A method for removing boron film in a tokamak nuclear fusion device

Abstract

The present invention relates to the technical field of controlled nuclear fusion, and specifically discloses a method for removing a boron film in a tokamak nuclear fusion device. The removal method comprises: in a chamber of the tokamak device, using a fluorine-containing gas compound to generate fluorine plasma under glow discharge conditions, and then using fluorine ions to react with the boron element in the boron film to chemically remove the boron film; and after the removal of the boron film, using hydrogen and helium combined glow cleaning to remove the fluorine ion residues on the inner wall of the vacuum chamber. The method provided by the present invention can effectively remove the boron film in the tokamak nuclear fusion device, and the reaction products can be discharged from the interior of the chamber in a gaseous form, without any deposited residues, and will not cause damage to the base material of the stainless steel inner wall.

Description

Technical Field

[0001] The invention relates to the technical field of controlled nuclear fusion, and in particular to a method for removing a boron film in a tokamak nuclear fusion device. Background Art

[0002] In the physical experiment of the Tokamak nuclear fusion device, the high-temperature plasma collides with the inner wall of the device chamber, causing the chamber wall to release various impurities into the plasma in the device chamber. The impurities will consume the energy of the plasma, thereby causing a huge loss of plasma energy. Growing a boron film layer on the inner wall of the nuclear fusion vacuum chamber can effectively inhibit the escape of metal impurities and oxygen and carbon impurities in the inner wall during the fusion reaction, and effectively prevent the probability of the chamber wall material being collided with by the plasma as an impurity and entering the chamber and mixing into the plasma, thereby reducing the plasma discharge energy loss and greatly improving the plasma performance. The development of this protection method has formed a technology called boronization. For example, the use of boron hydride compounds such as diborane (B2H6) and decaborane (B 10 H 14 ) or organic boride such as B(CH3)3 is used as a boron source to grow a boron film on the inner wall of the corresponding chamber material by chemical vapor deposition.

[0003] However, the ability of the boron film to suppress impurities has a certain lifespan in maintaining the effect. As the number of plasma discharges increases, the impurity suppression ability of the boron film will gradually reach saturation. In order to maintain the high-purity vacuum chamber environment of the fusion device, it is necessary to regularly perform additional boronization on the inner wall of the vacuum chamber. As the number of boronization increases, the thickness of the boron film on the inner wall of the vacuum chamber will gradually increase, and multiple boronizations will also cause the internal bonding force of the boron film to deteriorate, and there is a risk of falling off under the bombardment of plasma. Once the boron film falls off, it will seriously affect the discharge quality of the plasma.

[0004] In addition, due to periodic experimental adjustments or internal component upgrades, the reaction vacuum chamber is often opened regularly, resulting in the vacuum breaking of the vacuum chamber. The research team of the present invention has studied the effects of exposure of the boron film to air and found that after the boron film is exposed to the atmosphere for more than 120 hours, deliquescence occurs and even microcracks occur in some areas, and there is also a risk of boron film shedding.

[0005] Therefore, it is urgent to develop a method that can effectively remove the boron film without damaging the surface of the substrate material, so as to solve the technical problems of multiple boriding resulting in excessively thick boron film, poor bonding strength, and deliquescence and cracking of the boron film after cavity opening. Summary of the invention

[0006] In order to solve the problems existing in the prior art, the object of the present invention is to provide a method for removing the boron film in a Tokamak nuclear fusion device.

[0007] In order to achieve the object of the present invention, the technical solution of the present invention is as follows:

[0008] A method for removing a boron film in a tokamak nuclear fusion device, the method comprising the following steps:

[0009] (1) In a chamber of a tokamak device, a fluorine plasma is generated by using a fluorine-containing gas compound under glow discharge conditions, and then the boron film is chemically removed by using fluorine ions to react with the boron element in the boron film;

[0010] (2) After the boron film is removed, the residual fluorine ions on the inner wall of the vacuum chamber are removed by combined glow cleaning of hydrogen and helium.

[0011] Preferably, the fluorine-containing gas compound in step (1) is CF4 or NF3.

[0012] When CF4 is used to chemically remove the boron film in step (1), oxygen is introduced as an auxiliary gas while CF4 is introduced.

[0013] Furthermore, when CF4 is used to chemically remove the boron film in step (1), the glow discharge conditions are:

[0014] CF4 flow rate 30-150sccm

[0015] O2 flow rate 30-150sccm

[0016] Air pressure 0.1-0.6Pa

[0017] Current 0.1-0.7A

[0018] Voltage 400-650V

[0019] Time: 30-120 minutes.

[0020] Preferably, the ratio of CF4 to oxygen introduced is 1:1.

