- Open Access
“The Even-Lavie valve as a source for high intensity supersonic beam”
© Even. licensee Springer on behalf of EPJ. 2016
Received: 19 November 2015
Accepted: 16 December 2015
Published: 29 December 2015
We use extensive computer simulation to design (and test) a high pressure, fast acting, pulsed, valve that can produce short pulses of gas. We use flow simulations from shaped nozzles to optimize the beam density and finally use low density simulation to decide on the best skimmer shape and placement. All these details are crucial to operate a high intensity beam machine.
These limitations were overcome when pulsed gas sources became available [10–16]. If the gas pulse is short enough (shorter than a typical molecular reflection time from the walls of the vacuum chamber), and if the repetition rate is low enough to so that the mean gas pressure is less than 10−5 mbars in the source chamber, then the gas expands without hindrance, and no structured shock waves interfere with the jet propagation. Much smaller turbo-molecular pumps (300–500 l/sec) can now be used for low repetition rate experiments (10-50Hz), while bigger pumps (3000 l/sec) are still required for higher repetition rates (1–5 KHz) pulsed valve systems. The following parts will summarize our experience with one type of a pulsed valve that we developed, and is still the only one that can be used over a very wide temperature scale (40-5500 K). The valve is an electro- magnetic actuated device. Readily available computer simulation programs can now be used to optimize the electro- magnetic structure, the mechanical movement dynamics, the nozzle shape and the interaction of the high density resulting jet with skimmers. These processes will be explained and experimental results will be shown. These valves, known as the “Even-Lavie” valves are now widely used in more than 100 labs. Some details have been published before .
High pressure, wide temperature range pulsed valve
General dimensions consideration and material selection
We want the gas pulse to be short enough to reduce the required pump load and to allow the gas to expand into a good vacuum and neglect vacuum walls interaction during the pulse. Considering the high velocity of a light expansion gas (and Helium will be proven to be the best choice, with velocities of up to ~2300 m/sec at 500°K), and typical dimension of 0.1 m for the vacuum wall diameter, we get a gas reflection time of 40 microseconds. The gas pulse should be shorter than this typical time.
The short pulse time can be achieved by miniaturization of the moving parts (small masses), creating a magnetic structure with small air gaps (to maximize the generated field and force) and to minimize the amplitude of the mechanical movement.
The coil generating the field should have small inductance to allow for a fast rise time of the current pulse (microseconds).
The wide temperature range implies that there will be considerable thermal expansion in the valve components. Not all the materials in the valve have the same expansion coefficients, thus the method of sealing should be based on constant force applied by internal springs to compensate for thermal expansion.
We should strive to make the metallic parts inert dictating the use of stainless steel (304) for the valve body, high nickel alloy for the springs (Nimonic 90), and strong strength alloy (Inconel 625) for the thin walled pressure tube. The magnetic parts are made from a magnetic stainless steel (alloy 250) which is a compromise between the required high magnetic saturation alloy and sensitivity to corrosion (usually these two requirements are contradictory). Not all batches of the magnetic circuit alloys are created equal, depending on their history of thermal treatment. We have to test (and reject) each batch for its magnetic saturation by inserting it in a coil and measuring the resultant current flow, selecting only the batch with high permeability in high magnetic fields. The sealing material is Kapton that is flexible enough but also strong enough for the sealing forces required and can also withstand elevated temperatures.
Magnetic circuit simulation
Mechanical motion simulation
The masses of the plunger and the return spring.
The shape of the current pulse.
The local saturation of the magnetic properties of the plunger and plug.
The closure of the air gap in the magnetic circuit during the motion.
The recoil velocity when the plunger hits the immobile plug.
The recoil from the nozzle surface at the end of the motion cycle.
Valve construction details
Stainless gas inlet tube (1/16”).
Tightening spring (100 N) and pressure relief valve.
Kapton foil gasket (rear, 0.125 mm. thickness).
Ruby rear guiding ferrule.
Return spring (Nimonic 19 alloy).
Thin walled pressure vessel (Inconel 625 alloy).
Reciprocating plunger (magnetic stainless steel alloy 750).
