Low-$Z$ gas stripper as an alternative to carbon foils for the acceleration of high-power uranium beams

The RIKEN accelerator complex started feeding the next-generation exotic beam facility radioisotope beam factory (RIBF) with heavy-ion beams from 2007 after the successful commissioning of RIBF at the end of 2006. Many improvements made from 2007 to 2010 were instrumental in increasing the intensity of various heavy-ion beams. However, the available beam intensity of very heavy ion beams, especially uranium beams, is far below our goal of $1\mathrm{p}\mu \mathrm{A}$ ($6\times {10}^{12}\mathrm{particles}/\mathrm{s}$). In order to achieve this goal, upgrade programs are already in progress; the programs include the construction of a new 28-GHz superconducting electron cyclotron resonance ion source and a new injector linac. However, the most serious problem, that of a charge stripper for high-power uranium beams, still remains unsolved, despite extensive research and development work using large foils mounted on a rotating cylinder and a ${\mathrm{N}}_2$ gas stripper. A gas stripper is free from problems related to lifetime, though the equilibrium charge state in this stripper is considerably lower than that in a carbon foil, owing to the absence of the density effect. Nevertheless, the merits of gas strippers motivated us to develop a low-$Z$ gas stripper to achieve a higher equilibrium charge state even in gases. We measured the electron-loss and electron-capture cross sections of uranium ions in He gas as a function of their charge state at 11, 14, and $15\mathrm{MeV}/\mathrm{nucleon}$. The equilibrium charge states extracted from the intersection of the lines of the two cross sections were promisingly higher than those in ${\mathrm{N}}_2$ gas by more than 10. Simple simulations of charge development along the stripper thickness were performed by assuming the measured cross sections. The simulation results show that about $1\mathrm{mg}/{\mathrm{cm}}^2$ of He gas should be accumulated to achieve a charge state higher than that of ${\mathrm{N}}_2$ gas, notwithstanding the difficulty in accumulation of this helium amount owing to its fast dispersion. However, we now believe that the following two solutions can overcome this difficulty: a gas cell with a very large differential pumping system and a gas cell with a plasma window. Their merits and demerits are discussed in the paper.

Figure 1

Bird’s-eye view of RIBF. Two injectors, RIKEN linear accelerator (RILAC) and the AVF cyclotron (K70 MeV), are followed by four booster cyclotrons: RIKEN ring cyclotron [(RRC), K540 MeV], fixed-frequency ring cyclotron (fRC), intermediate-stage ring cyclotron (IRC), and superconducting ring cyclotron (SRC). The accelerated beams are transported to an in-flight RI beam separator BigRIPS.

Figure 2

Acceleration scheme for uranium beams at RIBF. The uranium beams from an 18-GHz electron cyclotron resonance ion source (ECRIS) are accelerated using two linear accelerators, radio-frequency quadrupole linac (RFQ) and RILAC, and four ring cyclotrons, RRC, fRC, IRC, and SRC, for RI beam production in BigRIPS. Two charge strippers are located downstream of the first and second cyclotrons, RRC and fRC, respectively. The numbers in the brackets below the cyclotrons denote the extraction energies in $\mathrm{MeV}/\mathrm{nucleon}$.

Figure 3

Simple estimation of cross sections of e-loss and e-capture (${\sigma }_{\mathrm{BEM}}^L$ and ${\sigma }_{\mathrm{SCH}}^C$) for uranium beam in the gases of ${\mathrm{N}}_2$, He, and ${\mathrm{H}}_2$, calculated as a function of the charge state by using the binary encounter model [25] and Schlachter’s formula [26]. The circles indicate the intersections of the two lines.

Figure 4

Schematic of experimental setup for the measurements of cross sections of e-loss and e-capture for uranium beams at 11, 14, and $15\mathrm{MeV}/\mathrm{nucleon}$ in RIBF beam lines between RRC and fRC. Uranium beams emerging from RRC passed through a carbon foil located in front of a pair of bending magnets (DAD1&DMD1), which were used to select the individual projectile charge state. After emerging from the He gas cell, the beams passed through the second bending magnet (DAD4&DMD4) into the Faraday cup at point F41 (FC-F41), which measured the intensity of the beam current of the charge state for e-loss, e-capture, no reaction, and so on. The Faraday cup at D17 (FC-D17) measured the current of the incoming ion with the aim of eliminating its fluctuation.

Figure 5

Measured cross sections of e-loss and e-capture as a function of charge state of uranium ions at 11, 14, and $15\mathrm{MeV}/\mathrm{nucleon}$ in He gas. The cross sections were extracted assuming the thickness of the gas cell to be $13.27\mu \mathrm{g}/{\mathrm{cm}}^2$. The intersections between the two lines for the cross sections of e-loss and e-capture are indicated by the circles with the corresponding numbers of ion charge below.

Figure 6

Cross sections used for simulation of charge evolution of uranium ions in He gas at $11\mathrm{MeV}/\mathrm{nucleon}$ along with thickness. The dotted lines show the calculated cross sections of e-loss and e-capture (${\sigma }_{\mathrm{BEM}}^L$ and ${\sigma }_{\mathrm{SCH}}^C$) using the binary encounter model and Schlachter’s formula. The solid lines show the scaled cross sections by 0.351 and 0.290 for e-loss and e-capture, respectively, in order to fit to the measured cross sections. The circles and squares denote the measured cross sections of e-loss and e-capture, respectively.

Figure 7

Mean charge evolution of uranium ions at $11\mathrm{MeV}/\mathrm{nucleon}$ with and without energy loss of the projectile as a function of thickness of He gas.

Figure 8

Schematic of plasma window for keeping the vacuum separated from atmosphere. It consists of three cathode holders, cooling plates, and an anode plate to fill the aperture with arc plasma.

Figure 9

Conceptual illustration of low-$Z$ gas stripper for ${\mathrm{U}}^{35+}$ beam with a pair of plasma windows (PWs). A gas cell with He input/output is located between the PWs. A pair of a root blower and two TMPs are located to connect the PWs with background vacuum for uranium beam.