A pre-protective filter for superconducting tokamak millimeter wave diagnosis

Abstract

The application provides a pre-protection filter for superconducting tokamak millimeter wave diagnosis, and belongs to the technical field of magnetic confinement fusion plasma diagnosis, which can ensure the safety of sensitive millimeter wave receivers while keeping the integrity of the frequency spectrum, polarization and phase information of the to-be-measured signal to the maximum. The application adopts a single-band multi-section waveguide filter structure, which includes a band-pass filter for low-frequency band interference, a notch filter for interference in the diagnostic frequency band, and a low-pass filter for high-frequency band interference. The application can effectively protect against interference of different auxiliary heating frequency points and realize high-fidelity transmission of the received signal.

Description

Technical Field

[0001] This invention belongs to the field of magnetic confinement fusion plasma diagnostic technology, specifically relating to a pre-protection filter for millimeter-wave diagnostics of superconducting tokamak. Background Technology

[0002] Tokamaks are currently the most promising devices for achieving safe, clean, and widely available magnetic confinement fusion energy. Future tokamak fusion power plants require long-pulse operation to reduce power generation costs; therefore, all tokamak fusion devices currently under construction and planned employ a fully superconducting design. Both current magnetic confinement fusion and future commercial fusion reactors will utilize low-maintenance, highly stable, non-invasive millimeter-wave diagnostics for monitoring safe operation and optimizing operating modes. However, both existing magnetic confinement devices and future fusion reactors will employ megawatt-level auxiliary heating devices, which can interfere with the millimeter-wave diagnostic system, reducing measurement accuracy and even shortening its lifespan. Millimeter-wave diagnostic pre-protection filters are a key technology for fusion reactor diagnostics and urgently need to be addressed.

[0003] like Figure 1 As shown, based on the frequency band of millimeter-wave diagnostics, the interference signals in a tokamak device can be divided into three frequency bands:

[0004] Low-frequency band (low-frequency interference band) interference, including ion cyclotron heating, low-hybrid heating, and other diagnostic interference, with frequencies lower than the millimeter-wave diagnostic working band;

[0005] Interference within the diagnostic operating band (diagnostic operating frequency band), including active detection of millimeter wave sources that overlap with the millimeter wave diagnostic operating band;

[0006] High-frequency band (high-frequency interference band) interference, including higher harmonics and stray radiation, high-frequency auxiliary heating frequency, and frequency higher than the diagnostic working band.

[0007] in, Figure 1 The red line represents the expected loss of the "millimeter-wave diagnostic pre-protection filter" across the entire band.

[0008] Regarding the above three types of interference, the existing technology mainly has the following shortcomings:

[0009] First, the frequency band coverage is incomplete, lacking a comprehensive, all-band integrated protection solution. Existing technologies are mostly designed for single or dual-frequency interference. For example, Chinese patent application CN105356021A discloses an integrated cavity bandpass filter and low-pass filter assembly, employing an integrated structure of a coaxial, skewer-shaped low-pass filter and a cross-shaped or comb-shaped bandpass filter, achieving integration of bandpass and low-pass. However, this solution only includes two stages of filtering and lacks a notch filter, making it unable to precisely suppress interference at specific frequencies within the operating band in an extremely narrow band. Chinese patent application CN103346746A discloses a novel filter for DPDs, using a combination of a notch filter and a low-pass filter composed of discrete components. However, this solution is a second-order LC circuit structure, also lacking a bandpass filter, and cannot effectively suppress low-frequency interference. Furthermore, it uses an active differential circuit, making it unsuitable for passive millimeter-wave transmission in tokamak devices under strong electromagnetic interference environments.

[0010] Second, the structural form is incompatible with the application scenarios of tokamak. Chinese patent application CN105356021A adopts a coaxial cavity integrated structure, which achieves miniaturization by fixing the inner conductor to the metal resonant pillar. However, this structure is suitable for conventional microwave frequency bands such as satellite communication (its core frequency is 2.95 GHz), and it is difficult to apply it directly to the millimeter wave frequency band (50-220 GHz). Moreover, its integrated design limits the filter order and cannot be flexibly expanded into a three-stage cascade. Chinese patent application CN103346746A adopts discrete component LC circuits, which are suitable for DPD feedback links in communication systems. However, this type of active circuit has poor reliability in the megawatt-level electromagnetic interference environment of tokamak and cannot maintain the polarization and phase information of millimeter wave signals, thus failing to meet the requirements of high-precision plasma diagnostics.

[0011] Third, there is a lack of coordinated design between thermal management and signal fidelity. The megawatt-level reflection leakage of the tokamak auxiliary heating system can cause the filter to heat up significantly. Existing technologies do not consider thermal barrier design, and the high temperature can easily be transmitted to the downstream nanowatt-level sensitive millimeter-wave receiver, causing performance drift or damage. At the same time, although existing quasi-optical frequency selective surfaces and other solutions can achieve high attenuation, they will destroy the signal polarization and phase characteristics, and are not suitable for high-precision diagnostics that require maintaining the integrity of spectrum, polarization and phase information.

[0012] Fourth, insufficient VSWR suppression in cascaded systems. In existing technologies, multi-stage filters are designed and installed independently, resulting in large signal echo reflections at the connection points between filters. VSWR suppression for cascaded structures has not been optimized, leading to deterioration of transmission characteristics within the passband.