Kapton insulated copper coil (0.6 mm wire diameter 6x6 winding).
Magnetic shield (alloy 17/4PH) and field concentrator.
Ruby front guiding ferrule.
Kapton foil gasket (front, 0.125 mm. thickness).
Front flange (stainless or copper).
Conical (or trumpet) shape expansion nozzle (Zirconia ceramic or hardened stainless steel).
The carrier gas is exposed to Inconel, Stainless steel of various grades and Kapton only, and is thus inert to most molecules. It is susceptible to metallic corrosion if Halides are used and the Kapton is sensitive to Ammonia or other Amines. Low concentration (1 %) of Halides (even Fluorine) can be compatible if water traces are removed. Low concentration of Amines can also be tolerated, but may require more frequent gaskets change. We recommend that the valve should be pumped down when not in use (say overnight) so as to reduce the corrosion rate (even with water vapors).
The reflected shock waves inside the nozzle are evident in the kink from the smooth curve, most evident in the bell shaped nozzles. The smoothest curve is achieved for the trumpet shaped nozzle.
Pulse profile in time at the valve exit
These measurements also indicate that a pulse time width of ~10microsec. is a practical lower limit for pulsed valves of this type and dimensions.
Velocity spread and achieved Mach numbers and temperatures
Skimmer design and position
Because high pressure shaped nozzles can produce high on-axis beam intensities, it is not surprising that modifications of the accepted skimmer design are required. Since the early days of low intensity continuous beams, skimmers have been recognized as more than a simple bystander that extracts the central portion of the beam. The effects of molecules scattering from the finite skimmer edges have long been recognized [1, 5, 31, 42–44], and the reduced transmission of the beam through the skimmer was termed “skimmer interference”. At times the term “skimmer clogging” was used to describe the dramatic reduction in skimmer transmission. To study these effects, we employed the same DSMC gas flow simulation program  which is eminently suitable for the gas density encountered at the skimmer. Here we show how the skimmer distance, skimmer opening, and skimmer edge sharpness influence the transmission of the beam through the skimmer.
It is obvious from the simulation that the skimmer shape is wrong and that the entering beam intensity is too high. A severe loss of beam intensity is caused by the shock wave structure near the skimmer entrance.
A surprising result is that in order to allow for a reasonable beam transmission one has to move the skimmer to a larger distance from the nozzle, as large as 150 mm. (or 750 nozzle diameters!) for Ne or 55 mm. for helium. Lower beam transmission is found for Ar.
Ionic clusters and radical sources
There are some experiments where we want to study ions, ionic clusters, meta-stable atoms or molecular radicals. There were several attempts to combine a nozzle with electrical or laser discharge scheme to produce these species. In most cases the excitation process warms the supersonic jet, or contaminates it with sputtered electrode material [47–52]. We have developed a new type of electrode less Dielectric Barrier Discharge [53–56] source that does not heat the jet or contaminates it . The source is attached to the front flange of the Even-Lavie valve and uses a built in disk magnet to confine the discharge electrons. It operates on a series of rapid high voltage pulses.
The high collected current represents very high flux (3*1014 atoms/second, at the peak, for a time duration of ~10 microseconds). The narrow time width indicates high Mach number of the beam (up to 50) so no appreciable heating of the beam occurs [35, 58, 59]. The excitation occurs inside the expansion nozzle where there are still many collisions in the gas. The meta-stable energy state is the lowest excited state of the atoms. When using gas mixture we find only the lowest meta-stables. Thus a mixture of helium and neon will produce only the neon excited atoms. Even trace impurities (10 ppm) will steal the energy from the excited helium and transfer it to lower molecular states with high efficiency. Water is a common impurity and needs to be removed if meta-stable Helium is to be created efficiently.
This DBD source can replace laser photo fragmentation in many cases, with the added advantage of producing cold jets.
We have shown that intense and cold (<10mK) pulsed jets of short durations (25microsec.) and high repetition rate can be produced. The high pressure used in the valve and some nozzle shaping contribute to the beam intensity. Changes in skimmer shape and positioning are required to take advantage of the higher beam intensities.
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