M. Dombsky, R. Baartman, J. Doornbos, T. Hodges, K. Jayamanna, R. Keitel, T. Kuo,

G. Mackenzie, M. McDonald, P. Schmor, Y. Yin, D. Yuan.

The properties of ion sources and the beams extracted from them are critical to the successful design of the ISAC facility at TRIUMF. To better understand these properties, an off line 60 kV test stand has been constructed to test and evaluate targets and ion sources intended for use at the future ISAC facility. The test stand also presents the opportunity to evaluate diagnostics, beam monitoring and system control techniques required for ISAC. The test stand vacuum chamber mimics the ISAC target module. Ion beams are produced using a fixed geometry multielectrode extraction column and transported through 5m of beamline using electrostatic elements. Mass analysis is achieved using a 45° magnetic dipole midway along the flight path. Diagnostic elements positioned throughout the beamline are used to determine beam properties.

Certain features of the ISAC facility being constructed at TRIUMF are significantly different from those of existing ISOL systems. With the anticipated higher proton currents (³ 10 mA) incident on the targets, there is a greater need for remote handling and extensive shielding of target/ion-source (TIS) modules. For ISAC, these criteria are achieved by suspending the target/ion source at the bottom of a ~2 m long shielding "plug" that is lowered into the target station vacuum vessel. The plug, complete with target, ion source and their required services (electrical conductors, cooling lines, gas feed lines), is transported by crane between the target station and the hot cell where the TIS modules are exchanged using manipulators. The extraction electrode system is also suspended at the bottom of the shielding plug and is pre-aligned to the ion source during installation. The high radiation fields anticipated at the target station preclude the use of electrical components, such as positioning motors, on the extraction electrode. Furthermore, mechanical positioning of the extraction electrode through ~2 m of shielding is excluded as impractical and imprecise. This gives rise to, perhaps, the most significant departure from traditional ISOL designs, the use of fixed geometry, multielectrode extraction systems for all ISAC ion sources. The optimum beam extraction will be accomplished by tuning the voltages of the component electrodes rather than the relative positions of the ion source and extraction electrode. While this approach solves some problems presented by a high radiation environment, it presents new problems in the need for precise reproducible alignment and voltage regulation. In order to ensure the success of the target/ion source/extraction systems designed for ISAC [1], an off-line developmental facility has been constructed that mimics certain features of the ISAC target station and allows for evaluation of the various ion sources proposed for ISAC.

The test stand is designed for £  60 keV ion beams of mass to charge ratio up to A/Q » 140 with a resolution of M/DM £ 3000. The TIS vacuum chamber is 1.4 m long ´ 0.6 m wide ´ 0.6 m high, with the ion beam axis 30 cm below the bottom lid surface (Figure 1). TIS and extraction systems are suspended from the removable chamber lid, which is equipped with a high voltage insulator and Faraday trunk for routing cooling and electrical services at high voltage. An equipment rack containing the required power supplies and feed gases is located immediately adjacent to the TIS chamber in a high voltage Faraday cage. Computer control of equipment at high voltage is achieved through the use of optically isolated links. All TIS components are designed to operate in £ 10^-6 torr vacuum, achieved using 500 L/s turbomolecular pumps at the TIS chamber and throughout the beam line. A plan view representation of the test stand is shown in Figure 2.

The initial beam optics elements consist of four electrostatic quadrupoles that match the beam from the ion source to the object slit of the mass-separator stage. The separator consists of a 45° magnetic bend with a 41 cm radius sandwiched between two electrostatic quadrupole triplets. The magnet has a 5 cm gap, a maximum field of 12 kGauss and parallel polefaces at angles of 22.5° with respect to the incoming and exiting beam. The first order optics is symmetric with respect to the center of the bend (Q1=Q6, Q2=Q5 and Q3=Q4). There are two electrostatic sextupoles (S1 and S2) before, and two octupoles (O1 and O2) after the bend which correct the second and third order aberrations associated with the horizontal and vertical angles. The separator can be used to give a high mass resolution of 3,000 for a phase space area of 8 mm-mrad by making the beam wide in the bending magnet in the bend plane. In this case Q1/Q6 and Q2/Q5 are horizontally defocusing while Q3/Q4 are horizontally focusing. Alternatively, the separator can accept a large phase space area of 80 mm-mrad for a resolution of 300. In that case Q1/Q6 and Q3/Q4 are horizontally focusing and Q2/Q5 are vertically focusing. Other tunes are possible.

The test stand will eventually be equipped with sufficient beam diagnostic equipment to measure ion source behaviour and beam properties in some detail. At the present time, the following are installed (see Figure 2). Initial diagnostic devices (position ID) are at the center of the initial quadrupole quadruplet section and consist of a beamstop followed by a selection of apertures which may be used to reduce acceptance or beam current. These pneumatic devices are water cooled and may be biased up to 300 V without affecting the reading of the incident beam current. The beam stop can be automatically activated by software defined trip levels, providing over-current protection for downstream devices.

