A radio frequency antenna for use in an ultra-high vacuum cavity and its equivalent circuit

Abstract

A radio frequency antenna installed in an ultra-high vacuum chamber includes an upper and lower interface flange module, a connecting straight wire module, and an open multi-segment quasi-circular arc antenna module. The upper and lower interface flange modules are used to ensure vacuum sealing; the connecting straight wire module is used to transmit excitation signals and impedance matching; the open multi-segment quasi-circular arc antenna module is used to provide three-segment elliptical arc antennas with an eccentricity of approximately 1. The entire antenna has an ultra-wide impedance bandwidth from DC to 127MHz, and can radiate an effective radio frequency signal with a polarization degree of 0.9987 and a signal strength of 53.6mG in the atomic interaction region under a low feed power of 3W. The present invention can not only reduce the radio frequency power demand by 90%, but also maintain the system vacuum degree better than 2.1×10 ‑8 Pa, and has good electromagnetic compatibility. The present invention is applied to a space ultracold atomic physics experimental device, and can provide a radio frequency antenna installed in an ultrahigh vacuum chamber for experiments of radio frequency induced evaporative cooling and radio frequency state preparation.

Description

Technical Field

[0001] The invention relates to the technical field of evaporative cooling, in particular to a radio frequency antenna used in an ultra-high vacuum cavity and an equivalent circuit thereof. Background Art

[0002] The pursuit of further ultra-low temperatures is one of the important driving forces of current cold atom technology. Laser cooling and trapping cool the temperature of atoms to the microkelvin scale, which has great application value in dark matter detection, precision measurement and deep space navigation. The ultracold field has opened up fields such as quantum simulation and quantum information. For alkali metal atoms, the mature solutions used in the laboratory to obtain a large number of high-density ultracold atomic molecules are optical trap evaporative cooling and magnetic trap evaporative cooling. Optical traps have a fast evaporation rate and are less difficult to deploy. However, since optical traps have no selectivity for atomic species, they cannot be used for the coordinated cooling of Bose-Fermi mixtures. The RF-induced magnetic trap evaporative cooling scheme is often used. While maintaining the frequency of the magnetic trap, the RF tail-cutting method is used to drive away atoms of the specified type to achieve coordinated cooling.

[0003] Conventional RF antennas for cold atoms use ring wires or parallel strip antennas. Considering the symmetry of the cavity and the need for subsequent RF state preparation, ring-like antennas can provide us with RF signals with better intensity and polarization performance. In order to avoid heating of atoms due to mutual inductance during evaporative cooling, the RF antenna should suppress the signal components generated along the axis of the magnetic trap coil, so the RF antenna can only feed the signal from the horizontal window. Due to the large edge-to-center distance, if the antenna is placed outside the metal cavity, the excitation power required to meet the experimental requirements will reach more than 30W. At the same time, due to the reflection of the metal cavity, most of the signals will be shielded and cannot be effectively fed into the vacuum cavity, and the leakage of high-power RF will cause serious electromagnetic compatibility problems. Summary of the invention

[0004] In view of the deficiencies of the above-mentioned prior art, the present invention proposes an experimental device for ultracold atomic physics in space, which can provide a radio frequency antenna and its equivalent circuit for use in an ultra-high vacuum cavity for experiments on radio frequency induced evaporative cooling and radio frequency state preparation in space. The transmission characteristics of the antenna of the present invention have an ultra-wide impedance bandwidth from DC to 127MHz, and can radiate an effective radio frequency signal with a polarization degree of 0.9987 and a signal strength of 53.6mG in the atomic interaction region under low feed power conditions of 3W, fully meeting the experimental requirements. In addition to reducing the radio frequency power requirement by 90%, the present invention can also maintain the system vacuum degree at 2.1×10 -8 Pa ultra-high vacuum level and good electromagnetic compatibility.

