Patent Application
20100140510
The invention relates to a method and to a device for cooling a gas, and particularly for liquefying a gas.
A variety of materials can be used to produce a plasma by bombardment with intensive laser light. Such a plasma, among other things, emits X-rays and extreme ultraviolet (EUV) light. However, it can also accelerate charged particles, such as electrons or protons.
Since the laser intensity required for producing plasma is very high, the irradiated material (target) is locally destroyed at the site of irradiation. Thus, for steady generation of plasma a source is required that provides a continuous supply of unused target material.
It is known from (Ö. Nordhage, “On a Hydrogen Pellet Target for Antiproton Physics with PANDA”, Dissertation University of Uppsala, ISBN 91-554-6649-4) to first liquefy hydrogen, divide the fluid flow into drops using a vibrating nozzle, and then solidify these drops into pellets by further cooling. These pellets are used as targets for the analysis of interactions with fast protons using a synchrotron and, in principle, could also be used for generating plasmas by laser bombardment.
The pellets produced in this way are disadvantageous, and the plasmas generated therefrom exhibit sharply fluctuating quality levels.
The objects of the invention are therefore to provide a method and a device for cooling a gas, the end product of which has fewer quality fluctuations and is therefore better suited as a raw material for the production of pellet targets for laser bombardment than the gas cooled according to the state of the art.
These objects are achieved according to the invention by a method according to the main claim and by a device according to the additional independent claim. Further advantageous embodiments are apparent from the dependent claims referring to these claims. A plasma source based on the device and advantageous uses of the same are the subject matters of the additional independent claims.
As part of the invention, a method for cooling a gas was developed. To this end, in a first cooling step, the gas is conducted through a line, which is in thermal contact with a first cooling medium.
In principle, any medium having a lower temperature than the gas is suited as the cooling medium, the temperature preferably being both homogeneous spatially across the medium and constant across time. For example, it can be a cooled solid body, through which the line runs. It can also be a fluid bath, through which the line runs. This fluid can in particular be a liquefied cryogas.
It is particularly advantageous to select a liquefied cryogas, which is able to exchange vapor with the surrounding area, as the first cooling medium. Since the evaporation heat absorbed during the generation thereof is also dissipated to the surrounding area with the escaping vapor, the temperature of the liquefied cryogas does not increase to above the boiling point thereof until it is completely evaporated.
In a second cooling step, the gas is conducted through a liquefier, which is in thermal contact with a second cooling medium. The second cooling medium is generally colder than the first. However, it can also have the same temperature as the first cooling medium, and in particular it can be identical therewith.
According to the invention, in the second cooling step, the gas is cooled by no more than twice, preferably by no more than 1.5 times, and particularly preferably by no more than 1 times the temperature difference between the melting point and the boiling point thereof. The temperature difference between the melting point and the boiling point is a material property inherent to the gas. It is, for example, 6K for H2, 14K for N2, and 4K each for Ar, Kr, and Xe.
This solution has an effect whereby the flow of the gas in, and downstream of, the liquefier is particularly homogeneous and particularly laminar. These advantages, however, become especially apparent in a particularly preferred embodiment of the invention, in which the gas is liquefied in the second cooling step: The flow of the liquefied gas is then free of areas in which it boils or freezes. It was found that a flow of liquefied gas conditioned in this way is particularly easy to further process, for example into drops having defined sizes and shapes: Temperature fluctuations of the liquefied gas cause velocity fluctuations of the gas jet exiting the liquefier. If this gas jet is divided into drops, these velocity fluctuations lower the quality thereof. The solution according to the invention reduces the temperature fluctuations and thus enables the production of liquid gas drops having a better quality.
According to the invention, the quality of the liquefied gas is best if a constant volume of this gas is present in the liquefier. This can advantageously be achieved in that the gas is fed to the liquefier exclusively during the gas phase. For this purpose, the gas must not be cooled upstream of the liquefier so much that the temperature drops below the boiling point. Ideally, the gas is fed to the liquefier at a temperature that is just slightly above the boiling point thereof, so that the temperature thereof in the liquefier has to be changed only by a small amount. The liquefier then substantially absorbs the difference in energy between the gas phase and the liquid phase of the gas. The gas should not freeze in the liquefier, because otherwise the liquefier can become clogged.
