Therefore, the state of charge of the ion beam is adjusted and accelerated through the Electron Beam Ion Source (EBIS) charge enhancer to match the RI beam generated by the Target Ion Source (TIS) of ISOL at 10 keV/u. With the completion of the online commissioning using a stable beam, the performance of the charge refinement and transmission of an RI beam using the EBIS charge breeder was verified.
RAON Heavy Ion Accelerator Facility
Other heavy ion accelerator facilities use one of these, but the RAON facility uses a combination of the ISOL system and the IF system differently. In the ISOL system, the RI beam generated at the target using protons is accelerated using SCL3 and SCL2, transported to the IF separator, and new RI elements are created.
ISOL System in RAON
Since the RI elements that can be generated are different for each method, a RAON device using combined ISOL and IF systems can generate a more exotic RI beam in the area indicated by the red ellipse in the figure. The EBIS charge impurizer meets the conditions for the ion beam generated in ISOL to enter the post-accelerator, mass-to-charge ratio (A/q) less than 6 and energy per nucleon 10 keV/u.
Overview of EBIS Charge Breeder
The basic idea of charge enhancement using the EBIS is to use an electron beam to remove the charge from ions. The basic principles of EBIS and charge enhancement of ions using EBIS are introduced, and the behavior and characteristics of the electron beam in the magnetic field are described.
Principle of EBIS and Charge Breeding
The first and second terms of equation 2.1) represent changes in the charge state due to electron impact ionization. The charge exchange cross section with the background gas for the low charge state is calculated from the formula in [41].
Electron Beam in Magnetic Field
It depends only on the size of the cathode and the magnetic field strength, as shown in Eq. And this can be deduced by the ratio of the magnetic field at the cathode position and the superconducting magnet.
Electric Potential with Space Charge of Electron Beam
The potential of the electron beam due to the space charge effect can be obtained from the Poisson equation, Eq. whereρ andε0 are the volume charge density and the vacuum permittivity, respectively. Both results show that the space charge effect of the electron beam decreases by about 1 kV from 20 kV, which is the voltage applied to the drive tube.
Ion Beam Properties for EBIS
In the ion beam experiments, the emittance of the beam entering the EBIS is measured, and it is transmitted with less than 100%. However, before the loss occurs, it is possible to predict the energy characteristics of the ion beam in the EBIS.
Ion Trap
The electron gun, the collector and the deflector are important for the electron beam utilization, starting with the ion capture section where the charge multiplication of the ion beam takes place. So, these guide coils are used to align the trajectory of the electron beam with the axis. Therefore, if the breeding area is in this 700 mm section, the behavior of the ion beam in the magnetic field can be stable, and the charge breeding can also be performed stably.
One of the essential parts in the ion trap section for the EBIS charge breeding experiment is the control coil. Therefore, the components of the ion trap section, including the drive tube, are generally mounted on the HV platform.
Electron Gun and Collector
In this case, when the repeller voltage is applied lower than the cathode voltage, the electron beam cannot pass through the repeller and is reflected. So the solar collector is isolated from the components of the ion trap section and the supporting rack is also made of an insulator. Compared to the EBIS platform, the potential of the repeller is around -10 kV and the potential of the breeding region is around +8 kV.
At this time, the voltage of the Einzel reflective lens used is 2 kV or less, and the voltage of each control electrode is within ±2 kV. The low voltage of the Einzel lens is used because the pickup is calculated at the position before the entrance of the repeller and the state.
Ion Transport Line
The beam coming from the test ion source is transferred to the EBIS by applying the voltage according to the energy of the ion beam and bending it 15 degrees. And they are sent to the diagnostic line by bending 15 degrees to measure using the dipole magnet of the EBIS. The slit width of 4 mm was used in the charge counting experiment of the ion beam.
Its width is experimentally adjusted according to the focal strength of the Einzel lens to send the ion beam from the switching site line to the Faraday Cup. Thus, the ion beam is focused in the direction of the electrodes with the positive voltage and defocused with the negative.
Vacuum System
The charge counting test is performed by transmitting and injecting a beam to make it suitable for the acceptance condition of the EBIS using the beam optics of the switching site line by the result of measurement by a diagnostic system. Then, in the residual gas experiment, the A/q spectrum of the residual gas ions is measured using the diagnostic line, and then the measurement of the charge-grown ions is performed using the probe ion beam. Therefore, it was verified that the charge counting experiment of the EBIS could be performed stably.