[0005] The technical solution of the present invention is as follows:

[0006] In one aspect, the present invention provides a radio frequency antenna for use in an ultra-high vacuum chamber, which is characterized by comprising:

[0007] Upper and lower interface flange modules, used for sealing connection with the ultra-high vacuum chamber;

[0008] Connect the straight wire module to transmit the excitation signal and impedance matching;

[0009] Arc antenna module, used for radio frequency signal transmission;

[0010] The upper and lower interface flange modules include a first SMA interface, an upper interface flange, a second SMA interface and a lower interface flange, the connecting straight wire module includes a first connecting straight wire and a second connecting straight wire, the arc antenna module is an open multi-segment ellipse formed by sequentially connecting a first arc antenna, a second arc antenna and a third arc antenna, and the eccentricities of the first arc antenna, the second arc antenna and the third arc antenna are all approximately 1, and the opening end of the first arc antenna and the tail end of the third arc antenna constitute the opening end of the open multi-segment ellipse;

[0011] The open ends of the open multi-segment ellipse are respectively connected to one end of the first connecting straight wire and one end of the second connecting straight wire, the other end of the first connecting straight wire is connected to the first SMA interface through the upper interface flange, and the other end of the second connecting straight wire is connected to the second SMA interface through the lower interface flange.

[0012] In order to achieve the above requirements, laser welding technology is used between the components to ensure vacuum tightness and low material outgassing rate. At the same time, the welding of the connector shielding layer and the titanium cavity can effectively shield the RF signal, reduce the leakage of RF signals along the window, and improve the electromagnetic compatibility of the system;

[0013] As an optimization of the above solution, the first SMA interface and the second SMA interface transmit the excitation signal through the coaxial line, the first SMA connector and the upper interface flange are threaded and laser welded, and the second SMA interface and the lower interface flange are threaded and laser welded to ensure the airtightness of the vacuum chamber.

[0014] In the above scheme, a connecting straight wire module is designed, including a first connecting straight wire and a second connecting straight wire, which are used to transmit the excitation signal and perform impedance matching;

[0015] As an optimization of the above solution, the first connecting straight wire and the second connecting straight wire are composed of an oxygen-free copper core, a ceramic sleeve and a titanium shielding layer, and are responsible for the transmission of radio frequency signals and impedance matching between the port and the antenna;

[0016] In the above scheme, an open multi-segment quasi-arc antenna module is designed, including a first arc antenna, a second arc antenna and a third arc antenna, which is used to provide three-segment elliptical arc antennas with an eccentricity of approximately 1 required by the radio frequency antenna.

[0017] As an optimization of the above solution, the first arc antenna and the third arc antenna are oblique to the optical axis, and the second arc antenna is placed vertically along the radial direction. Such a three-section arc antenna is made by tightly fixing a complete oxygen-free copper wire with a diameter of 1mm on the surface of a cylindrical mold with the same diameter as the cross optical path of the cooling light, and then extruding it along the high direction of the cylinder using a special mold. The diameter of the cylindrical mold is 23mm, and the special mold is the intersection line of the cross optical path of the cooling light. The length of each elliptical arc is about 23mm, the eccentricity is approximately 1, and the radius of curvature is about 13mm, corresponding to an arc degree of about 100°.

[0018] On the other hand, the present invention also provides an equivalent circuit of a radio frequency antenna for use in an ultra-high vacuum chamber, which is characterized in that it includes a first port impedance, a second port impedance, an antenna drive signal source, a first antenna capacitor, a second antenna capacitor, a third antenna capacitor, a fourth antenna capacitor and a fifth antenna capacitor, a first antenna inductor and a second antenna inductor;

[0019] The antenna driving signal source is connected in series with the first port impedance and connected to the ground wire;

[0020] The first antenna capacitor is connected in parallel with the first arc antenna, and the second antenna capacitor is connected in parallel with the third arc antenna; the third antenna capacitor, the fourth antenna capacitor and the fifth antenna capacitor are respectively connected in parallel with the second arc antenna; the first antenna inductor is located at the first arc antenna position, the third antenna inductor and the fourth antenna inductor are located at the third arc antenna position, the two antenna inductors are located at the third arc antenna, and all antenna inductors are connected in series.

