A prototype surface ionization source coupled with a fixed-geometry extraction electrode system has been commissioned on the ISAC ion source test stand at TRIUMF. The suitability of the ion source and extraction system for use in the ISAC facility has been determined by a series of emittance measurements of the extracted beams. The test stand optics were successfully commissioned using the prototype ion source; emittance measurements of the mass-separated beams demonstrated that 2^nd- and 3^rd-order beam aberrations (introduced by the magnetic dipole mass separation) could be corrected by the use of multipole electrostatic optics elements. An upper limit of the rms energy spread (2 eV ) was deduced from the emittance measurements. Emittance measurements were performed at beam energies of 10-50 keV, as well as for ion masses ranging from Li⁺ to Rb⁺, to demonstrate the feasibility of the prototype for a variety of beam energies and masses.

Introduction

The ISAC facility at TRIUMF, scheduled to start operation in 1999, is intended to provide high intensity accelerated radioactive beams produced by the on-line isotope separation (ISOL) method. The desired radioactive species will be produced by nuclear reactions induced by 10mA of 500 MeV protons bombarding thick targets held at high temperature (~2000° C) and will diffuse out of the target matrix into a closely coupled ion source where they are ionized and extracted as a 60 keV beam for subsequent transport and mass selection in a magnetic analyzer. In practice, maximum anticipated yields of the ion beams of interest (after losses due to decay in transit and limits on ionization efficiency) are of the order of 10¹⁰ ions/s.

For ISAC, the ion sources will be inaccessible during on-line operation and mechanical adjustments will only be possible by remote handling in hot cells. For this reason, ion sources and extraction columns must be simple, robust and capable of reproducible operation with a minimum of intervention. In a departure from traditional ISOL techniques, the ISAC ion sources are designed to use fixed geometry, multiple electrode extraction columns rather than a mechanically movable extraction electrode system. Optimum beam extraction will be achieved by tuning the voltages of the component electrodes rather than the relative positions of the ion source and extraction electrode. In order to ensure the success of the target/ion source (TIS) and extraction systems designed for ISAC, an off-line development facility (mimicking certain features of the ISAC target station) has been constructed to evaluate potential ion sources.

The ISAC Test Stand

The test stand consists of a vacuum chamber housing the TIS elements, a beam transport stage and a mass separator stage. A plan view representation is shown in Figure 1. The test stand was designed for ion beam energies up to 60 keV and mass to charge ratio up to 140 with a maximum resolution of M/DM = 3000. TIS and extraction systems are suspended in vacuum (£  10^-6 Torr) from a removable chamber lid, which is equipped with a high voltage insulator and Faraday trunk for routing cooling and electrical services at high voltage. Computer control of equipment at high voltage is achieved through the use of optically isolated links. The initial optics elements (four electrostatic quadrupoles) change the beam from the ion source to give the desired beam spot size and divergence at the object slit of the mass-separator stage. The separator consists of a 45° magnetic bend sandwiched between two electrostatic quadrupole triplets. The first order optics is symmetric with respect to the center of the bend. There are two electrostatic sextupoles before, and two octupoles after the bend which correct the second and third order aberrations associated with the dipole. A detailed description of the test stand is given in Reference 1.

The Surface Ionization Source & Extraction System

The first ion source scheduled for use at ISAC is a surface ionization source, chosen for its simplicity, robustness and capability of producing beams of alkali elements with high ionization efficiency and low energy spread. In surface ionization, atoms of elements with low ionization potential ( I) are converted to singly charged ions on contact with a metal surface of high work function (f ). In principle, for alkali elements such as K, Rb and Cs (I = 4.3, 4.2 and 3.9 eV respectively) in contact with Re surfaces (f = 5.1 eV), ionization efficiencies of 100% are possible.

The prototype ISAC surface source consists of a reentrant electrical conductor fabricated from electron beam welded concentric Ta tubes that are raised to temperatures ~2000° C by resistive DC heating. The reentrant geometry is used to pass the heating current in opposite directions through concentric conductors, minimizing magnetic fields that may adversely influence the extracted ion beam. With a fixed electrode geometry, it is impossible to counteract such effects by repositioning the extraction system, as is possible with movable electrodes. Alkali ion beams were generated by thermally decomposing alkali salts in a reservoir coupled to the ionizer tube. Beams of Rb and Cs were obtained using a Ta surface, while Li, Na and K beams were generated from a Re foil placed in the Ta ionizer.

The multielectrode extraction column was designed for variable beam energies up to 60 keV using the program IGUN [2]. 2 mm downstream of the 3 mm f ionizer exit aperture is an object defining Mo electrode of the same diameter, followed by a Mo tipped cooled 3.4 mm f extraction electrode (at 4 mm) and a final 11 mm f ground electrode (at 19 mm). The source and defining electrode are held at high voltage with the extraction electrode tuned at ~10% negative of the HV value with respect to the source. The surface source and extraction electrode column are shown in Figure 2.

