- Email: [email protected]
A Penning sputter ion source with very low energy spread
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 614 (2010) 174–178
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A Penning sputter ion source with very low energy spread Z. Nouri a, R. Li b, R.A. Holt a, S.D. Rosner a, a b
Department of Physics and Astronomy, University of Western Ontario, London ON, Canada N6A 3K7 TRIUMF, 4004 Wesbrook Mall, Vancouver BC, Canada V6T 2A3
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 December 2009 Accepted 19 December 2009 Available online 5 January 2010
We have developed a version of the Frankfurt Penning ion source that produces ion beams with very low energy spreads of 3 eV, while operating in a new discharge mode characterized by very high pressure, low voltage, and high current. The extracted ions also comprise substantial metastable and doubly charged species. Detailed studies of the operating parameters of the source showed that careful adjustment of the magnetic field and gas pressure is critical to achieving optimum performance. We used a laser-fluorescence method of energy analysis to characterize the properties of the extracted ion beam with a resolving power of 1 104, and to measure the absolute ion beam energy to an accuracy of 4 eV in order to provide some insight into the distribution of plasma potential within the ion source. This characterization method is widely applicable to accelerator beams, though not universal. The low energy spread, coupled with the ability to produce intense ion beams from almost any gas or conducting solid, make this source very useful for high-resolution spectroscopic measurements on fast-ion beams. & 2009 Elsevier B.V. All rights reserved.
Keywords: Penning sputter ion source Energy spread
1. Introduction The Penning ion source, also called the Penning (or Phillips) Ionization Gauge (PIG) ion source, owes its origin to the discovery of this gas discharge mode by Phillips [1], and has been used in cyclotrons and linear accelerators for many years [2]. The same discharge is the basis of ion pumps and some pressure gauges. In its most common form, a Penning cell consists of a linear arrangement of two disk cathodes separated by an anode, which is usually a hollow cylinder. Crucial to the design is an axial magnetic field, typically 0.02–0.15 T, which confines discharge electrons to spiral paths between collisions as they oscillate in the axial potential well. The lengthened electron path allows the Penning discharge to be self-sustaining at relatively low gas pressures compared to a normal glow discharge [3]. It is well known [3] that the interplay of gas pressure, magnetic field strength, and applied anode–cathode potential results in several different operating modes of the Penning discharge. In the high-pressure low-voltage high-current mode, which is of greatest interest here, the main discharge space between the anode and two cathodes is occupied by a weakly ionized quasi-neutral plasma very nearly at the anode potential. A steep potential gradient—the ‘cathode fall’—occurs very close to the cathodes, and is the source of a population of non-thermal fast electrons that are responsible for
Corresponding author. Tel.: + 519 661 3324; fax: + 519 661 2033.
E-mail addresses: [email protected] (Z. Nouri), [email protected] (R. Li), [email protected] (R.A. Holt), [email protected] (S.D. Rosner). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.12.060
most of the ionization in the discharge [4,5]. Positive gas ions from the discharge are accelerated in the opposite direction across the cathode fall and sputter material from the two cathodes that may then be ionized by the fast electrons. The high-pressure mode is also characterized by a relatively low anode–cathode voltage and a relatively high discharge current (see below for details). When the Penning cell is used as an axial-extraction ion source, one of the cathodes has a hole for ion extraction, and the other cathode is often referred to as an anticathode. (Radial extraction is also possible.) If the desired ion is to be generated by sputtering, both cathode and anticathode are formed from that material and an inert gas is used as the support gas for the discharge. Ions of both the support gas and sputtered material are extracted by an electric field produced by an external potential applied between an extraction electrode (which is often the first element of an einzel lens) and the cathode. Our source traces its history to a design developed by Baumann and Bethge at the University of Frankfurt [6]. This source could be used in either the high-voltage low-current mode or the low-voltage high-current mode. The transition from the former to the latter is accomplished by increasing the gas pressure inside the source. Much of their work was done in the high-voltage lowcurrent low-pressure mode to maximize the output and yield of multiply charged ions. They used a mass spectrometer with an energy resolving power E/DE of up to 400 to study the ion-beam energy spread in both modes [7] and to investigate indirectly the center potential in the ion source as a function of gas pressure, magnetic field, and discharge voltage, identifying four different modes [8]. In their observations of the high-voltage discharge
ARTICLE IN PRESS Z. Nouri et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 174–178
mode, carried out with a resolving power E/DE of only 100, the measured beam energy spreads of 50–100 eV were consistent with the instrumental resolution. For the studies of the high-current mode, they narrowed the slits to obtain E/DE=400 and measured an average full-width-half-maximum (FWHM) energy width of 24 eV. Since their energy resolution at 8 keV would have been 20 eV, this did not permit them to draw quantitative conclusions about the precise value of the ion-beam energy spread. Further development by the same group at Frankfurt led to two additional versions of the source, one incorporating an oven to vaporize solid materials and the other modified to allow long operating lifetimes in the sputtering mode [9]. The latter version, which is the one upon which our source is based, has the anode shortened to a thin disk, greatly reducing the amount of sputtered material that would be lost to the discharge process by being deposited on a long cylindrical anode. In this compact geometry the sputtered material primarily flows between the cathode and anticathode. Our primary interest in the PIG source relates to its application to fast-ion-beam laser-fluorescence measurements of atomic lifetimes, oscillator strengths, hyperfine structures, isotope shifts and transition wave numbers [10]. This work is primarily motivated by the needs of the astrophysical community for determining abundances of a wide range of metals found in stars and interstellar clouds. Many of the metals of interest (the Fe group and the lanthanides) are difficult to produce from ion sources relying on thermal vaporization because of the high temperatures required. Sputter ion sources are ideal since they rely on collisions to liberate the ions; however the literature indicates energy spreads that translate to Doppler widths of spectroscopic lines far too large for high-precision measurement. In practice spectroscopic linewidths of 100 MHz (0.003 cm 1) or less are required. For fundamental studies of few-electron atoms, the resolution requirements are even higher. Such linewidths correspond to energy spreads of only a few eV, along with the use of fast-ion beams in the 10 keV range or higher. Thus our goal was to explore the potential distribution in a Penning sputter ion source in order to produce ion beams with a low energy spread.
2. Apparatus and experimental methods The experimental setup for the ion and laser beams in the current measurements is typical of all our spectroscopic work [10] and will be described in detail below. Our requirements for highresolution spectroscopy on the extracted ion beam are stable, high ion currents, and especially a low energy spread of the extracted ions, as explained above. Acceleration to high energies causes a large Doppler shift but also narrows the linewidth by an effect called ‘kinematic compression’ [11]. Since the energy spread is presumed to arise from the different positions at which ions may be produced in the discharge, it is reasonable to explore a discharge mode with relatively low voltage. Hence we ultimately came to focus on the low-voltage high-current mode that is achieved at relatively high pressure. We began with a Penning sputter ion source (Fig. 1) obtained from Physicon Corporation [12], which is based closely on Version III of the source developed by Baumann and Bethge [9]. In this source the thin disk anode, which replaces the usual long cylindrical anode, allows the discharge space between cathode and anticathode to be decreased to 8.3 mm. The Al anode is 1 mm thick and has an 8.1 mm diameter aperture. In the present work we used Ti disks (diameter 10.9 cm) for the cathode and anticathode, and Ne as the support gas for the Penning discharge. Although we have also used Ar, Ne is preferable because it yields somewhat higher Ti + currents and because the Ti/Ne mass difference is large
175
Fig. 1. Schematic of Penning ion source and grounded extractor electrode. The volumes between the cathode, anode, and anticathode are filled with the support gas, which maintains the discharge. Note especially the Ta outlet plate inserted to maintain a large pressure difference between the interior of the source and its high-vacuum surroundings. The coils that produce the axial magnetic field are not shown.
enough to give a clean separation of the two elements in our ionbeam mass analyzer (a Wien velocity filter). The ion source has four interacting parameters, any three of which can be controlled externally: internal gas pressure, applied magnetic field, anode–cathode voltage, and discharge current. We use a crossover power supply for the discharge, which can operate in either constant-current or constant-voltage mode, with a 1 kO ballast resistor between the power supply and the anode to mitigate start-up transients and to help stabilize the discharge. The supply can provide up to 1 kV at 200 mA. Since the ion-beam energy determines the Doppler shift, and the ions are created in a plasma whose potential is close to that of the anode, it would seem that constant-voltage mode is preferable; however, in this mode the discharge current undergoes small fluctuations, which affect the voltage drop across the ballast resistor and hence the actual anode–cathode voltage. In practice these current fluctuations produce shifts in the resonant optical absorption wavelength that are small compared to the typical Doppler width. Baumann’s data for the Physicon source were typically taken with discharge parameters of 400 V and 50 mA [13]. We experimented with a wide variety of discharge conditions, as will be discussed in the next section. It is not possible to monitor the neutral gas pressure inside the source directly. We used an ionization gauge to measure the pressure in the high vacuum outside the source, and a Convectron [14] gauge in the gas supply line to the source. Baumann and Bethge typically ran their source with an operating vacuum of 1– 2 10 5 mbar, using oil diffusion pumps with a combined pumping speed of 700 l/s [13]. The standard Physicon source has a cylindrical outlet channel through the cathode that is 3.2 mm in diameter and 4.1 mm long. If we use a calculated [15] flow conductance of 0.403 l/s, this implies an internal source pressure of 17–35 mbar. In many applications, including ours, it is highly desirable to extract the ion beam into a region of high vacuum with as few collisions with neutral gas as possible in order to maximize the population of high charge states and/or highly excited metastable ions. (We use this term in the atomic-physics sense of excited electronic states that are forbidden to decay by electric-dipole selection rules, not in the mass-spectroscopy sense of molecular ions that dissociate in flight.) This motivated us to experiment with low gas pressures inside the source. However, running the
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ion source in the low-pressure mode produced low-current beams with large energy spread, consistent with the findings of Rohwer et al. [7]. In addition, we experienced serious problems of highvoltage breakdown that were ultimately traced to discharges through the neutral gas between the ion beam (which is at high potential arising from its space charge) and the extractor cone (the first element of the einzel lens). These discharges were eliminated by enlarging the hole in the extractor cone and reducing the gas pressure in the extraction region. As a result, we determined that, in order to achieve the dual goals of high metastable content and low energy spread, we would need to have a high gas pressure inside the source (to run in low-voltage high-current mode) and a very low pressure outside it (to avoid collisions with neutral gas as well as high-voltage breakdown). Our solution was to install a thin Ta disk (0.38 mm thickness) with a very small hole (0.38 mm diameter) between the cathode and the high vacuum. This does limit the total ion-beam current that can be extracted, but that is generally not a problem in collinear beam-laser experiments, where the laser beam size limits the portion of a (typically space-charge-limited) ion beam that can be excited. Thus the brightness of the ion beam is usually more important than the total current that can be delivered to a large target, e.g., the 1-cm-diameter Faraday cup used by Baumann and Bethge. From the 0.38 mm hole size and the nominal pumping speed of two baffled diffusion pumps, we estimate the internal Ne source pressure to be 0.3 mbar when the outside background pressure of Ne is 8 10 6 mbar measured 75 cm downstream from the source exit. Since this internal pressure is almost an order of magnitude greater than that used in the Frankfurt source, we conclude that we are operating the source in a discharge mode not previously studied. A comparison of gas pressures in the gas feed line for both sources supports this conclusion. The apparatus used to measure the Doppler width has been used extensively in our laboratory for measurements of hyperfine structure, isotope shifts, radiative lifetimes, and branching fractions [16]. In the present work, we used a cathode and anticathode of Ti. Ions produced in the source are extracted by a 10 kV potential difference and mass analyzed using a Wien filter primarily to remove ions of the Ne support gas so that they will not limit the useful current of Ti + via their space charge. A series of three einzel lenses and electrostatic deflectors transport the ion beam to the region where it interacts with an antiparallel collinear laser beam before being deflected into a Faraday cup. In this interaction region, the ions are accelerated by typically 478 V to bring them into optical resonance with the laser. Thus the laser-induced fluorescence (LIF) produced at resonance is confined to the region of the light-detection optics. This localization of the LIF is also important since the ion source produces many metastable ions with energies as high as 25,000 cm 1 above the ground state: if these ions interacted with the laser before reaching the light-detection optics, they would be undetectable. The LIF is collected by a set of radial quartz optical fibers arranged azimuthally around the collinear beams. These fibers pass through a specially designed vacuum feedthrough and are then directed through a short-pass optical color filter (UG-11) to a bialkali photomultiplier tube (Electron Tubes Ltd. 9235QB) with a gain of 1:8 106 . The filter has at least 70% transmission in the region 300–370 nm where most of the LIF occurs but gives excellent rejection of scattered laser light (420–460 nm). Since we were already set up to apply the fast-ion-beam laserinduced-fluorescence (‘beam-laser’) method [17], it was natural to use it to study the energy spread and absolute energy of our ion beams. This method provides a tremendous increase in resolving power over conventional electromagnetic energy spectrometers. For a laser beam intersecting an ion beam of speed b (in units of c)
at 1801, the laser frequency n‘ must be set to 1=2 1b n‘ ¼ n0 1þb
ð1Þ
to achieve resonance with a transition at frequency v0 in the ions’ rest frame. Since b 51, the speed b is related to the ion-beam energy E and mass M according to 2E 1=2 b¼ : ð2Þ Mc2 Thus the ‘sensitivity’ of the Doppler shift with respect to ionbeam energy is given, upon differentiating (1) with respect to E, by dn‘ n0 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 22:5 MHz=eV dE 2Mc2 E
ð3Þ
for typical parameters of M= 48 u, E =10.5 keV and v0 = c/ 435 nm. Note that the E 1/2 factor, implying that accelerating the ions narrows the spectral lines, expresses the effect of kinematic compression. One can see from (2) that it is simply due to the fact that the speed depends on the square root of the energy. Thus a small energy spread DE in the ion source results in a large velocity spread at low beam energy but a much smaller one at high energy, as a result of the decreasing slope of the E1/2 function. The ultimate limit of the energy resolution in the beam-laser method at this energy is 0.04 eV, due to the frequency jitter of the CW laser, which is 1 MHz. However, in most cases the natural linewidth of the atomic transition (due to spontaneous emission) sets the limit. For the Ti + transitions employed, the radiative lifetime is typically 4–7 ns, resulting in a limiting instrumental linewidth of 23–40 MHz, equivalent to an energy resolution of 1.0–1.8 eV. At a beam energy of 10.5 keV, this represents a resolving power of 0.6–1.0 104. In addition to measuring the energy spread of the ions, we can measure the ion beam energy with very high precision, and thus determine the plasma potential at which the ions were created. We use the laboratory frequency of the laser at resonance, n‘ , and a known value of the ion rest-frame transition frequency, v0 to find the speed of the ions from (1) and hence the absolute ion kinetic energy in the laboratory frame. The laser frequency is measured with a homebuilt traveling Michelson wavelength meter that uses a polarization-stabilized HeNe laser as a reference, and has an accuracy of 90 MHz, equivalent to measuring the absolute beam energy to 4 eV. This limitation can easily be overcome with higher-precision wavelength meters.
3. Results After experimenting with many combinations of the source parameters we reached several important conclusions: (1) A new very-high-pressure low-voltage high-current mode of the discharge produces the smallest Doppler linewidths. (2) As the source pressure is raised, the linewidth decreases, accompanied by an expected increase in peak signal strength. Eventually, however, no further improvement occurs and the source operation becomes less stable. (3) The smallest linewidths occur when the anode-cathode voltage is minimized for a given current. This is accomplished by varying the magnetic field, and the minimum is quite sharp. With an internal pressure of 0.3 mbar of Ne, the source can operate stably with a discharge current of 60 mA and an anode– cathode voltage of 175 V. The magnetic field at which this
ARTICLE IN PRESS Z. Nouri et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 174–178
operation is achieved can vary from 0.02 to 0.06 T. Typical ion currents of Ti + extracted at 10 kV were 150 nA, measured through a pair of 6 mm diameter apertures, separated by 8 cm, at 3 m from the ion-source exit hole. We also observe a significant current of Ti2 + , which is of astrophysical importance in hot stars. Furthermore, we have observed optical transitions in Ti + originating from levels with energies as high as 25,000 cm 1 above the ground state, confirming that the discharge mode is capable of producing energetic metastable populations. We find that a period of conditioning of 1 h is required to achieve the most stable operation, and that this can vary from day to day. Note that as the source operates, sputtered Ti is removed from the cathode and anticathode and deposited on the anode plate. During this process the cathode and anticathode become dished, changing the source geometry over time. The maximum time the source can operate before cleaning and possible replacement of electrodes is often limited by a metallic flake shorting the anode to a cathode or by the 0.38 mm hole in the outlet plate becoming clogged with sputtered Ti, and can be as high as 50 h. The source is easy to service and can usually be cleaned, refurbished, and reinstalled in 1 h. To measure a line profile for a given set of ion source parameters, the laser was tuned to the vicinity of an optical resonance in 48Ti + (the dominant isotope) and scanned over the line. In typical operation, the laser power was 20–50 mW at the entrance Brewster window, and the photomultiplier current was 500 nA on a background of 10 nA. This background arises primarily from light generated by collisions of beam ions with background gas, which has a pressure of typically 9 10 7 mbar (assuming it is mainly Ne) in the interaction region. Before translating the profile linewidth to an equivalent ionenergy spread in the source, it is necessary to account for the natural linewidth, and partial saturation of the absorption line, i.e. laser power broadening. To check for saturation, we observed the line profile for a given transition at two laser powers differing by a factor of 7 using a neutral density filter. The a2G7/2-z2Do5/2 transition at 23 027.804 cm 1 showed no change in linewidth and so was chosen for our study of line profile for different source conditions. For this transition, the lifetime of the upper state is 6.6 ns [18], correspond-
177
ing to a Lorentzian FWHM of 24 MHz, and thus makes a significant contribution to the observed linewidths of 75 MHz Fig. 2 shows the observed line profile for the 48Ti isotope as a function of source magnetic field, at fixed source pressure ( 0.7 mbar) and discharge current (60 mA). There is an optimum magnetic field at which the LIF signal is maximized, and the linewidth is only slightly larger than its minimum value at lower fields; at fields larger than the optimum, the signal declines and the linewidth increases, both rapidly. Fig. 3 shows the same profile as a function of source pressure at a fixed discharge current of 60 mA; at each pressure, the source magnetic field was varied to minimize the anode–cathode voltage since we saw evidence that this condition produced the best combination of small linewidths and large peak LIF signal. The linewidth has a minimum of 75 MHz at the highest pressure ( 0.5 mbar), then increases at first slowly and then rapidly as the pressure is reduced. The anode–cathode voltage at the pressure giving the narrowest linewidth was 233.5 V. The Doppler contribution to the minimum linewidth of 75 MHz was estimated at 65 MHz, or 2.9 eV, by simulating the experimental profile using a convolution of a Gaussian with a Lorentzian of width 24 MHz. Note that this contribution is not truly Gaussian (as would be obtained with a Maxwellian velocity distribution), but reflects the actual energy spread of ions extracted from the source, and typically shows an asymmetric low-energy tail which we attribute to ions produced in the cathode fall. As discussed in the previous section, we can use a measurement of n‘ and v0 to determine the energy of the ion beam from (1). Using this approach, we calculated an ion kinetic energy of 10712 eV at the central part of the interaction region where the potential is flat. With an extraction potential of 10 000 V and an interaction-region energy boost of 478 eV, this implies that the ions have an additional energy of 234 eV, which must be gained from the anode–cathode voltage. The very close agreement with the measured anode–cathode voltage of 233.5 V implies that the ions we observe are formed in a plasma that is very near anode potential, with very few forming in the cathode fall, although there is evidence for the latter in the longer ’tail’ of the lineshape on the high-frequency (low-energy) side, as mentioned above.
Fig. 2. Laser-induced-fluorescence spectrum of the 48Ti II a2G7/2-z2Do5/2 transition at 23027.804 cm 1 as a function of magnetic field in the ion source. The internal source pressure was held constant at an estimated pressure of 0.7 mbar and the discharge current was regulated to be 60 mA. Each curve is labeled at the left by the magnetic field in mT.
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Fig. 3. Laser-induced-fluorescence spectrum of the 48Ti II a2G7/2-z2Do5/2 transition at 23027.804 cm 1 as a function of Ne pressure. At each pressure, the source magnetic field was adjusted to minimize the anode–cathode potential difference. The discharge current was regulated to be 60 mA. Each curve is labeled at the left by the Ne pressure measured in the upstream gas feed line. The internal source pressure for the uppermost curve is estimated to be 0.5 mbar.
4. Conclusions
References
We have shown for the first time that an inexpensive, reliable, and nearly universal Penning sputter ion source can be operated in a stable mode that produces a very narrow ion-energy spread. Our measurements of the energy spread and absolute ion-beam energy by the beam-laser technique represent a very substantial improvement over electromagnetic energy-analysis methods. To operate the source in this new mode, it was necessary to modify it to provide a high differential pressure between its interior volume and the vacuum in the rest of the apparatus, and to adjust the magnetic field in the source very carefully. Additional important properties of this new discharge mode are the production of significant populations of metastable states and doubly charged species. The source is robust and requires only modest maintenance. It is especially versatile in that it can produce ions from any gas or from any conducting solid without the use of high-temperature ovens to vaporize the solid. This feature makes it useful with many refractory materials that are of astrophysical importance.
[1] C.E.S. Phillips, Proc. Roy. Soc. (London) A 64 (1898) 172–176. [2] B. Wolf, Handbook of Ion Sources, CRC Press, Boca Raton, FL, 1995 pp. 69–91. [3] W. Schuurman, Physica 36 (1967) 136–160. [4] S.P. Nikulin, Tech. Phys. 43 (7) (1998) 795–802. [5] L.A. Zyul’kova, A.V. Kozyrev, D.I. Proskurovsky, Tech. Phys. Lett. 30 (2004) 672–674. [6] H. Baumann, K. Bethge, Nucl. Instrum. Meth. 122 (1974) 517–525. ¨ [7] P. Rohwer, H. Baumann, K. Bethge, W. Schutze, Nucl. Instrum. Meth. 204 (1982) 245. ¨ [8] P. Rohwer, H. Baumann, W. Schutze, K. Bethge, Nucl. Instrum. Meth. 211 (1983) 543. [9] H. Baumann, K. Bethge, Nucl. Instrum. Meth. 189 (1981) 107 See especially Section 3.3 and Fig. 6.. [10] R.A. Holt, R. Li, R. Chatelain, S.J. Rehse, S.D. Rosner, T.J. Scholl, Phys. Scr. T 134 (2009) 014012 (8pp). [11] S.L. Kaufman, Opt. Commun. 17 (1976) 309. [12] Physicon Corporation, 12 Kendrick Road, Wareham MA 02571, U.S.A. [13] Heimart von Zweck, private communication. [14] Brooks Automation (Granville-Phillips), Inc., 15 Elizabeth Drive, Chelmsford, MA 01824, USA. [15] S. Dushman, Scientific Foundations of Vacuum Technique, 2nd ed., Wiley, New York, 1962. [16] S.D. Rosner, D. Masterman, T.J. Scholl, R.A. Holt, Can. J. Phys. 83 (2005) 841. ¨ A. Gaupp, K. Tillmann, W. Wittmann, Nucl. Instrum. Meth. 110 [17] H.J. Andra, (1973) 453. [18] A. Bizzarri, M.C.E. Huber, A. Noels, N. Grevesse, S.D. Bergeson, P. Tsekeris, J.E. Lawler, Astron. Astrophys. 273 (1993) 707.
Acknowledgements We thank the Natural Sciences and Engineering Research Council of Canada for financial support. We thank Harry Chen for expert electronics assistance and Brian Dalrymple and Frank Van Sas for expert machining.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A Penning sputter ion source with very low energy spread Z. Nouri a, R. Li b, R.A. Holt a, S.D. Rosner a, a b
Department of Physics and Astronomy, University of Western Ontario, London ON, Canada N6A 3K7 TRIUMF, 4004 Wesbrook Mall, Vancouver BC, Canada V6T 2A3
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 December 2009 Accepted 19 December 2009 Available online 5 January 2010
We have developed a version of the Frankfurt Penning ion source that produces ion beams with very low energy spreads of 3 eV, while operating in a new discharge mode characterized by very high pressure, low voltage, and high current. The extracted ions also comprise substantial metastable and doubly charged species. Detailed studies of the operating parameters of the source showed that careful adjustment of the magnetic field and gas pressure is critical to achieving optimum performance. We used a laser-fluorescence method of energy analysis to characterize the properties of the extracted ion beam with a resolving power of 1 104, and to measure the absolute ion beam energy to an accuracy of 4 eV in order to provide some insight into the distribution of plasma potential within the ion source. This characterization method is widely applicable to accelerator beams, though not universal. The low energy spread, coupled with the ability to produce intense ion beams from almost any gas or conducting solid, make this source very useful for high-resolution spectroscopic measurements on fast-ion beams. & 2009 Elsevier B.V. All rights reserved.
Keywords: Penning sputter ion source Energy spread
1. Introduction The Penning ion source, also called the Penning (or Phillips) Ionization Gauge (PIG) ion source, owes its origin to the discovery of this gas discharge mode by Phillips [1], and has been used in cyclotrons and linear accelerators for many years [2]. The same discharge is the basis of ion pumps and some pressure gauges. In its most common form, a Penning cell consists of a linear arrangement of two disk cathodes separated by an anode, which is usually a hollow cylinder. Crucial to the design is an axial magnetic field, typically 0.02–0.15 T, which confines discharge electrons to spiral paths between collisions as they oscillate in the axial potential well. The lengthened electron path allows the Penning discharge to be self-sustaining at relatively low gas pressures compared to a normal glow discharge [3]. It is well known [3] that the interplay of gas pressure, magnetic field strength, and applied anode–cathode potential results in several different operating modes of the Penning discharge. In the high-pressure low-voltage high-current mode, which is of greatest interest here, the main discharge space between the anode and two cathodes is occupied by a weakly ionized quasi-neutral plasma very nearly at the anode potential. A steep potential gradient—the ‘cathode fall’—occurs very close to the cathodes, and is the source of a population of non-thermal fast electrons that are responsible for
Corresponding author. Tel.: + 519 661 3324; fax: + 519 661 2033.
E-mail addresses: [email protected] (Z. Nouri), [email protected] (R. Li), [email protected] (R.A. Holt), [email protected] (S.D. Rosner). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.12.060
most of the ionization in the discharge [4,5]. Positive gas ions from the discharge are accelerated in the opposite direction across the cathode fall and sputter material from the two cathodes that may then be ionized by the fast electrons. The high-pressure mode is also characterized by a relatively low anode–cathode voltage and a relatively high discharge current (see below for details). When the Penning cell is used as an axial-extraction ion source, one of the cathodes has a hole for ion extraction, and the other cathode is often referred to as an anticathode. (Radial extraction is also possible.) If the desired ion is to be generated by sputtering, both cathode and anticathode are formed from that material and an inert gas is used as the support gas for the discharge. Ions of both the support gas and sputtered material are extracted by an electric field produced by an external potential applied between an extraction electrode (which is often the first element of an einzel lens) and the cathode. Our source traces its history to a design developed by Baumann and Bethge at the University of Frankfurt [6]. This source could be used in either the high-voltage low-current mode or the low-voltage high-current mode. The transition from the former to the latter is accomplished by increasing the gas pressure inside the source. Much of their work was done in the high-voltage lowcurrent low-pressure mode to maximize the output and yield of multiply charged ions. They used a mass spectrometer with an energy resolving power E/DE of up to 400 to study the ion-beam energy spread in both modes [7] and to investigate indirectly the center potential in the ion source as a function of gas pressure, magnetic field, and discharge voltage, identifying four different modes [8]. In their observations of the high-voltage discharge
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mode, carried out with a resolving power E/DE of only 100, the measured beam energy spreads of 50–100 eV were consistent with the instrumental resolution. For the studies of the high-current mode, they narrowed the slits to obtain E/DE=400 and measured an average full-width-half-maximum (FWHM) energy width of 24 eV. Since their energy resolution at 8 keV would have been 20 eV, this did not permit them to draw quantitative conclusions about the precise value of the ion-beam energy spread. Further development by the same group at Frankfurt led to two additional versions of the source, one incorporating an oven to vaporize solid materials and the other modified to allow long operating lifetimes in the sputtering mode [9]. The latter version, which is the one upon which our source is based, has the anode shortened to a thin disk, greatly reducing the amount of sputtered material that would be lost to the discharge process by being deposited on a long cylindrical anode. In this compact geometry the sputtered material primarily flows between the cathode and anticathode. Our primary interest in the PIG source relates to its application to fast-ion-beam laser-fluorescence measurements of atomic lifetimes, oscillator strengths, hyperfine structures, isotope shifts and transition wave numbers [10]. This work is primarily motivated by the needs of the astrophysical community for determining abundances of a wide range of metals found in stars and interstellar clouds. Many of the metals of interest (the Fe group and the lanthanides) are difficult to produce from ion sources relying on thermal vaporization because of the high temperatures required. Sputter ion sources are ideal since they rely on collisions to liberate the ions; however the literature indicates energy spreads that translate to Doppler widths of spectroscopic lines far too large for high-precision measurement. In practice spectroscopic linewidths of 100 MHz (0.003 cm 1) or less are required. For fundamental studies of few-electron atoms, the resolution requirements are even higher. Such linewidths correspond to energy spreads of only a few eV, along with the use of fast-ion beams in the 10 keV range or higher. Thus our goal was to explore the potential distribution in a Penning sputter ion source in order to produce ion beams with a low energy spread.
2. Apparatus and experimental methods The experimental setup for the ion and laser beams in the current measurements is typical of all our spectroscopic work [10] and will be described in detail below. Our requirements for highresolution spectroscopy on the extracted ion beam are stable, high ion currents, and especially a low energy spread of the extracted ions, as explained above. Acceleration to high energies causes a large Doppler shift but also narrows the linewidth by an effect called ‘kinematic compression’ [11]. Since the energy spread is presumed to arise from the different positions at which ions may be produced in the discharge, it is reasonable to explore a discharge mode with relatively low voltage. Hence we ultimately came to focus on the low-voltage high-current mode that is achieved at relatively high pressure. We began with a Penning sputter ion source (Fig. 1) obtained from Physicon Corporation [12], which is based closely on Version III of the source developed by Baumann and Bethge [9]. In this source the thin disk anode, which replaces the usual long cylindrical anode, allows the discharge space between cathode and anticathode to be decreased to 8.3 mm. The Al anode is 1 mm thick and has an 8.1 mm diameter aperture. In the present work we used Ti disks (diameter 10.9 cm) for the cathode and anticathode, and Ne as the support gas for the Penning discharge. Although we have also used Ar, Ne is preferable because it yields somewhat higher Ti + currents and because the Ti/Ne mass difference is large
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Fig. 1. Schematic of Penning ion source and grounded extractor electrode. The volumes between the cathode, anode, and anticathode are filled with the support gas, which maintains the discharge. Note especially the Ta outlet plate inserted to maintain a large pressure difference between the interior of the source and its high-vacuum surroundings. The coils that produce the axial magnetic field are not shown.
enough to give a clean separation of the two elements in our ionbeam mass analyzer (a Wien velocity filter). The ion source has four interacting parameters, any three of which can be controlled externally: internal gas pressure, applied magnetic field, anode–cathode voltage, and discharge current. We use a crossover power supply for the discharge, which can operate in either constant-current or constant-voltage mode, with a 1 kO ballast resistor between the power supply and the anode to mitigate start-up transients and to help stabilize the discharge. The supply can provide up to 1 kV at 200 mA. Since the ion-beam energy determines the Doppler shift, and the ions are created in a plasma whose potential is close to that of the anode, it would seem that constant-voltage mode is preferable; however, in this mode the discharge current undergoes small fluctuations, which affect the voltage drop across the ballast resistor and hence the actual anode–cathode voltage. In practice these current fluctuations produce shifts in the resonant optical absorption wavelength that are small compared to the typical Doppler width. Baumann’s data for the Physicon source were typically taken with discharge parameters of 400 V and 50 mA [13]. We experimented with a wide variety of discharge conditions, as will be discussed in the next section. It is not possible to monitor the neutral gas pressure inside the source directly. We used an ionization gauge to measure the pressure in the high vacuum outside the source, and a Convectron [14] gauge in the gas supply line to the source. Baumann and Bethge typically ran their source with an operating vacuum of 1– 2 10 5 mbar, using oil diffusion pumps with a combined pumping speed of 700 l/s [13]. The standard Physicon source has a cylindrical outlet channel through the cathode that is 3.2 mm in diameter and 4.1 mm long. If we use a calculated [15] flow conductance of 0.403 l/s, this implies an internal source pressure of 17–35 mbar. In many applications, including ours, it is highly desirable to extract the ion beam into a region of high vacuum with as few collisions with neutral gas as possible in order to maximize the population of high charge states and/or highly excited metastable ions. (We use this term in the atomic-physics sense of excited electronic states that are forbidden to decay by electric-dipole selection rules, not in the mass-spectroscopy sense of molecular ions that dissociate in flight.) This motivated us to experiment with low gas pressures inside the source. However, running the
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ion source in the low-pressure mode produced low-current beams with large energy spread, consistent with the findings of Rohwer et al. [7]. In addition, we experienced serious problems of highvoltage breakdown that were ultimately traced to discharges through the neutral gas between the ion beam (which is at high potential arising from its space charge) and the extractor cone (the first element of the einzel lens). These discharges were eliminated by enlarging the hole in the extractor cone and reducing the gas pressure in the extraction region. As a result, we determined that, in order to achieve the dual goals of high metastable content and low energy spread, we would need to have a high gas pressure inside the source (to run in low-voltage high-current mode) and a very low pressure outside it (to avoid collisions with neutral gas as well as high-voltage breakdown). Our solution was to install a thin Ta disk (0.38 mm thickness) with a very small hole (0.38 mm diameter) between the cathode and the high vacuum. This does limit the total ion-beam current that can be extracted, but that is generally not a problem in collinear beam-laser experiments, where the laser beam size limits the portion of a (typically space-charge-limited) ion beam that can be excited. Thus the brightness of the ion beam is usually more important than the total current that can be delivered to a large target, e.g., the 1-cm-diameter Faraday cup used by Baumann and Bethge. From the 0.38 mm hole size and the nominal pumping speed of two baffled diffusion pumps, we estimate the internal Ne source pressure to be 0.3 mbar when the outside background pressure of Ne is 8 10 6 mbar measured 75 cm downstream from the source exit. Since this internal pressure is almost an order of magnitude greater than that used in the Frankfurt source, we conclude that we are operating the source in a discharge mode not previously studied. A comparison of gas pressures in the gas feed line for both sources supports this conclusion. The apparatus used to measure the Doppler width has been used extensively in our laboratory for measurements of hyperfine structure, isotope shifts, radiative lifetimes, and branching fractions [16]. In the present work, we used a cathode and anticathode of Ti. Ions produced in the source are extracted by a 10 kV potential difference and mass analyzed using a Wien filter primarily to remove ions of the Ne support gas so that they will not limit the useful current of Ti + via their space charge. A series of three einzel lenses and electrostatic deflectors transport the ion beam to the region where it interacts with an antiparallel collinear laser beam before being deflected into a Faraday cup. In this interaction region, the ions are accelerated by typically 478 V to bring them into optical resonance with the laser. Thus the laser-induced fluorescence (LIF) produced at resonance is confined to the region of the light-detection optics. This localization of the LIF is also important since the ion source produces many metastable ions with energies as high as 25,000 cm 1 above the ground state: if these ions interacted with the laser before reaching the light-detection optics, they would be undetectable. The LIF is collected by a set of radial quartz optical fibers arranged azimuthally around the collinear beams. These fibers pass through a specially designed vacuum feedthrough and are then directed through a short-pass optical color filter (UG-11) to a bialkali photomultiplier tube (Electron Tubes Ltd. 9235QB) with a gain of 1:8 106 . The filter has at least 70% transmission in the region 300–370 nm where most of the LIF occurs but gives excellent rejection of scattered laser light (420–460 nm). Since we were already set up to apply the fast-ion-beam laserinduced-fluorescence (‘beam-laser’) method [17], it was natural to use it to study the energy spread and absolute energy of our ion beams. This method provides a tremendous increase in resolving power over conventional electromagnetic energy spectrometers. For a laser beam intersecting an ion beam of speed b (in units of c)
at 1801, the laser frequency n‘ must be set to 1=2 1b n‘ ¼ n0 1þb
ð1Þ
to achieve resonance with a transition at frequency v0 in the ions’ rest frame. Since b 51, the speed b is related to the ion-beam energy E and mass M according to 2E 1=2 b¼ : ð2Þ Mc2 Thus the ‘sensitivity’ of the Doppler shift with respect to ionbeam energy is given, upon differentiating (1) with respect to E, by dn‘ n0 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 22:5 MHz=eV dE 2Mc2 E
ð3Þ
for typical parameters of M= 48 u, E =10.5 keV and v0 = c/ 435 nm. Note that the E 1/2 factor, implying that accelerating the ions narrows the spectral lines, expresses the effect of kinematic compression. One can see from (2) that it is simply due to the fact that the speed depends on the square root of the energy. Thus a small energy spread DE in the ion source results in a large velocity spread at low beam energy but a much smaller one at high energy, as a result of the decreasing slope of the E1/2 function. The ultimate limit of the energy resolution in the beam-laser method at this energy is 0.04 eV, due to the frequency jitter of the CW laser, which is 1 MHz. However, in most cases the natural linewidth of the atomic transition (due to spontaneous emission) sets the limit. For the Ti + transitions employed, the radiative lifetime is typically 4–7 ns, resulting in a limiting instrumental linewidth of 23–40 MHz, equivalent to an energy resolution of 1.0–1.8 eV. At a beam energy of 10.5 keV, this represents a resolving power of 0.6–1.0 104. In addition to measuring the energy spread of the ions, we can measure the ion beam energy with very high precision, and thus determine the plasma potential at which the ions were created. We use the laboratory frequency of the laser at resonance, n‘ , and a known value of the ion rest-frame transition frequency, v0 to find the speed of the ions from (1) and hence the absolute ion kinetic energy in the laboratory frame. The laser frequency is measured with a homebuilt traveling Michelson wavelength meter that uses a polarization-stabilized HeNe laser as a reference, and has an accuracy of 90 MHz, equivalent to measuring the absolute beam energy to 4 eV. This limitation can easily be overcome with higher-precision wavelength meters.
3. Results After experimenting with many combinations of the source parameters we reached several important conclusions: (1) A new very-high-pressure low-voltage high-current mode of the discharge produces the smallest Doppler linewidths. (2) As the source pressure is raised, the linewidth decreases, accompanied by an expected increase in peak signal strength. Eventually, however, no further improvement occurs and the source operation becomes less stable. (3) The smallest linewidths occur when the anode-cathode voltage is minimized for a given current. This is accomplished by varying the magnetic field, and the minimum is quite sharp. With an internal pressure of 0.3 mbar of Ne, the source can operate stably with a discharge current of 60 mA and an anode– cathode voltage of 175 V. The magnetic field at which this
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operation is achieved can vary from 0.02 to 0.06 T. Typical ion currents of Ti + extracted at 10 kV were 150 nA, measured through a pair of 6 mm diameter apertures, separated by 8 cm, at 3 m from the ion-source exit hole. We also observe a significant current of Ti2 + , which is of astrophysical importance in hot stars. Furthermore, we have observed optical transitions in Ti + originating from levels with energies as high as 25,000 cm 1 above the ground state, confirming that the discharge mode is capable of producing energetic metastable populations. We find that a period of conditioning of 1 h is required to achieve the most stable operation, and that this can vary from day to day. Note that as the source operates, sputtered Ti is removed from the cathode and anticathode and deposited on the anode plate. During this process the cathode and anticathode become dished, changing the source geometry over time. The maximum time the source can operate before cleaning and possible replacement of electrodes is often limited by a metallic flake shorting the anode to a cathode or by the 0.38 mm hole in the outlet plate becoming clogged with sputtered Ti, and can be as high as 50 h. The source is easy to service and can usually be cleaned, refurbished, and reinstalled in 1 h. To measure a line profile for a given set of ion source parameters, the laser was tuned to the vicinity of an optical resonance in 48Ti + (the dominant isotope) and scanned over the line. In typical operation, the laser power was 20–50 mW at the entrance Brewster window, and the photomultiplier current was 500 nA on a background of 10 nA. This background arises primarily from light generated by collisions of beam ions with background gas, which has a pressure of typically 9 10 7 mbar (assuming it is mainly Ne) in the interaction region. Before translating the profile linewidth to an equivalent ionenergy spread in the source, it is necessary to account for the natural linewidth, and partial saturation of the absorption line, i.e. laser power broadening. To check for saturation, we observed the line profile for a given transition at two laser powers differing by a factor of 7 using a neutral density filter. The a2G7/2-z2Do5/2 transition at 23 027.804 cm 1 showed no change in linewidth and so was chosen for our study of line profile for different source conditions. For this transition, the lifetime of the upper state is 6.6 ns [18], correspond-
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ing to a Lorentzian FWHM of 24 MHz, and thus makes a significant contribution to the observed linewidths of 75 MHz Fig. 2 shows the observed line profile for the 48Ti isotope as a function of source magnetic field, at fixed source pressure ( 0.7 mbar) and discharge current (60 mA). There is an optimum magnetic field at which the LIF signal is maximized, and the linewidth is only slightly larger than its minimum value at lower fields; at fields larger than the optimum, the signal declines and the linewidth increases, both rapidly. Fig. 3 shows the same profile as a function of source pressure at a fixed discharge current of 60 mA; at each pressure, the source magnetic field was varied to minimize the anode–cathode voltage since we saw evidence that this condition produced the best combination of small linewidths and large peak LIF signal. The linewidth has a minimum of 75 MHz at the highest pressure ( 0.5 mbar), then increases at first slowly and then rapidly as the pressure is reduced. The anode–cathode voltage at the pressure giving the narrowest linewidth was 233.5 V. The Doppler contribution to the minimum linewidth of 75 MHz was estimated at 65 MHz, or 2.9 eV, by simulating the experimental profile using a convolution of a Gaussian with a Lorentzian of width 24 MHz. Note that this contribution is not truly Gaussian (as would be obtained with a Maxwellian velocity distribution), but reflects the actual energy spread of ions extracted from the source, and typically shows an asymmetric low-energy tail which we attribute to ions produced in the cathode fall. As discussed in the previous section, we can use a measurement of n‘ and v0 to determine the energy of the ion beam from (1). Using this approach, we calculated an ion kinetic energy of 10712 eV at the central part of the interaction region where the potential is flat. With an extraction potential of 10 000 V and an interaction-region energy boost of 478 eV, this implies that the ions have an additional energy of 234 eV, which must be gained from the anode–cathode voltage. The very close agreement with the measured anode–cathode voltage of 233.5 V implies that the ions we observe are formed in a plasma that is very near anode potential, with very few forming in the cathode fall, although there is evidence for the latter in the longer ’tail’ of the lineshape on the high-frequency (low-energy) side, as mentioned above.
Fig. 2. Laser-induced-fluorescence spectrum of the 48Ti II a2G7/2-z2Do5/2 transition at 23027.804 cm 1 as a function of magnetic field in the ion source. The internal source pressure was held constant at an estimated pressure of 0.7 mbar and the discharge current was regulated to be 60 mA. Each curve is labeled at the left by the magnetic field in mT.
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Fig. 3. Laser-induced-fluorescence spectrum of the 48Ti II a2G7/2-z2Do5/2 transition at 23027.804 cm 1 as a function of Ne pressure. At each pressure, the source magnetic field was adjusted to minimize the anode–cathode potential difference. The discharge current was regulated to be 60 mA. Each curve is labeled at the left by the Ne pressure measured in the upstream gas feed line. The internal source pressure for the uppermost curve is estimated to be 0.5 mbar.
4. Conclusions
References
We have shown for the first time that an inexpensive, reliable, and nearly universal Penning sputter ion source can be operated in a stable mode that produces a very narrow ion-energy spread. Our measurements of the energy spread and absolute ion-beam energy by the beam-laser technique represent a very substantial improvement over electromagnetic energy-analysis methods. To operate the source in this new mode, it was necessary to modify it to provide a high differential pressure between its interior volume and the vacuum in the rest of the apparatus, and to adjust the magnetic field in the source very carefully. Additional important properties of this new discharge mode are the production of significant populations of metastable states and doubly charged species. The source is robust and requires only modest maintenance. It is especially versatile in that it can produce ions from any gas or from any conducting solid without the use of high-temperature ovens to vaporize the solid. This feature makes it useful with many refractory materials that are of astrophysical importance.
[1] C.E.S. Phillips, Proc. Roy. Soc. (London) A 64 (1898) 172–176. [2] B. Wolf, Handbook of Ion Sources, CRC Press, Boca Raton, FL, 1995 pp. 69–91. [3] W. Schuurman, Physica 36 (1967) 136–160. [4] S.P. Nikulin, Tech. Phys. 43 (7) (1998) 795–802. [5] L.A. Zyul’kova, A.V. Kozyrev, D.I. Proskurovsky, Tech. Phys. Lett. 30 (2004) 672–674. [6] H. Baumann, K. Bethge, Nucl. Instrum. Meth. 122 (1974) 517–525. ¨ [7] P. Rohwer, H. Baumann, K. Bethge, W. Schutze, Nucl. Instrum. Meth. 204 (1982) 245. ¨ [8] P. Rohwer, H. Baumann, W. Schutze, K. Bethge, Nucl. Instrum. Meth. 211 (1983) 543. [9] H. Baumann, K. Bethge, Nucl. Instrum. Meth. 189 (1981) 107 See especially Section 3.3 and Fig. 6.. [10] R.A. Holt, R. Li, R. Chatelain, S.J. Rehse, S.D. Rosner, T.J. Scholl, Phys. Scr. T 134 (2009) 014012 (8pp). [11] S.L. Kaufman, Opt. Commun. 17 (1976) 309. [12] Physicon Corporation, 12 Kendrick Road, Wareham MA 02571, U.S.A. [13] Heimart von Zweck, private communication. [14] Brooks Automation (Granville-Phillips), Inc., 15 Elizabeth Drive, Chelmsford, MA 01824, USA. [15] S. Dushman, Scientific Foundations of Vacuum Technique, 2nd ed., Wiley, New York, 1962. [16] S.D. Rosner, D. Masterman, T.J. Scholl, R.A. Holt, Can. J. Phys. 83 (2005) 841. ¨ A. Gaupp, K. Tillmann, W. Wittmann, Nucl. Instrum. Meth. 110 [17] H.J. Andra, (1973) 453. [18] A. Bizzarri, M.C.E. Huber, A. Noels, N. Grevesse, S.D. Bergeson, P. Tsekeris, J.E. Lawler, Astron. Astrophys. 273 (1993) 707.
Acknowledgements We thank the Natural Sciences and Engineering Research Council of Canada for financial support. We thank Harry Chen for expert electronics assistance and Brian Dalrymple and Frank Van Sas for expert machining.