I. What is an EBIT?
An electron beam ion trap (EBIT) is a unique ion source as a means of producing and trapping highly charged ions, which was developed initially at Lawrence Livermore National Laboratory (LLNL) for spectroscopic studies. It was based on the earlier electron-beam ion source (EBIS) but with shorter trap length to limit instabilities inside the trap and hence increase the residence time of ions. This increased residence time is the key to producing highly charged ions. For a number of years, two such devices have been in operation at LLNL. One of them is called super-EBIT which can generate an electron beam of up to 200 keV and the other one is called EBIT2 with up to 25keV electron beam energy. They succeeded in making fully stripped uranium ions by using the super-EBIT. Similar types of ion sources as EBIT2 have also been constructed in Oxford , NIST , Berlin .
II. The Tokyo-EBIT
We have constructed a new electron beam ion trap (Tokyo-EBIT) at the Institute of Laser Science , the University of Electro-Communications , Tokyo. The new device has many features which differ from other EBITs although the fundamental concepts are the same. The below figure shows a schematic drawing of the Tokyo-EBIT.
The below is a picture of the Tokyo-EBIT.
Our EBIT has been named YEBISU (Young Electron Beam Ion Source Unit). Originally, YEBISU is one of the Seven Deities of Good Luck. Among them, YEBISU is the only Japanese domestic god. YEBISU is also a popular brand of beer in Japan.
II-a. Operating principles
The device consists of three parts, an electron gun , a cryostat region including drift tubes and an electron collector. The electrons emitted from the electron gun are accelerated upwards by the potential difference between the electron gun and the drift tubes, whilst being magnetically compressed. The resultant beam ionizes and traps ions in the drift tube region. The drift tube assembly is comprised of three successive cylindrical electrodes, which produce an axial trapping potential due to the positive potential applied to the end drift tubes with respect to the central one. On the other hand radial trapping is achieved by the space charge potential produced by the magnetically compressed electron beam. Eight ports surround the central drift tube. These ports are used for observations, neutral gas injection and laser introduction. Liquid helium and liquid nitrogen tanks are situated around and over the trap assembly. They act as cryo-pumps to evacuate the trap region. A superconducting coil is installed in the liquid helium tank to generate a magnetic field which compresses the electron beam. After exiting the drift tubes, the electron beam is decelerated by the potential difference between the drift tubes and the collector, and then collected by the electron collector. Both the electron gun and the electron collector assemblies are designed to be floated to -300 kV, and the drift tube assembly is to +40 kV from laboratory earth. Therefore it is possible to generate an electron beam with an energy of up to 340 keV at the trap region.
II-b. Electron gun
The electron gun was designed in house, while all other EBITs employ commercial ones. The gun is a pierce-type, which consists of four electrodes, cathode, focus, anode and snout. Its perviance was designed to be 0.44 micro-perviance (I/V3/2), which gives 100mA at the anode voltage of 3.8kV and 300mA at 7.8kV. To optimize the electric field inside the gun, a number of trajectory simulations using EGUN2 were performed. We adopted the Brillouin focusing as well as almost all EBIS/T. Therefore the magnetic field at the cathode designed to be zero and the magnetic field distribution along the beam axis was also optimized by the simulations. Cancellation of the magnetic field is achieved with the soft iron, which is the material of the snout electrode, and the bucking coil which is placed below the cathode. The cathode is a spherical concave shaped dispenser-type one, which is a porous tungsten matrix infiltrated BaO, CaO and Al2O3. The diameter of the cathode is 3 mm. The work function of the cathode materials is reduced by Os and Ru put over the surface of the cathode to reduce the temperature at which it works. The designed value of the maximum current output is 300 mA. At present, total emission of up to 250 mA has been obtained with a good agreement with the designed value of perviance.
II-c. Trap region (Drift tubes)
The trap structure is completely different from other EBITs. The drift tubes consist of 8 electrodes. At normal operation, all of DT2L, M and U are at the same potential of up to -500 V with respect to DT1 and 3. When different potentials are applied to each of DT2L, M and U, the trap becomes a cylindrical Penning trap. Furthermore, by applying a rf electric field to DT2M(a)-(d), ion motions can be excited for particular charge to mass ratios. This enables us to study cyclotron or axial motion and to measure charge to mass ratio. Normally, the superconducting magnet vessel is at 4.2K and serves as a cryopump similarly to other EBIS/T. In our case, however, the temperature of liquid helium can be decreased to below 2.4K by using the Joule-Thomson effect. The vapor pressure of H2 is about 10-6 torr at 4.2K and about 10-15 torr at 2.4K, hence the vacuum of the trap region is expected to be significantly improved since the pressure is considered to be dominated by the partial pressure of H2.