[0021] The first port impedance is located at the first SMA interface position, and the second port impedance is located at the second SMA interface position.

[0022] The impedance of the first port and the impedance of the second port are 50Ohm. The antenna drive signal source sends out a radio frequency signal. The entire equivalent circuit includes five antenna capacitors. The first antenna capacitor value is 2.52pF, the second antenna capacitor value is 2.52pF, the third antenna capacitor value is 0.51pF, the fourth antenna capacitor value is 0.51pF, and the fifth antenna capacitor value is 0.51pF. It includes four antenna inductors. The first antenna inductance value is 26.04nH, the second antenna inductance value is 26.04nH. The third antenna inductance value is 20.16nH, and the fourth antenna inductance value is 20.16nH. Finally, connect the ground wire.

[0023] As a further optimization of the above scheme, a finite element simulation was performed on the RF antenna model, and the spatial distribution of the RF magnetic field generated by the antenna in the ±5mm area of ​​the cavity center under the excitation of a 3W100MHz signal was analyzed.

[0024] As a further optimization of the above solution, the RF signal distribution of the RF antenna is graphically analyzed, including the RF magnetic field distribution along the vertical axis and the frequency distribution of the RF intensity at the center of the cavity, and the analysis is performed based on the diagram.

[0025] As a further optimization of the above scheme, the simulation data of the antenna S parameters in the range of 100kHz to 150MHz in the RF antenna are compared with the measured sampling results. In the antenna S parameter quantity, the reflection coefficient S11 and transmission coefficient S21 of the antenna are measured in the frequency range of 100kHz to 150MHz, and the measurement result graph is obtained, and then a comparative analysis is performed based on the simulation data graph.

[0026] Compared with the prior art, the technical effects of the present invention are:

[0027] 1) It has the advantages of low power consumption and high polarization degree, and can achieve the same experimental goals with one-tenth of the power consumption of the extracavity antenna;

[0028] 2) Meeting the requirements of high vacuum and high electromagnetic compatibility can play an important role in aerospace projects with strict restrictions.

[0029] 3) The transmission characteristics of the entire antenna have an ultra-wide impedance bandwidth from DC to 127MHz, and can radiate an effective RF signal with a polarization degree of 0.9987 and a signal strength of 53.6mG in the atomic interaction region under a low feed power of 3W.

[0030] 4) It can not only reduce the RF power requirement by 90%, but also maintain the system vacuum better than 2.1×10 -8 Pa, and has good electromagnetic compatibility, meeting the requirements of aerospace engineering.

[0031] 5) Applied to space ultracold atomic physics experimental devices, it can provide assistance for space radio frequency induced evaporative cooling and radio frequency state preparation experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the antenna and magneto-optical trap structure.

[0033] Figure 2 It is a schematic structural diagram of a radio frequency antenna used in an ultra-high vacuum cavity according to the present invention;

[0034] Figure 3 is an equivalent circuit diagram of a radio frequency antenna used in an ultra-high vacuum cavity of the present invention;

[0035] Figure 4 This is a spatial distribution diagram of the radio frequency magnetic field generated by the antenna in the ±5mm area of ​​the cavity center under the excitation of a 3W 100MHz signal in an embodiment of the present invention;

[0036] Figure 5 : is a distribution diagram of radio frequency signals in an embodiment of the present invention, wherein (a) is the distribution of radio frequency magnetic field along the vertical axis; (b) is the frequency distribution of radio frequency intensity at the center of the cavity;

[0037] Figure 6 It is a comparison chart of the simulation data and measured sampling results of the antenna S parameters in the range of 100kHz to 150MHz.