[0021] Therefore, it is further preferred that in step (1), when CF4 is used to chemically remove the boron film, the glow discharge conditions are:

[0022] CF4 flow rate 50sccm

[0023] O2 flow rate 50sccm

[0024] Air pressure 0.3Pa

[0025] Current 0.2A

[0026] Voltage 550V

[0027] Time: 60 minutes.

[0028] Wherein, when NF3 is used to chemically remove the boron film in step (1), the glow discharge conditions are:

[0029] NF3 flow rate 50-200sccm, preferably 100sccm,

[0030] Air pressure 0.1-0.6Pa, preferably 0.3Pa,

[0031] Current 0.1-0.7A, preferably 0.3A,

[0032] Voltage 380-600V, preferably 500V,

[0033] Time 30-120min, preferably 60min.

[0034] After the boron film is removed, step (2) uses hydrogen and helium combined glow cleaning to remove residual fluorine ions on the inner wall of the vacuum chamber. Specifically, hydrogen glow cleaning is first performed to remove residual fluorine ions, and then helium glow cleaning is performed to remove residual hydrogen.

[0035] Furthermore, the conditions for hydrogen glow cleaning are current 0.3A, voltage 490V, pressure 2.6Pa, and time 7-12h.

[0036] Furthermore, the conditions for helium glow cleaning are current 0.3A, voltage 280V, pressure 0.5Pa, and time 7-12h.

[0037] In practical applications, the step of detecting the boron film is also included before step (1) to determine whether the boron film needs to be removed. The detection method is specifically: using a built-in film thickness meter to detect the thickness of the existing boron film, and starting etching when the boron film thickness is greater than or equal to a preset value, wherein the preset thickness is preferably 700nm; or, monitoring the conductivity of the glow electrode surface, and starting etching when the conductivity decreases; or, detecting the morphology of the boron film, and starting etching when cracks appear on the surface of the boron film or the boron film falls off.

[0038] Furthermore, in order to better ensure that the residual fluoride ions are completely removed, in practical applications, the aforementioned method also includes placing a test sample before using fluoride glow cleaning, taking out the test sample for X-ray energy spectrum analysis after using hydrogen-helium glow combined cleaning, and if it is shown that there is no fluoride ion residual, then helium glow cleaning is performed, and if it is shown that there is still fluoride ion residual, then hydrogen glow cleaning is continued until no fluoride ion is residual. The raw materials or reagents involved in the present invention are all common commercially available products, and the operations involved are all conventional operations in the field unless otherwise specified.

[0039] Based on the common knowledge in this field, the above-mentioned preferred conditions can be combined with each other to obtain a specific implementation method.

[0040] The beneficial effects of the present invention are:

[0041] The present invention provides a method for chemically removing the boron film inside the chamber of a fusion device by using fluorine glow plasma. The reaction products can be discharged from the chamber in gaseous form without any deposit residue. The boron film removal effect is good and no damage is caused to the base material of the stainless steel inner wall. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0044] Figure 1 This is the EDS analysis result of the stainless steel surface of the chamber inner wall after the boron film is removed by CF4 in Example 1 of the present invention;

[0045] Figure 2 This is the EDS analysis result of the stainless steel surface of the inner wall of the chamber after the residual fluorine ions were cleaned by hydrogen-helium glow combined cleaning in Example 1 of the present invention.

[0046] Figure 3 This is the X-ray photoelectron spectroscopy (XPS) test result of the surface composition of the sample after the reaction of removing the boron film using CF4 in Comparative Example 1 of the present invention.

[0047] Figure 4 This is the EDS analysis result of the stainless steel surface of the inner wall of the chamber after the boron film is removed by NF3 in Example 2 of the present invention. DETAILED DESCRIPTION

[0048] The present invention provides a method for removing a boron film in a tokamak nuclear fusion device, the method comprising the following steps:

[0049] (1) In a chamber of a tokamak device, a fluorine plasma is generated by using a fluorine-containing gas compound under glow discharge conditions, and then the boron film is chemically removed by using fluorine ions to react with the boron element in the boron film;

[0050] (2) After the boron film is removed, the residual fluorine ions on the inner wall of the vacuum chamber are removed by combined glow cleaning of hydrogen and helium.

[0051] Wherein, the fluorine-containing gas compounds include but are not limited to CF4 and NF3.

[0052] In a specific embodiment of the present invention, when CF4 is used to chemically remove the boron film, oxygen is introduced as an auxiliary gas at the same time as CF4 is introduced. CF4 generates fluorine plasma under glow discharge conditions, and fluorine ions react with boron elements in the boron film to generate boron trifluoride, which is discharged from the chamber in gaseous form; the carbon element generated by the dissociation of fluoride gas reacts with oxygen to generate carbon dioxide, which is discharged from the vacuum chamber without polluting the interior of the chamber.

