| Momentum Imaging for TimE Resolved Studies – MISTERS |
A COLTRIMS Apparatus for Time Resolved Studies: Located in building 2, room 102
Applications at the High Harmonics Laser Lab and the ALS beamline
COLTRIMS is an acronym for COLd Target Recoil Ion Momentum Spectroscopy, which readily describes what it stands for: An internally cold gas jet target, produced via a supersonic expansion into a vacuum, and a device that resolves momentum of ions (and electrons) which are produced by ionizing radiation or particle impact of any kind from that gas target. The reaction volume, e.g. the intersection of the supersonic gasjet and the incoming projectile beam is located inside the momentum spectrometer. Since we would like to extend this technique to time resolved measurements we call it Momentum Imaging for TimE Resolved Studies (MISTERS).
UHV Chamber
The largest and most obvious part of a COLTRIMS apparatus is the vacuum chamber. While in ion beam or synchrotron radiation experiments the requirements concerning vacuum are rather low, the use of high intensity laser pulses imposes much stronger restrictions on the base pressure in the chamber. At field strength of a few times 1014 W/cm2 the ionization probability for any atom or molecule reaches unity. Therefore any atom or molecule that is in the focal volume will be ionized. To avoid space charge effects, which would destroy the resolution, a small focal volume in connection with a high vacuum is necessary. In case of ion-electron coincidences the requirements are even more severe. This is where we get into the ultra high vacuum regime of <10-10 mbar, e.g. a pressure of 10-10 mbar, which corresponds to still about 2400 particles per mm3.
Whereas the 10-10 mbar range is easily reached with a two stage setup of turbo molecular pumps, proper copper sealing and moderate baking (2 days, 100°C), the next factor of ten in vacuum is achieved only with some effort. This includes rigorously clean room conditions at assembly time, strict avoidance of virtual leaks, baking at an increased temperature and finally the right vacuum pumps such as cryo pumps, ion traps or titanium sublimation pumps. From test runs we know that in an empty chamber like this, baked at 300°C, pumped by two cryo pumps with 1500 l/s a pressure of 4 · 10-11 mbar can be achieved. Fig. 1 shows the vacuum system.
Figure 1: Schematic view of the ultra high vacuum (UHV) chamber. The pumping speeds of roughing and high vacuum pumps have to match each other and the total gas load of the system. We separated the vacuum system into a high load, low vacuum (source and collimator) and a low load, high vacuum part (main). The latter one consisted of three turbo molecular drag pumps attached to the main chamber with together 1200 l/s backed by another small 70 l/s turbo molecular drag pump. The UHV in the main chamber is separated by a skimmer from the source chamber where the gas jet is generated. There, naturally, a much higher gas load has to be taken into account, but fortunately a vacuum of a few times 10−4 mbar is already sufficient to create a supersonic expansion (image courtesy of RoentDek).
Gas Jet
The gas jet is made up of a modified Leybold coldhead with a 0.01 mm nozzle mounted to its tip. The gas passes through a copper tube and is precooled in the coldfinger before it is allowed to expand through the nozzle. In the resulting directed gas stream the so called zone of silence is formed. In this region the internal temperature of the gas in the zone of silence is to only a few Kelvin.
Its dimensions depend strongly on the ratio of gas supply pressure and the background pressure of the expansion chamber. Fig. 2 shows a schematic sketch of a supersonic expansion. A skimmer, appropriately positioned, can then cut out gas from the zone of silence, thereby creating a very directional, high velocity gas jet.
The diagnosis of the jet can be done with a moveable pitot tube positioned above the spectrometer, which has an ion gauge at its blind end. The pitot tube can be moved in and out of the jet and thereby gave a means of scanning the jet. Alternatively a differentially pumped chamber as a jet dump can be chosen
Figure 2: Schematic drawing of the adiabatic gas expansion, which forms a supersonic jet.
Spectrometer
The purpose of the spectrometer is to guide ions and electrons onto position sensitive detectors. It typically consists of two regions: The extraction region where a homogeneous electric field is generated to extract the charged particles from the interaction zone and an (electric) field free drift region. Both are separated by meshes of variable transmission. Apart from that, use of electrostatic lenses is made occasionally for low energetic ions (meV).
Thus, for measurements where small ion momentum perpendicular to the spectrometer axis is expected, we employ a long drift region for the ions to increase our resolution and make use of an electrostatic lens to spread the perpendicular momentum even more. In circularly polarized light on the other hand there is no orientation of the ions along with respect to the spectrometer. To increase our collection efficiency for these ions we therefore would have to use a shorter extraction region and apply higher fields.
In the high resolution setup we omit a metallic mesh in between the extraction and the ion drift region thereby creating a magnifying electrostatic lens. Using the electromagnetic field simulation software SIMION the spread of transverse momenta depending on extraction field and initial kinetic energy can be calculated and used in the data analysis for reconstruction of ion momenta. The achieved magnification is typically in the range of 4:3 up to 3:2.