In the first diagnostics chamber (D1), the separator object aperture is defined either by a selection of slots machined into a water cooled beamstop or by the combination of a broad slot defining the vertical aperture and an adjustable water cooled slit (made by Danfysik) defining the width of the horizontal aperture. These are followed by scanning wire profile monitors and a 3.6 cm f aperture Faraday cup with supressor electrode. An emittance-mass scanner (EMS) [2] with 3.7 cm travel may be used when the Faraday cup is retracted. The resolution can be affected by a too large vertical divergence; a retractable aperture to limit this will be installed just prior to the first quadrupole of the separator triplet.

The equipment of the object diagnostic box (D1) can be mirrored in the box (D2) which contains the image point. Any of the equipment used upstream may be transferred to this location. It will also have a second set of scanning profile monitors and a selection of apertures at the image point followed by an 8 cm diameter Faraday cup. At low beam current, scintillation screens may be installed at the image point to check the performance of the sextupoles and octupoles.

The energy spread of the beam will be measured by the method of Gaus et al. [3]; the apparatus is presently being manufactured. In the intermediate term, a linear scanning profile monitor will be developed for use in the focal plane and at other locations where beam spots are large. Emittance measuring equipment will be assembled for use after the image slit.

The test stand was also chosen to implement and evaluate a control system concept based on the EPICS control system tool kit [4]. The hardware architecture consists of an ethernet segment connecting SUN workstations, X-terminals and a layered system of front end processors. One VME based EPICS I/O computer (Motorola MV162) controls all test stand devices. Beam optics and diagnostics devices are controlled directly, whereas the vacuum system and the ion source are indirectly controlled by supervision of a programmable logic controller (MODICON 984 VME). An inexpensive power-supply controller card was designed as a building block for a distributed device-controller network. It is used to control beamline power supplies. Field buses supported on the test stand will be CANbus, Modbus Plus and CAMAC.

Conventional software development was kept to a minimum. The Modicon PLC is programmed in ladder logic. For the rest of the system, EPICS software tools were used to configure the EPICS distributed run-time data base, operator interface screens, data loggers, archivers, etc. C-code was developed to integrate the MODICON PLC and the power supply controller network into EPICS and to implement some fast scan loops. The control system is in the final stage of commissioning. The initial experience with EPICS, as an architecture, as a shared software system, and as a collaboration of many controls groups was very positive.

The beams necessary for the ISAC physics program will require a range of ion sources. The first source scheduled for evaluation on the ISAC test stand will also be used to commission and evaluate the test-stand optics, diagnostics and controls. The 15 cm f ´ 15 cm multicusp ion source described in Reference [1] has been installed and operated on the test stand. The source is filament driven, with a multicusp plasma containment field generated by permanent SmCo magnets. In preliminary studies using this source, 30 keV beams of N⁺ (120 mA) and N₂⁺ (150 mA) have been successfully extracted and focused through to the final diagnostic chamber at the end of the test stand; total current out of the source was ~500 mA. This source may prove to be an alternative to an ECR source for generation of light gaseous element beams. A similar 10 cm f microwave driven (2.45 GHz) multicusp source has already been demonstrated to produce stable, reliable and efficient beams of both positive and negative ions at TRIUMF. [5] For elements that are reactive with the filament material, the RF source with a quartz-lined plasma chamber offers an alternative means of evaluating the multicusp sources.

The second source scheduled for evaluation is a surface-ionization source shown in Figure 3. The ionizer consists of a reentrant electrical conductor fabricated from electron beam welded concentric Ta tubes. The reentrant geometry arises from the use of fixed extraction electrodes. Magnetic fields generated by the DC heating current in the ionizer may adversely influence the extracted ion beam. With a fixed electrode geometry, it becomes impossible to counteract such affects by repositioning the extraction system, as is possible with movable electrodes. To minimize magnetic fields, the ionizer heating current is passed in opposite directions through concentric conductors, thus canceling (or minimizing) the magnetic field at the extraction region. One of the concerns regarding this design is the possible deflection of the ionizer exit aperture due to thermal expansion of the conductors; beam loss due to the aperture moving off-axis with respect to the extraction electrodes could be unacceptable. Thermal stress analysis calculations predict a possible deflection of up to 20% of the aperture diameter, however, the exact amount will require experimental determination on the test stand.

Figure 3. shows the multielectrode extraction column designed for the surface ionization source. Immediately next to the 3 mm f ionizer exit aperture is a fixed Mo electrode (with an identical aperture diameter) that defines the object of the extraction system; all subsequent electrodes are aligned with respect to this electrode. Both ionizer and object electrode are biased to +60 kV. The next "puller" electrode is nominally at +55 kV, providing the initial extraction potential. Subsequent electrodes are the ground electrode and a final electron-suppression electrode that can be biased negative or tied to ground. Tuning of the extraction column is achieved by varying the bias applied to the puller and suppression electrodes.

