Million-watt-level radio frequency fast axial flow carbon dioxide laser

Abstract

The invention discloses a myriawatt-level radio frequency fast axial flow carbon dioxide laser. The myriawatt-level radio frequency fast axial flow carbon dioxide laser comprises a discharge electrode and a discharge glass tube with a hollow tube wall, the discharge electrode is arranged on the outer wall of the discharge glass tube; the discharge glass tube comprises a water inlet hole and a water outlet hole, and the water inlet hole and the water outlet hole are communicated with the wall of the hollow tube to form a cooling water path; the water inlet hole is formed in the exhaust end of the radio frequency fast axial flow carbon dioxide laser, and the water outlet hole is formed in the air inlet end of the radio frequency fast axial flow carbon dioxide laser; and the radio frequency adapter is connected with the radio frequency power supply and the discharge module and is used for adjusting the temperature of the exhaust end of the radio frequency fast axial flow carbon dioxide laser. By arranging the radio frequency adapter, the myriawatt-level radio frequency fast axial flow carbon dioxide laser can reduce the temperature of high-temperature gas discharged from the exhaust end of the discharge tube in a water cooling mode, it is guaranteed that the temperature of the gas inlet end and the temperature of the exhaust end of the discharge tube are kept stable, laser can obtain higher gain, and the output power of the laser is improved.

Description

Technical Field

[0001] The present invention relates to the field of lasers, and in particular to a 10,000-watt radio frequency fast axial flow carbon dioxide laser. Background Art

[0002] High-power radio frequency fast axial flow CO2 lasers above 10,000 watts are used as extreme ultraviolet lithography light sources and particle acceleration experiments. Currently, most CO2 lasers above 10,000 watts use a combination of fans and heat exchangers to cool the temperature of the internal discharge area through gas flow circulation.

[0003] During the operation of the laser, in order to ensure that the laser obtains a sufficiently high gain, the temperature of the air inlet and outlet of the discharge glass tube needs to be maintained stable. Since the gas mass flow rate that the CO2 laser fan can provide is close to the upper limit, it is difficult to achieve temperature stability by continuing to increase the fan speed in high-power CO2 lasers.

[0004] Therefore, it is difficult for existing cooling methods to meet the demand for increasing the laser output power by increasing the RF electric power deposition of the CO2 laser. Summary of the invention

[0005] The purpose of the present invention is to provide a 10,000-watt radio frequency fast axial flow carbon dioxide laser that can solve at least one of the above-mentioned technical problems. The specific solution is as follows:

[0006] According to the specific embodiment disclosed in the present invention, the present invention discloses a 10,000-watt radio frequency fast axial flow carbon dioxide laser, comprising:

[0007] RF power supply;

[0008] A discharge module comprises: a discharge electrode and a discharge glass tube with a hollow tube wall; the discharge electrode is arranged on the outer wall of the discharge glass tube; the discharge glass tube comprises: a water inlet hole and a water outlet hole, the water inlet hole and the water outlet hole are connected with the hollow tube wall to form a cooling water path; the water inlet hole is arranged at the exhaust end of the radio frequency fast axial flow carbon dioxide laser, and the water outlet hole is arranged at the air inlet end of the radio frequency fast axial flow carbon dioxide laser;

[0009] A radio frequency adapter is connected to the radio frequency power supply and the discharge module and is used to adjust the temperature of the exhaust end of the radio frequency fast axial flow carbon dioxide laser.

[0010] Preferably, the discharge electrode comprises:

[0011] A first discharge electrode connected to a ground line;

[0012] The second discharge electrode comprises: a plurality of sub-discharge electrodes, wherein the sub-discharge electrodes are sequentially connected through insulating blocks.

[0013] Preferably, there are multiple radio frequency adapters;

[0014] Each of the radio frequency adapters is grounded individually;

[0015] Each of the radio frequency adapters is respectively connected in series with the sub-discharge electrode and the radio frequency power supply, and a plurality of the radio frequency adapters are connected in parallel.