The suitability of the extraction system for radioactive ion beam generation was investigated by a series of emittance measurements for varying beam energies, beam currents and beam masses. Emittances were measured using an Allison-type electric sweep emittance scanner [3] situated at either the object (D1 in Figure 1) or image (D2 in Figure 1) positions of the mass separator stage. All quoted values are for the 86% emittance contour, the given errors are due to uncertainty in the level of background electronic noise.

Energy Spread

The test stand was commissioned using stable rubidium, which is easily and efficiently produced by the surface source; the two Rb isotopes (85 and 87) give a ready check on the dispersion. The initial optics (to D1) was commissioned by first leaving the quadrupole quadruplet off and only tuning the steering elements. Emittance measurements at D1 characterized mainly the ion source divergence. This was used to calculate tunes for the quadruplet. It was found that calculated tunes gave very closely the desired beam characteristics at D1, provided that steerers were used to center the beam through the quads. With slit D1 set at 0.8 mm, the separator was tuned to D2. Again, first order optics calculations gave an accurate match to the desired beam size at D2. However, without higher order correction, the emittance figure at D2 had the distorted shape shown in Figure 3a. An octupole just downstream of the 45° dipole was adjusted to give the emittance figure of Figure 3b, and a further correction by a sextupole situated just upstream of the dipole gave the emittance figure of Figure 3c. Crucial to the success of the correction scheme was ensuring that the beam was centered through the multipoles.

In a series of runs, the object slit D1 was progressively narrowed to determine the image width in the limit of vanishing emittance. With a dispersion of 1.25 m, the limiting image FWHM was found to be 0.16 mm. This corresponds to a FWHM resolution of 7800. Converting to an energy spread, it represents DE = 4.0 eV, since the beam energy was 31.5 keV. It is not known whether this arises from high voltage power supply ripple, magnetic field ripple, or intrinsic ion source energy spread. Investigations are ongoing.

Emittance Measurements at Varying Beam Energy

Emittance scans of an ~1 mA Rb beam as a function of beam energy (10-50 keV) were recorded. At each point, the beam current was maximized by tuning the voltage of the extraction electrode to maximize beam current on a retractable Faraday cup at the emittance measurement position. Maximum currents were obtained with the extraction electrode potential at ~8% negative with respect to the ion source HV. Results are shown in Figure 4, where brightness is defined as intensity divided by the square of the normalized emittance. Though the measured emittance varied from 11.2 p mm mrad (10 keV) to 6.0 p mm mrad (50 keV) the brightness is remarkably constant over the energy range, indicating that the fixed multielectrode extraction system does not appreciably select the extracted phase space.

Emittance Measurements at Varying Beam Current

Emittance scans of a 31.5 keV Rb beam were recorded for beam currents ranging from 1-15 mA at the D1 position. Minor adjustments of the extraction electrode were made for each run with the potential value varying from 7.5-8.6% of the source HV. The results are presented in Figure 5. These indicate that there is no significant intensity dependent effect up to the order of 10 mA.

Emittance Measurements of Various Beam Masses

To measure beam emittance as a function of beam mass, the emittance scanner was moved to the image location of the mass separator stage (D2 in Figure 1). Beams of 31.5 keV ⁷Li, ²³Na, ³⁹K, ⁸⁵Rb and ¹³³Cs with beam currents on the order of 10-100 nA, were extracted simultaneously and emittances were measured changing only the mass analyzing magnet field. The respective emittances were 8.2 ± 0.8, 9.0 ± 1.0, 8.4 ± 0.5, 7.1 ± 0.3 and 7.5 ± 0.4 p mm mrad. Emittance scans for Li, K and Cs are shown in Figure 6. The marked similarity of emittance values and emittance figure shapes indicates the performance of the source and extraction system is independent of beam mass.

Summary

A prototype ISAC surface ionization source with a fixed geometry multiple extraction electrode system has been characterized by a series of emittance measurements. The measurements indicate that such a system is feasible for on-line use and is capable of operating over a range beam energies, currents and masses. It is worth noting that the beams from this ion source were both stable and reproducible, giving practically identical emittance figures in runs some months apart.

References

[1] M. Dombsky, R. Baartman, J. Doornbos, T. Hodges, K. Jayamanna, R. Keitel, T. Kuo, Mackenzie, M. McDonald, P. Schmor, Y. Yin, D. Yuan, Nucl. Instr . Meth. B 126 50 (1997).

[2] W. B. Herrmannsfeldt, SLAC Report - 331 (1988)

[3] P. W. Allison, D. B. Holtkamp and J. D. Sherman, IEEE Trans. Nucl. Sci. NS-30, 2204 (1983).