[0038] In the figure: 1 - cooling light cross optical path module; 2 - coil module; 3 - upper and lower interface flange module; 4 - connecting straight wire module; 5 - arc antenna module; 6 - first SMA interface; 7 - upper interface flange; 8 - second SMA interface; 9 - lower interface flange; 10 - first connecting straight wire; 11 - second connecting straight wire; 12 - first arc antenna; 13 - second arc antenna; 14 - third arc antenna; 15 - first port impedance; 16 - second port impedance; 17 - antenna drive signal source; 18 - first antenna capacitor; 19 - second antenna capacitor; 20 - third antenna capacitor; 21 - fourth antenna capacitor; 22 - fifth antenna capacitor; 23 - first antenna inductor; 24 - second antenna inductor; 25 - third antenna inductor; 26 - fourth antenna inductor; 27 - ground antenna; DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is clearly and completely described below in conjunction with implementation examples and drawings, but this should not limit the protection scope of the present invention.

[0040] See also Figure 1 and Figure 2 . Figure 1 This is a schematic diagram of the antenna and magneto-optical trap structure. Figure 2It is a schematic diagram of the structure of the radio frequency antenna used in the ultra-high vacuum cavity of the present invention. As shown in the figure, a radio frequency antenna used in the ultra-high vacuum cavity includes a cooling light cross-optical path module 1, a coil module 2, an upper and lower interface flange module 3, a connecting straight wire module 4 and an arc antenna module 5. The cross-optical path module 1 and the coil module 2 are used to form a magneto-optical trap. The upper and lower interface flange modules 3 include an upper interface flange 7 and a lower interface flange 9, which are used to fix and weld with the titanium cavity to ensure vacuum tightness; the connecting straight wire module 4 includes a first connecting straight wire 10 and a second connecting straight wire 11, which are used to transmit excitation signals and perform impedance matching; the arc antenna module 5 includes a first arc antenna 12, a second arc antenna 23 and a third arc antenna 14, which are used to provide three sections of elliptical arc antennas with an eccentricity of approximately 1 required for the radio frequency antenna.

[0041] The first SMA interface 6 and the second SMA interface 8 transmit the excitation signal through the SMA interface coaxial line. The first SMA interface 6 and the upper interface flange 7 are threaded and laser welded to ensure the airtightness of the vacuum chamber. The second SMA interface 8 and the lower interface flange 9 are threaded and laser welded. The first connecting straight wire 10 and the second connecting straight wire 11 are composed of an oxygen-free copper core, a ceramic sleeve and a titanium shielding layer, which are responsible for the transmission and impedance matching of the RF signal between the port and the antenna. The first arc antenna 12, the second arc antenna 13 and the third arc antenna 15 together form an open multi-segment arc-like antenna as a RF transmitting component. The first arc antenna 12 and the third arc antenna 14 are oblique to the optical axis, and the second arc antenna 13 is placed vertically along the radial direction. Such a three-section arc antenna is made by tightly fixing a complete oxygen-free copper wire with a diameter of 1 mm on the surface of a cylindrical mold with the same diameter as the cooling light cross optical path 1, and then extruding it along the high direction of the cylinder using a special mold. The diameter of the cylindrical mold is 23 mm, and the special mold is the intersection line of the cooling light cross optical path 1. The length of each elliptical arc is about 23 mm, the eccentricity is approximately 1, and the radius of curvature is about 13 mm, corresponding to an arc degree of about 100°.

[0042] See also Figure 3 . Figure 3It is the antenna equivalent circuit diagram. It includes port impedance, drive signal source, antenna inductance and capacitance. The value of the first port impedance 15 and the second port impedance 16 is 50Ohm, the antenna drive signal source 17 sends out a radio frequency signal, and the entire equivalent circuit includes five antenna capacitors. The value of the first antenna capacitor 18 is 2.52pF, the value of the second antenna capacitor 19 is 2.52pF, the value of the third antenna capacitor 20 is 0.51pF, the value of the fourth antenna capacitor 21 is 0.51pF, and the value of the fifth antenna capacitor 22 is 0.51pF. It includes four antenna inductors. The value of the first antenna inductor 23 is 26.04nH, and the value of the second antenna inductor 24 is 26.04nH. The value of the third antenna inductor 25 is 20.16nH, and the value of the fourth antenna inductor 26 is 20.16nH. Finally, the ground wire 27 is connected.