The liquefier can be, for example, a cooled solid body, into which a line for the gas has been introduced. It is particularly advantageous to conduct the second cooling medium through the liquefier. In this way fewer vibrations are transmitted to the gas than, for example, by the cooling of the liquefier using a chiller. It was found that vibrations transmitted by the liquefier impair the homogeneity and laminarity of the gas flow in the liquefier and should therefore be eliminated.
The liquefier, and the gas conducted through it, can be cooled in a particularly low-vibration manner if a gaseous second cooling medium is selected. A single gas particle does not then have sufficient momentum to cause a measurable movement of the solid liquefier. Collectively the momentum transfers by the particles are statistically distributed such that they compensate each other almost completely, and the liquefier is not moved in the overall by a measurable degree.
The vapor phase over a liquefied cryogas, for example, is suitable as the gaseous second cooling medium. The evaporation heat is dissipated with the vapor. If the pressure in the reservoir of the second cooling medium is constant, the evaporation rate and thus the temperatures of both the remaining liquefied gas supply and the vapor phase are nearly constant. The evaporation rate, and thus the temperature of the vapor phase of the second cooling medium, can be roughly regulated by way of the pressure in the reservoir. At the same time, the pressure determines the flow of the second cooling medium, which is crucial for the cooling power in the liquefier and therefore for the volume of gas that can be cooled per unit of time.
Advantageously, the gas and the second cooling medium are conducted in opposite directions through the liquefier. This limits the maximum temperature difference occurring between the two flows, which can cause turbulence in the flow of the gas, particularly if it is liquefied in the second cooling step.
In a particularly advantageous embodiment of the invention, before entering the liquefier, the gas is brought into thermal contact with the second cooling medium exiting the liquefier, in an intermediate cooler, for example by way of a heat exchanger. This reduces the temperature difference that must be bridged in the second cooling step in the liquefier. At the same time, the residual cold energy of the second cooling medium exiting the liquefier is utilized, so that less of this cooling medium is consumed.
In a further advantageous embodiment of the invention, the gas is heated between the first cooling step and the intermediate cooler, for example by way of an electric heater. In this way, the inlet temperature of the gas into the liquefier can be regulated particularly precisely, and also quickly. The temperature of the gas directly after the first cooling step can only be changed relatively slowly because the entire supply of the first cooling medium would have to be heated or cooled, for this purpose. The cooling caused in the intermediate cooler depends on the outlet temperature of the second cooling medium from the liquefier and cannot, therefore, be influenced directly, without changing the temperature conditions in the liquefier at the same time.
The temperature of the second cooling medium can be roughly regulated by the pressure in the cryogas reservoir, which in turn determines the evaporation rate thereof. Advantageously, the second cooling medium is heated before entering the liquefier, for example by way of an electric heater. This heating can be carried out both more precisely and more quickly than an indirect temperature change that is brought about by changing the pressure in the reservoir.
If the gas is liquefied using the method according to the invention, a particularly advantageous embodiment of the invention provides for conducting it through a vibrating nozzle in the liquefied state. This nozzle preferably vibrates parallel to the direction of flow of the liquefied gas and preferably has an amplitude between 100 and 1000 nm. In this way, the liquefied gas can be converted into a flow of drops having consistent shapes and sizes. The velocity of the outflowing liquefied gas is determined by the pressure at which the gas is supplied to the first cooling step and can be regulated by varying this pressure.
It was found that undesirable vibrations during the cooling of the gas according to the state of the art were the limiting factor for the quality of the drops and for the consistency of the quality. Such vibrations were introduced into the gas and the apparatus, for example, by using cooling heads. The vibration-free cooling according to the invention avoids such disturbances and thus enables the production of drops having a quality that is not only in higher, but also more consistent (particularly size) than was possible according to the state of the art.