[0016] Preferably, the radio frequency adapter comprises:

[0017] A first adjustable inductor, wherein the first adjustable inductor is connected in series with the radio frequency power supply;

[0018] A second adjustable inductor is connected in parallel with the radio frequency power supply.

[0019] Preferably, the equivalent circuit model impedance Z of the discharge module satisfies the following relationship:

[0020]

[0021] Among them, R d is the plasma resistance, j represents the imaginary part;

[0022] ω is the angular frequency of electrons in the gas medium of the discharge tube;

[0023] C0 is the capacitance of the discharge electrode composed of equivalent capacitances of multiple dielectrics connected in series.

[0024] Preferably, the capacitance C0 of the discharge electrode satisfies the following relationship:

[0025]

[0026] C i =ε i A / 4πkl i , i=1,2,3;

[0027] Where, C1 represents the equivalent capacitance of the glass medium;

[0028] C2 represents the equivalent capacitance of the air medium;

[0029] C3 represents the equivalent capacitance of cooling water; k represents the electrostatic force constant;

[0030] ε i and l i represent the relative electrostatic constant and thickness of the corresponding medium respectively.

[0031] Preferably, the first adjustable inductor L 串 , satisfying the following relationship:

[0032]

[0033] The second adjustable inductor L 并 , satisfying the following relationship:

[0034]

[0035] Among them, Z s is the output impedance of the radio frequency power supply;

[0036] A represents the discharge area of ​​the discharge electrode.

[0037] Preferably, when the flow rate of the cooling water channel is 20 L / min, the temperature difference between the water inlet and the water outlet is 10±1°C, and the plasma temperature at the exhaust end is less than 220°C.

[0038] Preferably, the discharge electrode is arranged on the outer wall of the discharge glass tube in a spiral or straight line shape.

[0039] Preferably, the coolant used in the cooling water circuit is deionized water or a liquid with a conductivity of less than 10 μS / cm.

[0040] Compared with the prior art, the above solution of the embodiment disclosed in the present invention has at least the following beneficial effects:

[0041] The present invention provides a radio frequency adapter so that a 10,000-watt radio frequency fast axial flow carbon dioxide laser can reduce the temperature of high-temperature gas coming out of the exhaust end of a discharge tube by water cooling, thereby ensuring that the temperatures of the air inlet and exhaust ends of the discharge tube remain stable, enabling the laser to obtain sufficiently high gain and improving the output power of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The drawings herein are incorporated into the specification and constitute a part of the specification, showing embodiments consistent with the disclosure of the present invention, and together with the specification, are used to explain the principles disclosed in the present invention. Obviously, the drawings described below are only some embodiments disclosed in the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0043] Figure 1 A schematic diagram of the overall structure of a 10,000-watt radio frequency fast axial flow carbon dioxide laser provided in an embodiment of the present invention;

[0044] Figure 2 for Figure 1 A cross-sectional view of

[0045] Figure 3 A circuit schematic diagram of multiple radio frequency adapters provided in an embodiment of the present invention. Description of the drawings:

[0047] 1 discharge module; 11 discharge glass tube; 112 air inlet end; 113 exhaust end; 114 water inlet hole; 115 water outlet hole; 116 cooling water path; 12 discharge electrode; 121 first discharge electrode; 122 second discharge electrode.

[0048] 2 radio frequency adapter; 21 insulating block; 22 first adjustable inductor; 23 second adjustable inductor.

[0049] 3RF power supply. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solutions and advantages disclosed in the present invention clearer, a 10,000-watt radio frequency fast axial flow carbon dioxide laser disclosed in the present invention will be further described in detail below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments disclosed in the present invention, rather than all of the embodiments. Based on the embodiments disclosed in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection disclosed in the present invention.

[0051] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings, and "multiple" generally includes at least two.

[0052] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0053] It should be understood that although the terms first, second, third, etc. may be used to describe in the embodiments of the present application, these should not be limited to these terms. These terms are only used to distinguish. For example, without departing from the scope of the embodiments of the present application, the first may also be referred to as the second, and similarly, the second may also be referred to as the first.