[0043] See also Figure 4 . Figure 4 This is the spatial distribution diagram of the RF magnetic field generated by the antenna in the ±5mm area of ​​the cavity center under the excitation of a 3W 100MHz signal. In the figure, the RF signal excited in the cavity shows an absorption mode that decays with the axial distance, and the signal intensity decays parallel to the antenna plane, with good uniformity and directivity. The strongest signal is Figure 4 Near the antenna plane area in the lower right corner, the RF intensity is as high as over 90mG. In the atomic cloud confinement area of ​​±1mm in the center, the RF intensity is attenuated by about half to 50mG. At this time, the RF intensity can still fully meet the RF evaporative cooling requirements.

[0044] See also Figure 5 , (a) The solid line corresponds to the intensity distribution of the RF magnetic field under the 50MHz excitation signal, and the shadow represents the range of RF signal intensity when the excitation source frequency changes from 100kHz to 100MHz. From the information of each component in the figure, it can be found that the RF signal is mainly distributed in the horizontal direction B z and B x In terms of components, it shows good symmetry with high center and low sides. In particular, in the highlighted area from -4.3mm to +4.7mm in the center, the horizontal component field strength reaches more than 50mG, which is about 3.3 times the RF intensity outside the cavity. The spatial distribution is uniform and the polarization direction is concentrated, so more power can be used to effectively expel atoms in the magnetic trap, and the expulsion probability reaches 99.99%. (b) shows the change trend of each component of the RF magnetic field at the center of the cavity with the excitation frequency from 100kHz to 100MHz. It can be found from the figure that the RF signal at the center of the cavity does not change much with the frequency, with the horizontal component B H The dominant component is not less than 53.6 mG, and the vertical component B V The weakest, with an intensity no higher than 2.68mG. The polarization degree of the RF signal here is no lower than 0.9987.

[0045] See also Figure 6 , Figure 6 The figure below is a comparison of the simulation data and the measured sampling results of the antenna S parameters in the range of 100kHz to 150MHz. In the antenna S parameter quantity, we measured the reflection coefficient S11 and transmission coefficient S21 of the antenna in the frequency range of 100kHz to 150MHz, and obtained Figure 6 The line graph in the figure is the simulation data, and the scatter plot with error bars represents the measured data of the vector network analyzer. The figure marks two special points A (100MHz, -11.7dB) and B (127.1MHz, -10dB) on the measured data, representing the experimental required bandwidth and impedance bandwidth respectively. By comparing the measured results with the simulation results, it is found that the two are highly consistent, with a deviation of less than 1dB (<10MHz). The S11 coefficient increases monotonically with frequency, and the maximum energy reflection coefficient appears at point A, the upper limit of the required bandwidth, which is -11.7dB. The antenna is a low-frequency antenna with an impedance bandwidth of DC to 127.1MHz and a relative required bandwidth of 199.69%, which meets the ultra-wideband requirements. The S21 coefficient decays with increasing frequency, and the transmission loss is less than -0.75dB within the measurement bandwidth.

[0046] The method used in the present invention is:

[0047] This invention is a radio frequency antenna installed in an ultra-high vacuum chamber in the cold atomic physics cabinet of the Chinese space station. The finite element simulation method is mainly used to simulate the antenna performance. The corresponding radio frequency antenna is manufactured according to the modeling and specific process, and the various performance indicators of the antenna are tested and experimentally verified on an integrated experimental platform. The results show that the transmission characteristics of the antenna are consistent with the simulation results. It has an ultra-wide impedance bandwidth from DC to 127MHz. It can radiate an effective radio frequency signal with a polarization degree of 0.9987 and a signal strength of 53.6mG in the atomic interaction region under the condition of a low feed power of 3W, which fully meets the experimental requirements. In addition to reducing the radio frequency power demand by 90%, the present invention can also maintain the system vacuum degree at 2.1×10 -8 Pa ultra-high vacuum level and good electromagnetic compatibility, meeting the requirements of aerospace engineering.