[0053] The glow discharge conditions for chemical removal of boron film using CF4 are as follows:

[0054] CF4 flow rate 30-150sccm

[0055] O2 flow rate 30-150sccm

[0056] Air pressure 0.1-0.6Pa

[0057] Current 0.1-0.7A

[0058] Voltage 400-650V

[0059] Time: 30-120 minutes.

[0060] More preferably:

[0061] CF4 flow rate 50sccm

[0062] O2 flow rate 50sccm

[0063] Air pressure 0.3Pa

[0064] Current 0.2A

[0065] Voltage 550V

[0066] Time: 60 minutes.

[0067] In another specific embodiment of the present invention, the glow discharge conditions when the boron film is chemically removed using NF3 are as follows:

[0068] NF3 flow rate 50-200sccm

[0069] Air pressure 0.1-0.6Pa

[0070] Current 0.1-0.7A

[0071] Voltage 380-600V

[0072] Time: 30-120 minutes.

[0073] Since the Tokamak nuclear fusion device requires a high-purity vacuum environment, after the boron film to be removed is removed using the above steps, it is necessary to use a combined glow cleaning of hydrogen and helium to remove the fluorine ion residues on the inner wall of the vacuum chamber.

[0074] During removal, the conditions for hydrogen glow cleaning are current 0.3A, voltage 490V, pressure 2.6Pa, and time 7h; the conditions for helium glow cleaning are current 0.3A, voltage 280V, pressure 0.5Pa, and time 7h; during cleaning, hydrogen glow cleaning is first used to remove the fluorine ions remaining in the etching step, and then helium glow cleaning is used. The helium glow removes residual hydrogen under the bombardment of helium plasma to ensure a high-quality vacuum environment in the vacuum chamber.

[0075] In practical applications, before removing the boron film, a step of detecting the boron film is also included to determine whether the boron film needs to be removed. The specific detection method is: using a built-in film thickness meter to detect the thickness of the existing boron film, and starting etching when the boron film thickness is greater than or equal to a preset value, wherein the preset thickness is preferably 700nm; or, monitoring the conductivity of the glow electrode surface, and starting etching when the conductivity decreases; or, detecting the morphology of the boron film, and starting etching when cracks appear on the surface of the boron film or the boron film falls off.

[0076] In actual operation, during the step of removing the boron film, a built-in film thickness meter in the vacuum chamber can be used to monitor the thickness of the boron film in real time. When the thickness of the boron film is zero, the bombardment of the fluorine plasma is stopped.

[0077] It should be noted that the above real-time monitoring is only for improving the removal efficiency of the boron film and reducing the operating cost. Since the reaction between the fluorine glow plasma and the stainless steel is extremely weak, even if it is not stopped in time, it will not cause damage to the base material of the inner wall of the stainless steel.

[0078] Furthermore, in order to better ensure that the residual fluorine ions are completely removed, in actual applications, the aforementioned method also includes placing a test sample in the chamber, and after using hydrogen-helium glow combined cleaning, taking out the test sample for X-ray energy spectrum analysis. If it shows that there are no fluorine ions remaining, the cleaning is stopped. If it shows that there are still fluorine ions remaining, the hydrogen-helium glow combined cleaning is continued until there are no fluorine ions remaining.

[0079] The test sample is placed in the vacuum chamber at a distance of 5-10 cm from the inner wall, and is generally placed through a differential cavity so that the vacuum environment of the vacuum chamber will not be destroyed when the sample is taken or placed.

[0080] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0081] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0082] The preferred embodiments of the present invention will be described in detail below in conjunction with the examples. It should be understood that the following examples are provided only for the purpose of illustration and are not intended to limit the scope of the present invention. Those skilled in the art may make various modifications and substitutions to the present invention without departing from the purpose and spirit of the present invention.

[0083] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0084] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0085] Example 1

[0086] In this embodiment, the boron film in the Tokamak nuclear fusion device that has been boronized multiple times and has a cumulative thickness of more than 700 nm is chemically removed.

[0087] Removal methods include:

[0088] (1) A mixture of carbon tetrafluoride (CF4) and oxygen (O2) is used to chemically remove the boron film under the action of glow light.

[0089] The glow discharge conditions are as follows: CF4 flow rate 50 sccm, O2 flow rate 50 sccm, gas pressure 0.3 Pa, current 0.2 A, voltage 550 V, reaction time 60 min.

[0090] After the reaction, the boron film was completely removed, and the EDS analysis of the inner wall surface of the stainless steel was as follows: Figure 1 shown.

[0091] (2) Using hydrogen and helium combined glow to clean the surface fluorine ions: First, hydrogen glow cleaning can remove the residual fluorine ions in the etching step, and then helium glow cleaning is used. The helium glow removes the residual hydrogen under the bombardment of helium plasma to ensure a high-quality vacuum environment in the vacuum chamber.