[0054] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a product or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such product or device. In the absence of more restrictions, the elements defined by the sentence "comprises a" do not exclude the presence of other identical elements in the product or device including the elements.

[0055] During the DC discharge process of a kilowatt-class low-power DC CO2 laser, the direction of the electric field is parallel to the direction of the flow field, so the water cooling path is not in the discharge loop, which does not affect the DC glow discharge. Therefore, for a kilowatt-class low-power DC CO2 laser, a three-layer glass tube structure can be used, and the middle layer of the glass tube is cooled by water cooling to maintain the temperature of the discharge area.

[0056] Unlike kilowatt-class low-power DC CO2 lasers, in RF high-power fast axial flow CO2 lasers above 10,000 watts, since the direction of the electric field is perpendicular to the direction of the flow field, the discharge electrode surrounds the discharge glass tube. If cooling is still performed by passing cooling water through the glass tube, electromagnetic induction of the cooling water will be caused, resulting in mismatch with the RF power supply and inability to inject RF power.

[0057] Therefore, the present invention provides a 10,000-watt radio frequency fast axial flow carbon dioxide laser. By setting a radio frequency adapter, the 10,000-watt radio frequency fast axial flow carbon dioxide laser can reduce the temperature of the high-temperature gas coming out of the exhaust end of the discharge tube by water cooling, thereby ensuring that the temperature of the air inlet and exhaust ends of the discharge tube remains stable, so that the laser can obtain a sufficiently high gain and improve the output power of the laser.

[0058] The following is combined with Figure 1-3 Alternative embodiments of the present invention are described in detail.

[0059] like Figure 1-2 As shown, a 10,000-watt radio frequency fast axial flow carbon dioxide laser comprises:

[0060] The discharge module 1 further comprises: a discharge glass tube 11 having a hollow tube wall and a pair of discharge electrodes 12 .

[0061] The discharge electrode 12 is disposed on the outer wall of the discharge glass tube 11 , and an air dielectric layer exists between the discharge electrode 12 and the discharge glass tube 11 .

[0062] The RF power supply is used to provide electrical energy for the RF fast axial flow CO2 laser.

[0063] The radio frequency adapter 2 is connected to the radio frequency power supply and the discharge module 1 and is used to adjust the temperature of the exhaust end 113 of the radio frequency fast axial flow carbon dioxide laser.

[0064] In one embodiment of the present invention, the discharge electrode 12 is disposed in a spiral or linear shape on the outer wall of the discharge glass tube 11. Both ends of the discharge electrode 12 are sleeved on the tube wall of the discharge glass tube 11 through a fixing assembly.

[0065] Furthermore, the discharge glass tube 11 includes a water inlet 114 and a water outlet 115 .

[0066] The water inlet 114 and the water outlet 115 are connected to the hollow part of the discharge glass tube 11 to form a cooling water path 116. The water inlet 114 is arranged near the exhaust end of the RF fast axial flow CO2 laser, and the water outlet 115 is arranged near the inlet end of the RF fast axial flow CO2 laser. During operation, by adding a cooling path to the gas flow field cycle, the temperature of the high-temperature gas coming out of the exhaust end can be reduced, and the cooled gas can be recycled back to the inlet end, thereby maintaining the temperature of the inlet end of the discharge glass tube stable.

[0067] In the embodiment of the present invention, in order to ensure that the cooling liquid on the inner wall of the discharge glass tube does not affect the discharge state during operation, the cooling liquid is deionized water, or other liquids with a specific conductivity of less than 10 μS / cm.

[0068] During the operation of a 10,000-watt RF CO2 laser using a gas flow circulation cooling mode, as the RF power is deposited, the temperature at the exhaust end gradually increases and finally remains in a stable temperature range. In order to ensure laser gain, the temperature at the exhaust end needs to be maintained below 220°C.