Claims

1. A radio frequency antenna for use in an ultra-high vacuum chamber, characterized in that: include: Upper and lower interface flange modules (3) are used for sealing connection with the ultra-high vacuum chamber; Connecting a straight wire module (4) for transmitting an excitation signal and impedance matching; The circular arc antenna module (5) is used for transmitting radio frequency signals; The upper and lower interface flange modules (3) include a first SMA interface (6), an upper interface flange (7), a second SMA interface (8) and a lower interface flange (9); the connecting straight wire module (4) includes a first connecting straight wire (10) and a second connecting straight wire (11); the arc antenna module (5) is an open multi-segment ellipse formed by sequentially connecting a first arc antenna (12), a second arc antenna (13) and a third arc antenna (14); the eccentricities of the first arc antenna (12), the second arc antenna (13) and the third arc antenna (14) are all approximately 1; the opening end of the first arc antenna (12) and the tail end of the third arc antenna (14) constitute the opening end of the open multi-segment ellipse; The open ends of the open multi-segment ellipse are respectively connected to one end of the first connecting straight wire (10) and one end of the second connecting straight wire (11); the other end of the first connecting straight wire (10) is connected to the first SMA interface (6) via the upper interface flange (7); the other end of the second connecting straight wire (11) is connected to the second SMA interface (8) via the lower interface flange (9).

2. The radio frequency antenna for use in an ultra-high vacuum chamber according to claim 1, characterized in that: The first SMA interface (6) and the upper interface flange (7), as well as the second SMA interface (8) and the lower interface flange (9), are both threadedly fastened and laser welded to ensure the airtightness of the vacuum chamber; and the first SMA interface (6) and the second SMA interface (8) both transmit excitation signals to the outside through a coaxial line.

3. The radio frequency antenna for use in an ultra-high vacuum chamber according to claim 1, characterized in that: The first connecting straight wire (10) and the second connecting straight wire (11) are both composed of an oxygen-free copper core, a ceramic sleeve and a titanium shielding layer, and are responsible for the transmission of radio frequency signals and impedance matching between the port and the antenna.

4. The radio frequency antenna for use in an ultra-high vacuum chamber according to claim 1, characterized in that: The first arc antenna (12) and the third arc antenna (14) are obliquely intersected with the optical axis, and the second arc antenna (13) is placed vertically along the radial direction. The arc antenna module (5) is formed by tightly fixing an oxygen-free copper wire with a diameter of 1 mm on the surface of a cylindrical mold with the same diameter as the cooling light cross optical path (1), and then extruding it along the height direction of the cylinder using a mold. The diameter of the cylindrical mold is 23 mm, and the mold is an intersection curve of the cooling light cross optical path (1). The length of each arc is about 23 mm, the eccentricity is approximately 1, and the radius of curvature is about 13 mm, corresponding to an arc angle of 100°.

5. An equivalent circuit of a radio frequency antenna used in an ultra-high vacuum chamber according to any one of claims 1 to 4, characterized in that: It includes a first port impedance (15), a second port impedance (16), an antenna driving signal source (17), a first antenna capacitor (18), a second antenna capacitor (19), a third antenna capacitor (20), a fourth antenna capacitor (21), a fifth antenna capacitor (22), a first antenna inductor (23), and a second antenna inductor (24); The antenna driving signal source (17) is connected in series with the first port impedance (15) and connected to a ground line (27); The first antenna capacitor (18) is connected in parallel with the first circular arc antenna (12), and the second antenna capacitor (19) is connected in parallel with the third circular arc antenna (14); the third antenna capacitor (20), the fourth antenna capacitor (21) and the fifth antenna capacitor (22) are respectively connected in parallel with the second circular arc antenna (13); the first antenna inductor (23) is located at the first circular arc antenna (12), the third antenna inductor (25) and the fourth antenna inductor (26) are located at the third circular arc antenna (14), and the second antenna inductor (24) is located at the third circular arc antenna (14), and all antenna inductors are connected in series.

6. The equivalent circuit of the radio frequency antenna used in the ultra-high vacuum chamber according to claim 5, characterized in that The first port impedance (15) is located at the first SMA interface (6), and the second port impedance (16) is located at the second SMA interface (8).

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