[0092] Experimental conditions of hydrogen-helium glow treatment

[0093]

[0094] After energy spectrum analysis (EDS) and hydrogen-helium glow cleaning, the residual fluorine ions can be removed. EDS analysis of the inner wall surface of stainless steel is as follows: Figure 2 shown.

[0095] Comparative Example 1

[0096] The difference between this comparative example and comparative example 1 is that the glow discharge conditions in step (1) are as follows: CF4 flow rate 100sccm, O2 flow rate 50sccm, gas pressure 1.5Pa, current 0.5A, voltage 470V, reaction time 60min. This condition has no obvious effect on the removal of the boron film. This indicates that the gas pressure is high or the gas ratio is not appropriate (if the amount of oxygen introduced is insufficient, carbon elements will accumulate and cover the surface of the boron film, causing the fluorine ion etching effect to deteriorate), and the boron film cannot be effectively removed. The X-ray photoelectron spectroscopy (XPS) test results of the sample surface composition after the reaction under this condition are shown in Figure 3 It can be seen that there is still an obvious characteristic peak of B1s on the surface, indicating that the boron film has not been effectively removed.

[0097] Example 2

[0098] This embodiment chemically removes the boron film on the inner wall of a chamber in a tokamak nuclear fusion device where the vacuum has been broken.

[0099] Removal methods include:

[0100] (1) Using nitrogen trifluoride (NF3), the boron film is chemically removed under the action of glow light.

[0101] The glow discharge conditions were as follows: NF3 flow rate 100 sccm, gas pressure 0.3 Pa, voltage 500 V, current 0.3 A, and reaction time 60 min.

[0102] After the reaction, the boron film was completely removed, and the EDS analysis of the inner wall surface of the stainless steel was as follows: Figure 4 shown.

[0103] The results equivalent to those of this embodiment can also be obtained by using all the summarized process conditions of nitrogen trifluoride. Because this embodiment is the preferred condition, the NF3 flow rate is reduced or increased on this basis, but as long as the gas pressure is maintained at 0.1-0.6Pa, a certain concentration range of NF3 can be guaranteed. By adjusting the current 0.1-0.7A and the voltage 380-600V, a stable glow can be generated, the fluorine ion concentration is maintained in a certain range, and the effective removal rate range for the boron film is maintained. By adjusting the reaction time, the boron film can be completely removed.

[0104] (2) Using hydrogen and helium combined glow to clean the surface fluorine ions: First, hydrogen glow cleaning can remove the residual fluorine ions in the etching step, and then helium glow cleaning is used. The helium glow removes the residual hydrogen under the bombardment of helium plasma to ensure a high-quality vacuum environment in the vacuum chamber.

[0105] Experimental conditions of hydrogen-helium glow treatment

[0106]

[0107] After energy dispersive spectroscopy (EDS) and hydrogen-helium glow combined cleaning, the residual fluorine ions can be removed.

[0108] The above description is only a specific embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments described herein, but should conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for removing a boron film in a tokamak nuclear fusion device, characterized in that: The removal method comprises the following steps: (1) In the chamber of the tokamak device, CF4 or NF3 is used to generate fluorine plasma under glow discharge conditions, and then the fluorine ions react with the boron element in the boron film to chemically remove the boron film; (2) After the boron film is removed, a combined glow cleaning of hydrogen and helium is performed, firstly hydrogen glow cleaning is performed to remove residual fluorine ions, and then helium glow cleaning is performed to remove residual hydrogen; When CF4 is used to chemically remove the boron film in step (1), oxygen is introduced as an auxiliary gas while CF4 is introduced, and the ratio of CF4 to oxygen is 1:

1. The glow discharge conditions are: CF4 flow rate 30-150sccm O2 flow rate 30-150sccm Air pressure 0.1-0.6Pa Current 0.1-0.7A Voltage 400-650V Time: 30-120min; When the boron film is chemically removed by NF3 in step (1), the glow discharge conditions are: NF3 flow rate 50-200sccm Air pressure 0.1-0.6Pa Current 0.1-0.7A Voltage 380-600V Time: 30-120 minutes.

2. The removal method according to claim 1, characterized in that: When CF4 is used to chemically remove the boron film in step (1), the glow discharge conditions are: CF4 flow rate 50sccm O2 flow rate 50sccm Air pressure 0.3Pa Current 0.2A Voltage 550V Time: 60 minutes.

3. The removal method according to claim 1, characterized in that: In step (2), the conditions for hydrogen glow cleaning are current 0.3A, voltage 490V, pressure 2.6Pa, and time 7-12h. 。 4. The removal method according to claim 1, characterized in that: In step (2), the conditions for helium glow cleaning are current 0.3A, voltage 280V, pressure 0.5Pa, and time 7-12h.

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