[0069] At this time, the temperature gradient between the exhaust end and the intake end can be roughly expressed as:

[0070]

[0071] Among them, P 注入 is the electric power deposition in the discharge area;

[0072] C 气体 is the specific heat capacity of gas, v 气体 is the gas flow rate, ρ 气体 is the gas density;

[0073] S is the diameter of the discharge glass tube.

[0074] It can be seen that if you want to increase the electric power deposition under the condition of constant temperature gradient, you need to increase the mass flow rate of gas per unit time in the discharge area, that is, increase the fan speed. At present, the flow field wind speed in high-power carbon dioxide lasers is close to the speed of sound. If the wind speed reaches supersonic speed or above, shock waves, sound barriers and other phenomena will appear, which will seriously affect the discharge process. And continuing to increase the fan speed in high-power carbon dioxide lasers is also very difficult in the existing industry.

[0075] Therefore, when the cooling water circuit of this embodiment is used to reduce the temperature of the exhaust end, the power p of the cooling water circuit is 水 The temperature gradient between the exhaust end and the intake end can be expressed as:

[0076] p 水 =C 水 G 水 ΔT 水

[0077]

[0078] Among them, C 水 is the specific heat capacity of the coolant;

[0079] G 水 is the mass flow rate of the coolant;

[0080] ΔT 水 is the temperature difference between the water outlet and the water inlet.

[0081] It can be seen from the formula that at the gas flow rate v 气体 Under certain conditions, as the electric power deposition in the discharge area increases, 注入 The discharge module in this embodiment can maintain the temperature stability of the air inlet and exhaust ends to a certain extent, and keep the temperature at the exhaust end below 220°C.

[0082] For a common industrial water chiller using a flow rate of 20 L / min, when the injected electrical power is increased to 14 kW, the temperature difference between the water outlet and the water inlet in this embodiment can be maintained at about 10°C.

[0083] As an optional embodiment, the discharge glass tube with a hollow tube wall can be replaced by a discharge glass tube and a sealing glass tube nested outside the discharge glass tube. The space formed between the two layers of glass tubes is used as a cooling water path, and the water inlet and outlet holes are arranged on the outer glass tube wall.

[0084] In the embodiment of the present invention, the cooling water path causes the temperature of the discharge glass tube to change, and the impedance of the discharge module will change due to the introduction of the coolant medium parameter factor. In addition, due to the different spatial distribution of the discharge module temperature, the spatial distribution of the load impedance will also be different.

[0085] Therefore, in order to achieve maximum power deposition in each area of ​​the discharge module after adding the cooling water channel, the present embodiment designs a plurality of RF adapters for matching different temperature areas.

[0086] like Figure 3 As shown, in this embodiment, the first discharge electrode 121 is grounded, and the second discharge electrode 122 is evenly divided into three sub-discharge electrodes. Each sub-discharge electrode is connected by an insulating block 21, and each sub-discharge electrode has a separate RF adapter 2, which is used to solve the problem of electromagnetic induction of cooling water causing mismatch with the RF power supply 3 and inability to inject RF power.

[0087] Specifically, each RF adapter 2 is grounded separately, each RF adapter 2 is connected in series with a sub-discharge electrode and a RF power source 3 respectively, and three RF adapters 2 are connected in parallel with each other.

[0088] Each RF Adapter includes:

[0089] A first adjustable inductor 22, wherein the first adjustable inductor 22 is connected in series with the RF power supply 3;

[0090] The second adjustable inductor 23 is connected in parallel with the RF power source 3 .

[0091] It is known that the discharge current I(t) during the gas discharge process can be expressed as:

[0092]

[0093] Among them, m, e, T, and v(T) represent the electron mass, charge, temperature, and collision frequency, respectively.

[0094] n(T), A, U, and d represent the electron density, discharge area of ​​the electrode, voltage amplitude, and discharge distance, respectively.

