas diffusion, space charge effect and mainly the rf fringe field to degrade the achieved beam quality at the RFQ exit. A miniature rf quadrupole ($\mu \mathrm{RFQ}$) has been implemented at that exit to remove the degrading effects and provide beams with 1 eV of energy spread and around $1.75\pi \mathrm{mm}\mathrm{mrad}$ of emittance for 4 Pa gas pressure. This solution enables also to transmit more than 60% of the incoming ions for currents up to $1\mu \mathrm{A}$. Detailed studies of this development are presented and discussed in this paper. Transport of beams from SHIRaC towards the HRS has been done with an electrostatic quadrupole triplet. Simulations and first experimental tests showed that more than 95% of ions can reach the HRS. Because SPIRAL-2 beams are of high current and very radioactive, the buffer gas will be highly contaminated. Safe maintenance of the SHIRaC beam line needs exceptional treatment of radioactive contaminants. For that, special vinyl sleep should be mounted on elements to be maintained. A detailed maintenance process will be presented.

Figure 1

Schematic of SHIRaC design: 3D layout of SHIRaC beam line with an expanded view of its optics system.

Figure 2

Space charge effect on the beam quality: (right) simulated results of the beam current effect on the beam emittance; (left) longitudinal energy spread $\mathrm{\Delta }\mathrm{E}$ dependency to the beam current [17].

Figure 3

Space charge effect on the longitudinal energy spread: experimental results of the beam current effect on the longitudinal energy spread at the RFQC exit [16].

Figure 4

Confinement effect of rf voltage amplitude on the longitudinal energy spread: variation of longitudinal energy spread $\mathrm{\Delta }\mathrm{E}$ with the Mathieu parameter q (i.e. rf voltage amplitude) [16].

Figure 5

Layout of the developed extraction section and relative positioning of the $\mu \mathrm{RFQ}$ from the main RFQ.

Figure 6

Effect of the rf voltage amplitude on the longitudinal energy spread $\mathrm{\Delta }\mathrm{E}$ for $1\mu \mathrm{A}$ ion beam: variation of $\mathrm{\Delta }\mathrm{E}$ as a function of Mathieu parameter q.

Figure 7

Illustration of the space charge effect on the longitudinal energy spread with and without $\mu \mathrm{RFQ}$ and for Mathieu parameter $\mathrm{q}=0.4$ and ${\mathrm{P}}_{\mathrm{RFQ}}=2.5\mathrm{Pa}$: Variation of $\mathrm{\Delta }\mathrm{E}$ as a function of the beam current without $\mu \mathrm{RFQ}$ (left), variation of $\mathrm{\Delta }\mathrm{E}$ as a function of the beam current with $\mu \mathrm{RFQ}$ (right).

Figure 8

Effect of RFQ pressure ${\mathrm{P}}_{\mathrm{RFQ}}$ on the longitudinal energy spread $\mathrm{\Delta }\mathrm{E}$ for $1\mu \mathrm{A}$ of beam current and $\mathrm{q}=0.4$: variation of $\mathrm{\Delta }\mathrm{E}$ as a function of ${\mathrm{P}}_{\mathrm{RFQ}}$ (left), beam energy histogram taken at 4 Pa of ${\mathrm{P}}_{\mathrm{RFQ}}$ (right).

Figure 9

Diagrams of geometrical beam emittance for cooled beam with $1\mu \mathrm{A}$ beam current and 4 Pa ${\mathrm{P}}_{\mathrm{RFQ}}:\mathrm{X}-{\mathrm{X}}^{\prime }$ phase space distribution (top), transversal X-axis histogram plot (bottom left), transversal ${\mathrm{X}}^{\prime }$-axis histogram plot (bottom right).

Figure 10

Schematic view of the electrostatic quadrupole triplet: three quadrupoles with collimator in each quadrupole’s side.

Figure 11

Beam profile along the transverse axis for various beam currents and with the optimum cooling condition mentioned above: The image represents the transverse beam profile obtained via a CCD camera (top left); contour plot of the transverse beam profile obtained by an RGB analysis method (top right); projection of the image intensity according to the transverse axis for various beam currents (bottom).

Figure 12

Protection process of the turbomolecular pumps from radioactive gaseous waste.

Figure 13

Safety maintenance of the RFQ module: Establishment of confinement vinyl and withdraw of the module (Step 1), creation of two volumes (Step 2) and separation of two subsets (Step 3).

Figure 14

Technical processes for safety withdraw of elements from the device beam line: setup of the vinyl at the extremity of the insulator (left); separation of volumes by welding the vinyl (right).