The drops produced at the vibrating nozzle can be used to generate a plasma, for example by bombardment with intense laser pulses, the plasma radiating X-rays and/or extreme ultraviolet (EUV) light.
For such applications, and other applications taking place in a vacuum, in a particularly advantageous embodiment of the invention, the liquefied gas is transferred from the vibrating nozzle into a vacuum. To this end, it is particularly advantageous to solidify the liquefied gas, which has been divided into drops having consistent shapes and sizes due to the vibration of the nozzle. This takes place by surface evaporation, which causes additional cooling to below the freezing point. The result is then a homogenous flow of solid pellets, which can also be transported in the vacuum across extended distances. This is particularly advantageous for bombardment with intense laser pulses. Since the plasma generated in the process emits considerable amounts of heat and high-energy radiation, bombardment should be carried out at a spatial distance from the source of the pellets such that this source is not damaged.
Advantageously, the transfer into the vacuum from the vibrating nozzle takes place at a distance; in particular, the liquefied gas can be transferred into the vacuum by a chamber, in which the same gas is present in a gaseous state, the gas in the chamber preferably being close to the triple point. This shall be understood in particular as the combination of a pressure between 0.2 times and 3 times the triple point pressure and a temperature between 1 times and 1.2 times the triple point temperature. In this way, the homogeneity and the travel direction of the flow of liquefied gas, or the drops produced therefrom, are preserved to as great an extent as possible during the transfer into the vacuum.
For the same reason, it is advantageous to transfer the liquefied gas through a nozzle into the vacuum, the inside diameter of the nozzle steadily decreasing in the direction of the vacuum. To this end, the inside diameter of the nozzle upon entry of the gas should be greater by a factor of no more than 10, and preferably by a factor between 3 and 4, than on the vacuum side. Thus the velocity of the liquid drops, or pellets, will be homogeneously distributed, and the pellet flow will be collimated tightly around the target travel direction.
This homogeneity and collimation are at the maximum when the pressure gradient approaches the minimum along the nozzle. This can be achieved if the diameter of the nozzle decreases exponentially along the length thereof and the nozzle is also advantageously at least ten times longer than the outlet diameter thereof.
In a particularly advantageous embodiment of the invention, the gas is transferred into the vacuum in at least two stages. Advantageously, a pressure of 10−5 mbar or more, and preferably 10−4 mbar or more, is provided in the first stage, so that the shape of the gas drops is maintained when frozen into pellets. Advantageously, in the last stage, a pressure of 10−6 mbar, and preferably 10−4 mbar or less is provided, as these are typical base pressures for accelerator systems and EUV light generation systems, and the pellets can then be transferred directly into these systems. In order to implement the stages, for example several vacuum chambers having graduated pressures, which each are connected to each other by openings or nozzles through which the pellets can pass, are disposed one after the other in the travel direction of the pellets. The pressure difference between the chambers can be utilized as a propulsion force for the pellets. If a triple point chamber is present, the transition from the triple point chamber into the first vacuum chamber provides the pellets with significantly greater acceleration than the transitions between the remaining chambers. If the chambers are disposed beneath each other, however, the earth's gravitational field can also be used as an additional propulsion force, this effect being comparatively low (approximately 15 m/s velocity increase per 10 m of drop distance).
After the transfer into the vacuum, the pellets advantageously have a velocity of at least 50 m/s, and preferably at least 100 m/s. During generation of plasma from the pellets by means of laser bombardment, the pellets are evaporated and thereby consumed. The velocity of the pellet flow and the distances between the pellets in the pellet flow determine the possible repetition rate.
He, N2, Ar, Kr, and Xe in gaseous and liquid forms are particularly suitable cooling agents. The method according to the invention can, for example, be used to produce liquid drops and solid pellets from H2, N2, Ar, Kr, and Xe.
As part of the invention, a device for cooling a gas was also developed, which is particularly suitable for carrying out the method according to the invention. This device is characterized by a liquefier, comprising at least two lines, through which the gas and a primary cooling medium can flow in opposite directions, and which are in thermal contact with each other.