[0095] Thus, the expression of the impedance of the equivalent circuit model of the discharge module can be obtained:

[0096] Z=R d +iX d =md[v(T)+iω] / n(T)e 2 A

[0097] R d =mdv(T) / n(T)e 2 A

[0098] X d =mdω / n(T)e 2 A

[0099] Among them, R dis the plasma resistance; X d is the reactance;

[0100] ω is the angular frequency of electrons in the gas medium of the discharge tube;

[0101] The discharge module is represented by a plasma resistor connected in series with the capacitor C0 of the discharge electrode during discharge, such as Figure 3 As shown, the capacitance C0 of the discharge electrode is composed of the equivalent capacitances C1, C2, and C3 of the quartz glass medium, the air medium, and the deionized water medium in series. The expression of C0 is:

[0102]

[0103] C i =ε i A / 4πkl i , i=1,2,3;

[0104] Where, C1 represents the equivalent capacitance of the glass medium;

[0105] C2 represents the equivalent capacitance of the air medium;

[0106] C3 represents the equivalent capacitance of cooling water; k represents the electrostatic force constant;

[0107] ε i and l i represent the relative electrostatic constant and thickness of the corresponding medium respectively.

[0108] The plasma resistance can be calculated by the ideal gas equation P = Nk in the discharge tube. B T (N is the total number of gas molecules, k B is the Boltzmann constant, T is the thermodynamic temperature), and the threshold condition for the mixed gas to maintain self-sustaining discharge is E / N ≥ 3.5x10 - 16 Vcm 2 , the plasma resistance R is calculated d .

[0109] Therefore, the impedance expression of the discharge structure equivalent circuit model of the 10,000-watt RF-excited axial fast-flow CO2 laser can be obtained as follows:

[0110]

[0111] Where P = Nk B T;P L Injection power

[0112] d is the discharge distance, T is the thermodynamic temperature

[0113] ω is the angular frequency of the RF electric field;

[0114] k represents the electrostatic force constant;

[0115] l1, l2, and l3 represent the thickness of the quartz glass medium, the air medium, and the deionized water medium, respectively;

[0116] ε1, ε2, and ε3 represent the relative electrostatic constants of the quartz glass medium, the air medium, and the deionized water medium, respectively.

[0117] According to the impedance matching theory of RF circuit, when the load impedance and RF power supply impedance Z s (Z s =50Ω), the circuit power transmission efficiency is the highest. Combining the series-parallel connection of the first adjustable inductor and the second adjustable inductor, the following equations can be established:

[0118]

[0119] Among them, L 串 is the first adjustable inductor;

[0120] L 并 is the second adjustable inductor;

[0121] Z s is the output impedance of the RF power supply.

[0122] When Z s =50Ω, by solving the equation:

[0123] The first adjustable inductor L 串 The numerical expression of is:

[0124]

[0125] The second adjustable inductor L 并 The numerical expression of is:

[0126]

[0127] Where k represents the electrostatic force constant;

[0128] A represents the discharge area of ​​the discharge electrode.

[0129] Therefore, according to the temperature on the three-segment discharge electrode and the parameters of the discharge module, the values ​​of the first adjustable inductance and the second adjustable inductance on each RF adapter can be calculated respectively.

[0130] In the process of obtaining the RF adapter parameters, this embodiment fully considers the influence of the coolant on the RF impedance matching, so that the water cooling system can be used to maintain the temperature of the air inlet and exhaust ends of the discharge tube of the 10,000-watt RF fast axial flow carbon dioxide laser stable, reduce the temperature gradient of the lower and upper air outlets inside the discharge tube, ensure the maximum power deposition in the discharge area, and improve the output power of the laser.

[0131] The adjustable inductance in this embodiment refers to the static matching of the entire impedance loop after the discharge working state is stable. This matching method is more suitable for fixed load situations. It is not intended to use the adjustable capacitance to adjust the impedance of the entire loop during the operation of the laser to obtain a higher dynamic matching range of impedance matching. Dynamic matching is suitable for situations where the load changes frequently.

[0132] In summary, when the flow rate of the connected water chiller is 20L / min, by using the RF adapter of the present invention, the temperature difference between the water inlet and the water outlet of the 10,000-watt RF fast axial flow carbon dioxide laser of the present invention can be 10±1°C, and the plasma temperature at the exhaust end is less than 220°C, meeting the requirements of industrial development for increasing the power of carbon dioxide lasers.