Other ion sources scheduled for testing include a FEBIAD plasma source that is a modified version of an older generation ISOLDE source and a compact 2.45 GHz ECR ion source designed for operation inside the TIS vacuum chamber. The FEBIAD source was donated by ISOLDE and is currently being adapted for the test stand. The ECR has been designed, but not yet manufactured.

Prior to the commissioning of the ISAC test stand, measurement of beam properties had already commenced using existing test facilities. The beam emittance of a surface ionization source was measured using the emittance-mass scanner (EMS) developed at TRIUMF.[2] Beams of 15 keV Na⁺, as well as K⁺ and Rb⁺ from impurities, were generated by thermal decomposition of a small amount of Na₂CO₃ inside a heated 5 mm f Ta tube containing a Re ionizing foil. Using a fixed 3 electrode extraction column, a normalized emittance of 0.07 ± 0.01 pmm-mrad was measured for a 100 mA Na⁺ beam. A plot of an EMS emittance scan containing three alkali species is shown in Figure 4.

Further work using existing test facilities has involved the production of Be beams. There is currently significant interest in both radioactive beams and targets of ⁷Be for purposes of measuring the astrophysically important ⁷Be(p,g)⁸B reaction.[6] At TRIUMF, attempts were made to generate Be⁺ by heating metallic Be to temperatures ~1000° C inside the quartz-lined plasma chamber of the RF multicusp ion source. An initial mass scan of the extracted beam showed no evidence of Be⁺. Since the vapour pressure of Be is ~ 10^-4 torr at this temperature, it is likely that the Be oxidized to the more refractory BeO; at 1000° C, the vapour pressure of BeO is only ~ 10^-11 torr. To convert the beryllium to a volatile form, CF₄ was introduced at a rate of £ 0.2 cc/min into the ion source, resulting in the production of beryllium fluoride ions. Mass scans identified both BeF⁺ and BeF₂⁺ as shown in Figure 5; the BeF⁺ peak at the A=28 position is a mixture with N₂⁺ from residual gas but, the BeF₂⁺ at A=47 is clearly visible. It is possible that the reaction taking place is:

BeO + CF₄ ¾ ® BeF₂ + COF₂

This reaction is thermodynamically spontaneous, based on its free energy (DG_f° = - 139.7 kJ/mole). BeF₂ sublimes easily, with a vapour pressure above 10 torr at ~ 900° C, allowing successful extraction of beryllium fluoride species.

The ISAC ion source test stand has been constructed and commissioning is in progress. Beams from the first ion source have been successfully extracted and focused through the system. Additional ion sources are ready for evaluation while others are in the manufacture or design stages. It is envisaged that the test stand will provide the answers necessary for successful designs of the future ISAC target/ion source systems.

The authors gratefully acknowledge the contributions of L. Buchmann, D. Cameron, T. Denham, M. Leross, D. Ross, G. Waters and A. Zyuzin.  

[1] P. G. Bricault, Rev. Sci. Instrum. 67 (1996) 1277.

[2] D. Yuan, K. Jayamanna, T. Kuo, M. McDonald and P. Schmor, Rev. Sci. Instrum. 67 (1996) 1275.

[3] A. D. Gaus, W. T. Htwe, J. A. Brand, T. J. Gay and M. Schulz, Rev. Sci. Instrum. 65 (1994) 3739.

[4] L. R. Dalesio, J. O. Hill, M. Kraimer, S. Lewis, D. Murray, S. Hunt, W. Watson, M. Clausen and J. Dalesio, Nucl. Instr. and Meth. A 352 (1994) 179.

[5] K. Jayamanna, D. Yuan, T. Kuo, M. McDonald, P. Schmor and G. Dutto, Rev. Sci. Instrum. 67 (1996) 1061.

[6] J. N. Bahcall and M. Pinsonneault, Rev. Mod. Phys. 64 (1992) 885.

Figure 1: The TIS vacuum chamber. The surface-ionization source is shown with its extraction system: (A) high voltage insulator, (B) target, (C) surface-ionization source, (D) extraction electrodes.

Figure 2: Plan view of the ISAC test stand: (TIS) target/ion source chamber, (ID,D1,D2) diagnostics locations, (Q) quadrupoles, (S) sextupoles, (O) octupoles, (A) apertures.

Figure 3: Detail of the surface source and extraction column: (1) reentrant ionizer, (2) object electrode, (3) puller electrode, (4) ground electrode, (5) electron suppresser.

Figure 4: Simultaneous 3 component emittance scan obtained with the TRIUMF EMS scanner.

Figure 5: Mass scan obtained after CF₄ addition to the RF multicusp ion source. Total beam current ~100 mA. Mass peaks at 29, 30, 32 and 48 are tentatively identified as HBeF⁺, NO⁺, O₂⁺ and HBeF₂⁺. There is also a minor contribution to mass peaks at 28, 29 and 30 from silicon isotopes sputtered form the quartz liner of the plasma chamber.