This means has the effect of allowing a small temperature difference to be established between these two lines, along the entire lengths of the two lines, during operation of the liquefier. If the gas is in the hottest, or coldest, state thereof, so is the second cooling medium. In this way, a small temperature gradient can be achieved over the cross-section of the first line, which improves the homogeneity and laminarity of the flow in this first line.
This effect is even further increased in an advantageous embodiment of the invention, in that the two lines are disposed parallel or concentrically with respect to each other.
In a particularly advantageous embodiment of the invention, a pre-cooling medium is disposed upstream of the liquefier, so that the medium can be brought into thermal contact with the gas. This solution has the effect of allowing the gas to cover the majority of the temperature difference between the original temperature thereof and the desired target temperature while in thermal contact with the pre-cooling medium. If in addition to the original temperature of the gas, the maximum temperature reduction thereof in the liquefier is specified, a lower end temperature can be reached.
Advantageously, the pre-cooling medium is disposed in a receptacle, which is traversed by a line through which gas can flow. Such an arrangement maximizes the thermal contact between the gas and the pre-cooling medium.
The receptacle advantageously has a ring shape. A ring shape in this context shall be understood as any closed shape, which delimits a region at the interior thereof. This region does not have to be circular, but can also have an oval or angular shape; it is protected from thermal radiation from the surrounding area by the pre-cooling medium.
Advantageously, the supply of the primary cooling medium is disposed in this defined region. Thus, excessively rapid heating is avoided. For example, the pre-cooling medium can be inexpensive liquid nitrogen and the primary cooling medium can be much more expensive liquid helium. In such an arrangement, thermal radiation from the surrounding area consumes additional nitrogen, but not additional helium.
In a particularly advantageous embodiment of the invention, the device comprises a nozzle, which is disposed downstream of the liquefier and comprises means for producing vibration. Piezoelectric means are particularly suitable as means for producing the vibration. A laminar flow of liquefied gas produced in the liquefier can be broken up into drops in a defined manner by this nozzle with a suitable selection of the vibration amplitude.
In a further advantageous embodiment of the invention, the device comprises at least one vacuum chamber that is disposed downstream of the nozzle. The drops of liquefied gas exiting the nozzle can then freeze into pellets during the transition into the vacuum chamber. It was found that solid pellets can cover a longer distance between the site of generation thereof and the site of the use thereof than drops of liquefied gas. The pellets can then form a pellet target, for example, which interacts with radiation from a radiation source in a defined interaction zone. Such a pellet target has the advantage that the target material can be continuously supplied to the interaction zone.
In order to preserve the shape of the drops during freezing, advantageously several vacuum chambers having graduated pressures can be disposed one after the other.
Advantageously, the vacuum chamber is disposed spatially distanced from the nozzle, for example by a triple point chamber, in which conditions close to the triple point of the gas are present. Thus, after exiting the nozzle, the fluid drops are allowed to stabilize before they are solidified into pellets.
In an advantageous embodiment of the invention, the inlet into vacuum chamber is configured as a further nozzle, the inside diameter of which decreases steadily in the direction of the vacuum. To this end, the inside diameter of the further nozzle on the inlet side should be larger by a factor of no more than 10 than on the vacuum side. Thus the velocity of the liquid drops, or pellets, entering the vacuum chamber will also be homogeneously distributed, and the pellet flow will be collimated tightly around the target course.
As part of the invention, a plasma source was realized. This comprises a radiation source, which is directed at an interaction zone, and the device according to the invention for cooling a gas. The device is disposed such that it is able to emit cooled gas into the interaction zone. A laser is particularly suitable as the radiation source.
It was found that such a plasma source can be continuously operated and at the same time is more durable than plasma sources according to the state of the art. The gas serving as the target material, which is consumed upon bombardment with the beam from the radiation source, can be continuously supplied, independently of the state thereof. If this is output by the device in the form of pellets, the device can advantageously be disposed at a great distance (roughly 1 m or more) from the interaction zone. Thus, it is not damaged by the heat and radioactive radiation of the plasma.