[0133] Finally, it should be noted that the various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other. For the system or device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part description.

[0134] The above embodiments are only used to illustrate the technical solutions disclosed in the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments disclosed in the present invention.

Claims

1. A 10,000-watt radio frequency fast axial flow carbon dioxide laser, characterized in that: include: RF power supply; A discharge module comprises: a discharge electrode and a discharge glass tube with a hollow tube wall; the discharge electrode is arranged on the outer wall of the discharge glass tube; the discharge glass tube comprises: a water inlet hole and a water outlet hole, the water inlet hole and the water outlet hole are connected with the hollow tube wall to form a cooling water path; the water inlet hole is arranged at the exhaust end of the radio frequency fast axial flow carbon dioxide laser, and the water outlet hole is arranged at the air inlet end of the radio frequency fast axial flow carbon dioxide laser; A radio frequency adapter is connected to the radio frequency power supply and the discharge module and is used to adjust the temperature of the exhaust end of the radio frequency fast axial flow carbon dioxide laser.

2. The radio frequency fast axial flow carbon dioxide laser according to claim 1, characterized in that: The discharge electrode comprises: A first discharge electrode connected to a ground line; The second discharge electrode comprises: a plurality of sub-discharge electrodes, wherein the sub-discharge electrodes are sequentially connected through insulating blocks.

3. The radio frequency fast axial flow carbon dioxide laser according to claim 2, characterized in that: There are multiple radio frequency adapters; Each of the radio frequency adapters is grounded individually; Each of the radio frequency adapters is respectively connected in series with the sub-discharge electrode and the radio frequency power supply, and a plurality of the radio frequency adapters are connected in parallel.

4. The radio frequency fast axial flow carbon dioxide laser according to claim 3, characterized in that: The radio frequency adapter comprises: A first adjustable inductor, wherein the first adjustable inductor is connected in series with the radio frequency power supply; A second adjustable inductor is connected in parallel with the radio frequency power supply.

5. The radio frequency fast axial flow carbon dioxide laser according to claim 1 or 4, characterized in that: The impedance Z of the equivalent circuit model of the discharge module satisfies the following relationship: Among them, R d is the plasma resistance, j represents the imaginary part; ω is the angular frequency of electrons in the gas medium of the discharge tube; C0 is the capacitance of the discharge electrode composed of equivalent capacitances of multiple dielectrics connected in series.

6. The radio frequency fast axial flow carbon dioxide laser according to claim 5, characterized in that: The capacitance C0 of the discharge electrode satisfies the following relationship: C i =ε i A / 4πkl i ,i=1,2,3; Where, C1 represents the equivalent capacitance of the glass medium; C2 represents the equivalent capacitance of the air medium; C3 represents the equivalent capacitance of cooling water; k represents the electrostatic force constant; ε i and l i represent the relative electrostatic constant and thickness of the corresponding medium respectively.

7. The radio frequency fast axial flow carbon dioxide laser according to claim 4, characterized in that: The first adjustable inductor L 串 , satisfying the following relationship: The second adjustable inductor L 并 , satisfying the following relationship: Among them, Z s is the output impedance of the radio frequency power supply; A represents the discharge area of ​​the discharge electrode.

8. The radio frequency fast axial flow carbon dioxide laser according to claim 1, characterized in that: When the flow rate of the cooling water channel is 20 L / min, the temperature difference between the water inlet and the water outlet is 10±1°C, and the plasma temperature at the exhaust end is less than 220°C.

9. The radio frequency fast axial flow carbon dioxide laser according to claim 1, characterized in that: The discharge electrode is arranged on the outer wall of the discharge glass tube in a spiral or straight line shape.

10. The radio frequency fast axial flow carbon dioxide laser according to claim 1, characterized in that: The cooling liquid used in the cooling water circuit is deionized water or a liquid with a conductivity of less than 10 μS / cm.

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