The ion transport line must also be in the high vacuum region for the stable transmission of the ion beam. The test zone source, the diagnostic line and the EQT line also used one TMP in each section, as shown in the above drawings, to construct a vacuum system, so there is no beam.
Electrical System
The cathode platform of the electron gun determines the energy of the electron beam in the breeding area. The power supply for this ion source platform used a high voltage power supply, SHV120R-40kV-P from ConverTech to control the energy of the ion beam. High voltage amplifiers, TREK P0621P-H (+30 kV) for the lens and TREK 2220 (±2 kV) for the actuators, are used because the voltage must be changed according to the timing of the ion beam injection and extraction.
The line beam optical system of the ion source is also used for the use of a test ion beam. They do not use amplifiers, because the direction of the ion beam is unidirectional.
Control System
Unlike the EBIS platform, the cathode platform uses a device called cRIO for control instead of the PXIe. After integrating network variables with the EBIS platform operating system, the EPICS IOC is configured in the PXIe system as illustrated in Figure. The heater power supply and the two modules are connected to the ground platform network switch hub via fiber optic cables. .
The ground platform control sequence is built using LabVIEW software on PXIe, and the EPICS IOC is built using CAS modules such as EBIS PXIe, as shown in Fig. EPICS IOCs are built for PXIe control of EBIS platforms and ground platforms, and vacuum controllers are installed for the EBIS vacuum system.
Measurement System
The image of the beam passing through the reflector can be obtained by reflecting the light from the mirror and measuring it with a camera. In the range, the transverse angle of the beam in each hole and the emission and Twiss parameters are calculated. Angle⟩=∑Withingridpixel Angle(pixel)×I(pixel). in the same way as one. 3.2), and the maximum and minimum values of the angle are additionally found.
The exact conversion factor must be used to accurately calculate the properties of the beam in. In addition, we have shown the configuration and results of the important vacuum system in the EBIS experiment.
Electron Beam Transmission
The current of the electron beam was measured with a direct current converter (DC-CT) on the cathode and the collector, and its result is shown in the figure. After the short pulse test, the long pulse electron beam is measured. with rise time set to. An electron beam current of 1.51 A at the cathode and a transmission efficiency of 94.7% are achieved, similar to that of a single shot with a short pulse.
These results show that the electron beam of the EBIS charge generator in the magnetic field of 2 T can be operated to ionize and multiply ions. Correct on-axis alignment increases electron beam current and transmission efficiency.
Charge Breeding of Residual Gases
Through the experiments so far, the electron beam transmission was completed in 2 T and 6 T magnetic fields. And in the following experiments, the magnetic field is set to 6 T in the SC magnet to compress the electrons and increase the electron beam current density. These results indicate that the electron beam charge multiplication process in EBIS is working as expected.
In the residual gas experiment, the effectiveness of EBIS was confirmed by measuring the charge propagation effect of the residual gas by varying the propagation time and electron beam current. Consequently, longer propagation time and electron beam current result in a more efficient charge propagation effect, so the peak with lower A/q in the spectrum increases.
Charge Breeding Experiments using various Test Ion Beams (Rb, Cs)
The charge state of the trapped ions was grown for 30 ms using an electron beam of 1 A. The result shows that as the current of the electron beam increases, the ions in the higher charge state increase. These experiments confirmed the effect of the charge multiplication in the EBIS charge counter under various conditions.
In addition, the effective charge propagation current density of Cs ions was calculated, which confirmed that it was in agreement with experiment. In addition, the charge propagation effect of EBIS with respect to propagation time and electron beam current was confirmed from experimental results under different conditions.
Experiments with Stable Ions Transported from ISOL Beamline (Cs, Sn, Na)Sn, Na)
Cs ions extracted by EBIS were transmitted in the ISOL beam through the EQT line instead of the diagnostic line. The highly charged Cs ions exit through the EBIS branch point and are transmitted to the ISOL beamline A/q splitter. Charge growth was performed as before using the Cs test ion beam of EBIS for both cases with and without He gas in the RFQ-CB.
Before using the ISOL system with the RI beams, the performance of the RAON EBIS charge counter was confirmed by the electron beam and the charge counter experiments of the EBIS. The vacuum requirement for the charge counting experiment of the EBIS is the UHV condition (∼10−11mbar).