The plasma source advantageously comprises a cooling trap, which receives the portion of the gases that does not interact with the beam from the radiation source. The cooling trap is preferably disposed downstream from the zone in which the gas interacts, or the pellets interact, with the beam, in the direction of flow of the gas, or in the travel direction of the solid pellets. The trapping of excess gas residue and unused pellets in the cooling trap improves the vacuum in the plasma source and reduces the pumping power required for maintaining this vacuum.
The plasma generated with the plasma source can generate X-ray and extreme ultraviolet (EUV) radiation. In such a plasma, acceleration of charged particles, such as electrons and protons, to energies of several 100 MeV is also possible. In this way, the plasma source can serve as a source of EUV radiation, for example in a device for the photolithographic structuring of semi-conductors. However, it can also be used in a particle accelerator, for example. The zone in which the cooled gas (preferably in the form of pellets) and the accelerator beam interact should then be built with an extremely compact design.
Hereafter, the subject matter of the invention will be described in more detail based on figures, without thereby limiting the subject matter of the invention. Shown are:
The gas 4 can optionally be heated by a heater 7 before it is brought into thermal contact with the primary cooling medium 6 exiting the liquefier 9 by a heat exchanger 8. Subsequently, the gas enters the liquefier 9. From the primary cooling medium 6, the vapor phase is fed to the liquefier by way of a line 10. The liquefier 9 converts the gas 4 into a fluid flow, which is broken up into drops by a vibrating nozzle 11. These drops can then be solidified during the transfer into the vacuum by means of surface evaporation.
In one exemplary embodiment of the method, hydrogen is used as the gas 4. The pre-cooling medium 2 is liquid nitrogen, the primary cooling medium 6 is liquid helium. In the first cooling step, the hydrogen is cooled down from room temperature (293 K) to 81 K. In this embodiment, the heater 7 is not used. Upon exiting the reservoir of the liquid helium, the vapor phase of the helium has a temperature of 4.5 K, and upon entering the liquefier 9 it has a temperature of 5.05 K. Upon entering the heat exchanger 8, this has a temperature of 16.2 K and it cools the entering hydrogen to 21 K before the hydrogen is cooled to 16.9 K in the liquefier and thereby liquefied.
The vibrating nozzle 11 breaks the jet of liquid hydrogen into drops 12 having the same size, which are introduced into a triple point chamber (70 mbar, 14 K) filled with gaseous hydrogen.
Two different types of vibrating nozzles were used: glass nozzles enclosed in brass having internal diameters between 12 and 40 μm and stainless steel nozzles having internal diameters between 16 and 40 μm. Glass nozzles have smoother internal surfaces and, because they are transparent, allow visual inspection of the function thereof during operation. Stainless steel nozzles can be manufactured reproducibly, and the outlet openings thereof have a better (which is to say smaller) length-to-diameter ratio. As a result, less pressure is required to drive liquefied gas through stainless steel nozzles.
The spatial and angular distribution of the pellets and the velocities thereof were observed with CCD cameras. After the transfer into the first vacuum, pellets having a size of 30 μm have an average velocity of approximately 70 m/s. Over a period of several seconds, the size of the pellets is stable to within 1%, over a period of several hours it is still within 10%.
The radius Rc of the nozzle for the transfer into the vacuum is tapered in this embodiment exponentially to the coordinate x along the nozzle:
R
c
=R
1
+R
2·exp(−δ·χ)
where δ is a free tapering parameter, and R1 and R2 depend as follows on the radius Rmax on the inlet side and on the radius Rmin on the vacuum side:
The device comprises a laser (not shown), the beam 25 of which interacts with the drops 12, which are frozen into pellets, in an interaction zone. Unused pellets are collected in a cooling trap 27.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 017 212.7 | Apr 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE2008/000557 | 4/2/2008 | WO | 00 | 11/23/2009 |