Another unusual feature of geometry is the extraordinarily large cut-out (3 cm × 3.7 cm) along the laser beam axis. The parallel laser beam traverses the spectrometer through that cut-out before it falls on a mirror and is back reflected and focused (fig. 3). Since we intend using a 1″ parabolic mirror with 5 cm focal length, it has to reside within the spectrometer plates, which themselves are 11 cm squares of half millimeter copper sheet metal (with a centered 80 mm hole). Enough clearance is needed to allow for some adjustment of the mirror position via a XYZ-translation stage. The mirror is put separately on an electric potential to minimize distortions of the electric field at the focal point. The spectrometer is terminated on both sides by metallic meshes with a 240 μm grid constant and 80 % transmission.
Figure 3: Drawing of the interaction region inside the momentum spectrometer. Note the incoming laser pulse hits the movable mirror in the background first and focuses the light back into the gas jet.
Magnetic Field
To increase the acceptance angle for electrons without applying very high voltages, a homogeneous magnetic field is overlaid. For this purpose two large coils in a Helmholtz setup are commonly used. The Helmholtz geometry is made up of two identical coils placed relative to each other at a distance of their radius.
To extent the usable magnetic field while keeping the diameter fixed we will add a second pair of smaller correction coils in such a way that they lie on an imaginary sphere defined by the two large coils. The two magnetic fields add together and while the homogeneity is not as perfect the region of tolerable small fluctuations (<1%) can be almost doubled (fig. 4). In our case the large coils have a diameter of 1 m and 119 windings of a coated 2.5 mm Cu wire. The correction coils have a diameter of 0.208 m and 17 windings each and had a relative distance of 1.04 m. As long as the electric field and the magnetic field are exactly parallel to each other their influence on the motion of charged particles can be treated separately, because the magnetic field induced Lorentz force is a vector product of the velocity along the electric field vector and the magnetic field vector.
Figure 4: Relative change of the magnetic field on the spectrometer axis with correction coils.
Detector
We use commercially available position sensitive delayline detectors in conjunction with micro channel plates to detect ions and electrons (RoentDek GmbH). Micro channel plates serve as electron multipliers and supply the primary timing information for the trigger logic. A MCP is a few millimeter thin glass wafer, penetrated by 5-25 μm channels at some angle between 0°−15° to the surface normal, covering approximately 50 % of the total area.
The surface of the plate is covered with a metallic coating to create an electrode on either side. As a MCP is an electron multiplier the detection side always has to be the cathode. Accordingly the back side of the MCP will be more positively charged to attract the electron avalanche.
The resistance between both sides of a MCP is on the order of a few tens of MΩ up to 1000 MΩ. The actual resistance depends on the applied voltage, on the ambient pressure and on the water deposition on the channel walls.
A single MCP channel provides an electron multiplication of 103 − 104. Operating two MCPs in series increases the gain to 106 − 107 and with three MCPs the electron multiplication saturates at a few times 108. The maximum gain can only be achieved if the electron avalanche is pulled away from the MCP backside by a more positively charged anode – in our case the delayline anode. Otherwise the space charge at the end of the MCP channel would prevent an effective multiplication.
We employ 83 mm MCPs of 1 mm thickness in a chevron stack. The channel diameter is 25 μm and their bias angle to the surface normal 8°. The resistance of each MCP is around 20 M. The pulse width was 7 ns FWHM. Our MCPs are not sensitive to scattered laser radiation or its higher harmonics.
The delayline anode utilizes a simple, neat principle to supply the 2d position of the electron avalanche from the MCP. Two parallel wires wound around a metallic holder plate form one of two layers, each responsible for one axis. The wire pairs are electrically isolated from the holder and each other by threaded ceramic half-rods of different diameter (see fig. 5). Each wire pair splits up in a signal wire and a reference wire. Both are on a positive potential with respect to the MCP back side, the signal wire voltage though, is increased by another +50V to attract the electron cloud produced by the MCP. The resulting voltage dip on the signal wire propagates then in both directions along the wire using the reference wire as a relative ground. This geometry assures a constant impedance.
Dispersion and damping, imaginary and real part of the impedance account for spreading and decreasing of the signal. Nonetheless are the anode signals in the offline data analysis the more important ones, as the signal to noise ratio is usually much better and the anode multihit capability yields information even for double hits within the MCP dead time.
The 0.5 mm wires of each pair are spaced by 1 mm and so is every coil of the wire pair. Due to an interpolation effect a spatial resolution of 0.1 mm (FWHM) can be reached. This is because the distance of a few millimeters between the outermost layer and the MCP assures spreading of the electron cloud over a few turns of each layer. By determining the center of mass of the pulse the position of the electron cloud can be determined at a considerably higher resolution than the wire spacing suggests at first glance.
Besides the very intuitive rectangular anode a hexagonal delayline anode with three layers has been developed. The additional layer provides redundant information which is especially helpful for multihit operation in terms of reducing dead time and resolving ambiguous positions.
Figure 5: Delayline detectors with 80mm channel plates. The mesh close to the MCP separates the field free drift region from the high potential of the MCP front side, where the particle kinetic energy is boosted to increase its detection efficiency.