An important component of the reaccelerated beam facility (ReA) is an electron-beam ion trap (EBIT) charge breeder, which converts singly charged radioactive ions injected from the gas “stopping” cell to highly charged ions. The use of highly charged ions allows one to reach the high ion velocity required for injection of beams into the ReA’s radio-frequency quadrupole accelerator with little effort. Rare isotopes can have short half-lives and are expected from the gas cell with a wide range of yields, ranging from a few to 108 per second. Hence, a charge breeder of rare isotopes must have high capture and breeding efficiency, large charge capacity, and short breeding times to minimize losses of rare isotopes. An EBIT can meet these requirements.
An EBIT is an ion trap that uses electron-impact ionization by a high-density electron beam of tens of keV to significantly increase the charge state of injected atoms or ions. In order to obtain the required density for efficient ionization, the electron beam is compressed with a strong magnetic field. Typically, an electron beam of a current of approximately 100 mA and a few millimeters in diameter is compressed by a few-Tesla field to a hair-thin diameter of less than 100 µm. This corresponds to a current density in the range of 1000 A/cm2. Highly charged ions are confined with an energy of less than 200 eV in an axial electrostatic potential well created by cylindrical trap electrodes and by the attractive radial space-charge potential from the electron beam. At the NSCL’s EBIT, ion beams are quasi-continuously injected along the magnetic field axis. The ions accumulate in the trap once they have passed the first potential barrier and have become ionized by the electron beam on their first round trip through the potential well. After charge breeding, this first barrier is lowered for a short time to extract the highly charged ions.
Photograph of the trap electrode structure of the NSCL’s ReA EBIT
breeder. The trap is composed of 23 Titanium individual electrodes
and has an overall length of 1.2 m.
NSCL has developed a unique EBIT design, which maximizes the capture efficiency of continuously injected ion beams while maintaining the capability of producing very rapidly high charge states. The ReA EBIT features a high-current electron gun, a superconducting magnet with two variable magnetic-field regions of up to 6 T, and a long trap structure. The electron gun was designed to generate currents of up to 2.4 Amperes. The magnet uses a long solenoid and a pair of Helmholtz coils. The solenoid, facing the incoming ions, can provide a low magnetic field strength to intentionally keep the electron-beam diameter large over an extended region. This large diameter maximizes the spatial overlap between the electron and ion beams to increase the probability of electron-impact ionization during the injection process. The trap electrode structure consists of 23 individual electrodes and allows for an up to 0.8 m long potential well. The long trapping region provides sufficient time for the injected ions to interact with the electron beam. The Helmholtz coils can generate a strong magnetic field to reduce the electron-beam diameter and create a region of high-current density needed for fast charge breeding. The Helmholtz geometry was chosen to provide visible access to trapped highly charged ions for atomic or nuclear spectroscopy. The high-current electron beam in combination with a long trapping region makes for a large charge capacity of the trap, which increases the limit at which the electron beam becomes neutralized by positively charged ions.
Photograph and engineering model of the NSCL’s ReA EBIT
breeder. The plot shows, in light green, a magnetic field
configuration expected to yield high capture efficiency and, in dark
blue, a typical electrostatic potential applied to trap electrodes.
The ReA EBIT is currently in the commissioning phase. In preparation for injection of rare isotopes, we have recently started injecting K+ ions from a plasma test ion source, and extracting charge bred ions to the Q/A separator for preliminary charge breeding studies and reacceleration tests. Detailed Monte-Carlo simulations of the injection and charge breeding processes are also underway to guide the commissioning work. We expect first proof-of-principle charge breeding of rare isotopes in winter 2013.
Here is a summary of recent milestones: