DRESDEN ELECTRON BEAM ION SOURCES: LATEST DEVELOPMENTS∗ G.Zschornack† , M.Kreller, A.Silze Technische Universit¨at Dresden, Dresden, Germany V.P.Ovsyannikov, F.Grossmann, R.Heller, U.Kentsch, M.Schmidt, A.Schwan, F.Ullmann DREEBIT GmbH, Dresden, Germany Abstract We give an overview of the Dresden EBIT/EBIS ion source family as platform technology for various applications in basic research and technology. Available are three types of compact room-temperature ion sources as well as a powerful superconducting ion source. The ion sources can produce all ion charge states of elements with Z < 30 and neon-like ions of heavier elements such as iridium or others. New technical developments are presented. The ”W”series of the EBIT/EBIS family is introduced featuring an integrated Einzel lens and a Wien filter modul. Furthermore a new element injection technique based on a Liquid Metal Ion Source in combination with a quadrupole bender is described.
INTRODUCTION During the last two decades highly charged ions have become of increasing interest in basic research with fields such as atomic physics, materials research, astrophysics, fusion research, radiation biology, metrology and others. Additionally a growing number of proof-of-principle experiments is known to demonstrate the application potential of highly charged ions (HCI) for technological applications. For example actual topics are accelerator physics, surface analysis, lithography, nanostructuring, information technology and medicine. For the mentioned applications the quest is to use ion sources allowing in-situ observation of basic processes by spectroscopy (x-rays, UV, EUV, visible light) and to extract ions with energies in the keV region to avoid damages of targets during different applications. In principle HCIs can be produced in very different ways. A well introduced technique is to strip preaccelerated low charged ions to higher charge states. Basically with this technique all charge states of all elements can be produced but for a wide spectrum of applications swift ions are not appropriate due to their high damage potential in the projectile-target interaction and their capability to penetrate into deep target layers. Hence the most common ion sources for the production of slow HCIs are ElectronCyclotron Resonance Ion Sources (ECRIS) and Electron Beam Ion Sources (EBIS). Hereby it must be stated that ∗ Work supported by the EFRE fund of the EU and by the Freistaat Sachsen (Project Nos. 12321/2000 and 12184/2000). The authors also thank L.Bischoff and W.Pilz for their support by the utilization of the LMIS tecnique. †
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ECRIS are more often used than EBIS. But for applications with HCIs of the highest ion charge states and at experiments with special demands on the beam properties and quality EBIS are clearly the better option. A comparison of selected parameters of both sources is given in Table 1. In this paper we will report on the development of an ion source platform technology, the Dresden EBIT/EBIS ion source family [1]. Table 1: Comparison of ECRIS and EBIS as sources for HCIs parameter
ECRIS
EBIS
electron energy
continuous distribution
sharp and tunable ∆E < 1 eV
ion charge states
low Z: up to bare nuclei higher Z: q ≈ Z/2 up to mA
all ion charge states of all Z
100...200 mm mrad
2...10 mm mrad
DC ion currents emittance
pA...nA (up to µA per pulse
THE PLATFORM TECHNOLOGY Three generations of high-innovative room temperature EBIS/EBIT ion sources of highly charged ions (Dresden EBIT, EBIS and EBIS-A) and a superconducting version (Dresden EBIS-SC) have been developed by the collaboration of the Technische Universit¨at Dresden and the DREEBIT GmbH Dresden since 1999 [1]. The basics of the Dresden EBIT/EBIS technology are patented solutions for the realization of compact, stable working and smallbudget electron beam ion sources [2, 3]. The functional principle of these ion sources is described in [4]. A 3D drawing of the room-temperature ion sources is shown in Fig. 1. In Fig. 2 the functional principle of the Dresden EBIT/EBIS source family is shown. The electron energy Ee necessary to ionize a certain ion charge state is the sum of the cathode potential UK and the potential U2 on the middle drift tube. An axial trap potential is realized by the potentials U1 and U3 whereby the potential U3 can be
Table 2: Comparison of ECRIS and EBIS as sources for HCIs. B – magnetic field on axis; Ee – electron energy; Ie – electron beam current; L – ion trap length; N – number of drift tubes; je – electron current density; SC – superconducting
Figure 1: 3D drawing of the Dresden EBIS/EBIT roomtemperature ion source family
parameter
EBIT
EBIS
EBIS-A
EBIS-SC
B/mT max. Ee /keV max. Ie /mA je /A cm−2 L/cm N magnet
250 15 50 < 300 2 3 SmCo
400 25 100 < 300 6 3 NdFeB
600 30 200 < 600 6 3 NdFeB
6000 30 1000 > 1000 20 8 SC
Figure 2: Scheme of the source potentials of an ion source equipped with three drift tubes
switched periodically to open and to close the trap for a pulsed ion extraction regime. For the generation of DC currens (leaky mode) the potential U3 should be lowered to a well defined quasi-open potential configuration. In Table 2 the most important parameters of all four ion sources are summarized. The electron beam currents of the room-temperature ion sources are the highest recommended currents for an effective ion production. Higher electron beam currents are possible but not useful, e.g. the Dresden EBIS-A is also possible to work with electron beam currents of up to 450 mA. However for this operation regime the electron beam density remarkably decreases and the highest charge states can not be reached. A description of the experimental technique to verify the effective electron beam density in the ion sources can be found in [5]. In this experiment with a non-cryogenic Dresden EBIS an electron beam radius of 57 µm (80% current) was measured at an electron beam energy of 16 keV and an electron beam current of 30 mA. This corresponds to an electron beam density of 232 A/cm2 . The emittances of extracted ion beams is measured with a new developed pepperpot emittance meter [1] as shown in Fig. 3. The emittance meter is an ion beam imaging system which allows to detect beams of ions, neutrals, electrons as well as X-rays in real time. The system is mounted onto a
Figure 3: Schema of the pepperpot emittance meter
single DN160CF flange featuring a linear motion vacuum feedthrough, SHV high voltage feedthroughs, a glass viewport und a camera fixture. The system generates an image of the beam by primarily converting the incident beam to secondary electrons using a MCP (Micro-Channel Plate). The secondary electrons are accelerated towards a phosphor screen reproducing the beam shape as visible light which can be monitored via a CCD camera. The measured RMS emittance of ion beams extracted from Dresden EBIT/EBIS sources is typically between 2 and 10 mm mrad. The highest available electron beam energy is 30 keV (see 2). The decision to restrict the electron beam energy to this limit allows a very compact ion source design and allows to produce ions over a wide spectrum of ion charge states. Fig. 4 shows details about it for the different ion source models. Only the highest ion charge states of the heavier elements can not be produced by the available
source models. But it should be mentioned that the need for the highest ion charge states of the highest atomic numbers is small and only of interest for special experiments in basic research. These demands can be satisfied by some special high-energy EBITs as they exist at the LLNL, in Heidelberg, Tokyo or Shanghai.
Detailed numbers of produced ions in the pulse and in the leaky mode can be found in [1, 6].
THE ION SOURCE TEST BENCH For the characterization of the different EBIS/T a ion source test bench is available with the following features: • ion acceleration of up to 1.5 MeV, • ion decceleration down to 10 eV x q, • charge state separation by a q/A magnet, • beam fine focussing down to the sub-mm region, • emittance measurements via pepperpot emittance meter. Currently this test bench employs a Dresden EBIS-A for source diagnostics but also for irradiation experiments wih highly charged ions. A drawing of the ion source test bench is presented in Fig. 5. A more detail description can be found in [7].
Figure 5: Test bench beam line for ion source characterization
A NEW SOURCE GENERATION: THE W-TYPE Figure 4: Available Ion charge states at different electron energies. The limit of 15 keV is typical for the Dresden EBIT, 25 keV correspond to the Dresden EBIS and 30 keV to the Dresden EBIS-A and EBIS-SC ion sources. The upper part gives the ultimative limit to ionize a certain ion. In the lower part the ion charge state region is given which can be produced in an optimal manner, e.g. when the electron beam has 2.7 times of the electron binding energy of the weakest bound electron in the ion. The ion sources can operate in a pulsed mode with pulse lengths of some tens of ns up to about 100 µs. Roomtemperature EBIS/T also produce DC beams with particle numbers in the order of up to > 1010 ions per second.
Figure 6: 3D drawing of a wien filter for use together with ion sources of the Dresden EBIS/T family
In ion beam lines it is standard to separate beams extracted from an ion source by a magnetic q/A analyzer. As an alternative we have developed a Wien filter as compact charge state separator with a resolving power of up to > 80 [8]. A drawing of this filter is shown in Fig. 6. The construction of the filter allows the mounting of it on all ion sources of the Dresden EBIS/EBIT type resulting in a very compact solution for HCI beam lines. Ion sources supplied with a Wien filter are named as ”W” series, for instance Dresden EBIS-W (see Fig. 7). Ion sources of the ”W”-series also feature an integrated Einzel lens leading to optimal focusing results of the extracted ion beam. A 3D model of a ”W”-EBIS is shown in Fig. 7. The Wien filter can be equipped with apertures of different diameters (0.5, 1 or 1.5 mm) in dependence on the resolution and transmission requirements of the setup. The use of a ”W”-type ion source allows to realize very compact ion beam lines. Hence a beam line with an ion charge state separation, a Faraday-cup and a small target chamber features an overall length of less than 1 m.
Figure 8: Injection of elements into the the ion trap by a combination of a LMIS with a quadrupole beam bender lines from direct exciation and radiative recombination processes in the EBIT final ion charge states up to Au53+ can be determined. This demonstrates the functional principle of the ion injection technique. Hence EBIS/EBIT ion sources can be used as charge breeder in experiments with radioactice short-lived isotopes.
Figure 7: Drawing of a Dresden EBIS(W) with integrated Wien filter (on the right)
Table 3: Ion currents measured in the leaky mode at a Dresden EBIS(W) ion ions/s
H+
Xe8+
Xe26+
Xe44+
Xe46+
2 · 109
2 · 107
8 · 105
7 · 103
3 · 103
LMIS-BASED ELEMENT INJECTION TECHNIQUES In the past the element injection into the EBIT/EBIS sources was realized by gases, by MEVVA sources or with different volatile chemical compounds [9]. We have tested a new type of injection as shown in Fig. 8. Therefore we used a Liquid Metal Ion Source (LMIS) generating a beam of low charged ions which was injected into a Dresden EBIS via an electrostatic quadrupole. Such a technique allows to inject elements which are usually difficult to inject into EBIT/EBIS devices. This concerns elements such as Bi, Ge, Er, In, Au, Ce, Sb, Pt, Pr and others. Fig. 9 shows an X-ray spectrum measured after injection of Au1+ ions into a Dresden EBIT. Analyzing X-ray
Figure 9: X-ray spectrum from gold ions trapped in a Dresden EBIT after external injection. black spectrum – background spectrum before gold injection; red spectrum – spectrum of trapped gold ions
DRESDEN EBIS-SC: A NEW SUPERCONDUCTING ION SOURCE GENERATION To increase the extractable currents of highly charged ions and supplementing the Dresden EBIS/EBIT technology the prototype of a new superconducting ion source, the Dresden EBIS-SC, has been developed and is being tested. For this EBIS design the most modern principles of electron optics and magnetic field formation are applied. The
Dresden EBIS-SC operates with a refrigerator unit without liquid gases. The source design (see Fig. 10) is very compact and based on a DN350CF construction. Important design parameters are given in table 2.
SUMMARY With the Dresden EBIT/EBIS ion source family a platform technology for the application of highly charged ions in basic research as well as for applied technology is available. The different ion sources satisfy various requirements for a manyfold of investigations in the field of X-ray, UVand EUV-spectroscopy on highly charged ions and studies on ion-surface interactions. They complement high-tech devices such as Focused Ion Beams (FIB) [10] or Timeof-Flight Secondary Ion mass Spectrometer (TOF-SIMS). With the Dresden EBIS-SC a new powerful EBIS is available which suits requirements for applications in accelerators and applications requiring more intense beams of highly charged ions. The increasing efficiency of EBIS sources favours them beside ECRIS for applications in accelerators for the medical particel therapy such as synchrotrons [11, 12], CYCLINACs [13] and Rapid Cycling Medical Synchrotrons [14].
REFERENCES Figure 10: 3D drawing of the Dresden EBIS-SC. In the representation the electron gun side is seen including the feedtroughs for various electrical potentials, pressure measurement, gas inlet and drift tube heating.
Table 4: Design parameter of the Dresden EBIS-SC parameter
value
magnetic field max. electron beam current max. electron energy eff. electron beam density max. eletrical trap capacity distance cathode-anode ion trap length number of drift tubes
≤6T 1A 30 keV > 1000 A/cm2 ≈ 6 · 1010 e variabel ≤ 20 cm 8 (individually controllable)
The Dresden EBIS-SC features technical details to optimize the source operation. The cathode can be changed without affecting the vacuum in the sources. Additionally the distance cathode-anode can be set from the outside to optimize the source opoeration for special regimes. The drift tubes have room-temperature to guarantee the best working conditions for the operation in the high-current regime with electron beam currents up to 1 A. A special heater system is integrated in the drift tube construction for preparing high-quality vacuum conditions in the surrounding area of the electron beam. In first test experiments the source run with electron beam currents of 750 mA at an electron beam transmission of 0.9984. First pulses of hydrogen, carbon and argon ions have been extracted.
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