[0013] In summary, current technologies have not yet established an effective full-band pre-protection mechanism for tokamak millimeter-wave diagnostics. Serious consequences, including signal contamination and system burnout, have indeed occurred in multiple devices both domestically and internationally. To experimentally verify the reliability of the diagnostic system for safe operation of fusion reactors and to assess the characteristic time of system performance degradation, there is an urgent need to develop an integrated, high-fidelity, thermally robust millimeter-wave pre-protection filter that covers low-frequency, in-diagnostic-band, and high-frequency interference. Summary of the Invention

[0014] To address the aforementioned technical issues and simultaneously achieve high-damping filtering for low-frequency interference signals, extremely narrow-band damping filtering for interference signals within the operating band, high-damping filtering for high-frequency interference signals, highly integrated filter design, and effective thermal management, this invention provides a pre-protection filter for millimeter-wave diagnostics in superconducting tokamaks. This filter protects nanowatt-level sensitive millimeter-wave receivers from reflection and leakage interference from megawatt-level auxiliary heating millimeter waves.

[0015] Since tokamak auxiliary heating systems typically operate at multiple discrete frequencies, while superconducting tokamak millimeter-wave diagnostic systems require high-precision measurements within a fixed narrow band, this invention employs a single-band multi-section waveguide filter structure. This structure includes a bandpass filter for low-frequency interference, a notch filter for interference within the diagnostic band, and a low-pass filter for high-frequency interference. This effectively protects against interference at different auxiliary heating frequencies and achieves high-fidelity transmission of the received signal. While ensuring the safety of the sensitive millimeter-wave receiver, this invention also maximizes the integrity of the spectrum, polarization, and phase information of the signal under test.

[0016] To achieve the above objectives, the present invention adopts the following technical solution:

[0017] A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak includes, in sequence along the signal transmission direction, a notch filter, a bandpass filter, and a low-pass filter. The notch filter, bandpass filter, and low-pass filter are connected by a short waveguide. The notch filter is used to remove interference signals within the diagnostic operating frequency band, the bandpass filter is used to remove low-frequency interference signals, and the low-pass filter is used to remove high-frequency interference signals. The notch filter, bandpass filter, and low-pass filter all adopt a modular structure with resonant cavities fabricated within the waveguide.

[0018] Furthermore, the notch filter includes a notch filter housing and a fourth-order rectangular resonant cavity structure disposed inside the notch filter housing. The fourth-order rectangular resonant cavity structures are arranged in series and connected to each other through a standard rectangular waveguide.

[0019] Furthermore, the notch filter is equipped with input ports and output ports.

[0020] Furthermore, the bandpass filter includes a bandpass filter housing and a sixth-order symmetrical cavity structure disposed inside the bandpass filter housing, wherein the resonant frequencies of the six resonant cavities are uniformly distributed within the passband frequency band of the bandpass filter.

[0021] Furthermore, low-reflection transmission within the passband frequency range is achieved through enhanced coupling between adjacent resonant cavities, and the bandpass filter is equipped with input and output ports.

[0022] Furthermore, the low-pass filter includes, in sequence along the signal transmission direction, an input port, a ridge-shaped low-pass filter structure, a matching structure, and an output port. The ridge-shaped low-pass filter structure adopts a periodically arranged ridge structure.

[0023] Furthermore, the matching structure integrates a phase coupling structure.

[0024] Furthermore, the input and output ports of the notch filter, bandpass filter, and lowpass filter all use standard rectangular waveguide interfaces.

[0025] Furthermore, a thermal barrier layer is provided between the output port of the notch filter and the input port of the bandpass filter.

[0026] Furthermore, the operating frequency band covers the 50-170 GHz millimeter wave band.

[0027] Beneficial effects:

[0028] 1. This invention achieves integrated protection across the entire frequency band for three types of interference signals, employing a three-tiered structure of notch filter, bandpass filter, and lowpass filter to form a complete pre-protection system:

[0029] Notch filters are used to remove interference signals within the diagnostic operating frequency band. Because notch filters provide extremely narrow-band damping for interference signals at specific frequencies, typically greater than 60 dB, they attenuate the power of interference signals within the operating frequency band to less than one millionth of their original power. This allows other signals within the diagnostic operating frequency band to pass through with low loss while precisely and strongly attenuating interference signals, effectively removing interference from the operating frequency band.

[0030] Bandpass filters are used to remove low-frequency interference signals. Because bandpass filters have excellent frequency selectivity, especially for low-frequency signals below the passband frequency, they provide excellent attenuation, typically greater than 40 dB, reducing the power of low-frequency interference signals to less than one ten-thousandth, thus effectively removing low-frequency interference.

[0031] Low-pass filters are used to remove interference signals in the high-frequency band. Even though band-pass filters have the ability to attenuate high-frequency interference signals, the high-pass and low-impedance characteristics of millimeter-wave waveguides result in insufficient high-frequency damping, which is insufficient to protect the sensitive millimeter-wave receiver at the back end. Therefore, low-pass filters must be added to block interference signals higher than the diagnostic operating frequency band, such as high-frequency electronic cyclotron heating. Typically, the damping depth is greater than 40 dB, which means that the power of the high-frequency interference signal is attenuated to less than one ten-thousandth, thereby ensuring effective removal of high-frequency band interference.

[0032] The third-order filter uses a notch filter, a bandpass filter, and a low-pass filter in that order to suppress signal echo reflection at the filter interface and effectively suppress standing waves.

[0033] 2. This invention employs a single-band multi-section waveguide filter structure. The waveguide transmission is a passive structure, resulting in low passive intermodulation distortion and strong resistance to strong electromagnetic interference, making it suitable for megawatt-level leakage environments in tokamak. It naturally maintains the integrity of the spectrum, polarization, and phase information of the signal under test, meeting the stringent requirements for signal fidelity in high-precision plasma diagnostics. The operating frequency band can cover the 50-170 GHz millimeter-wave band, breaking through the application limitations of existing coaxial structures in the millimeter-wave band.

[0034] 3. This invention employs a specific arrangement of notch filter → bandpass filter → lowpass filter, and uses short waveguide connections. This cascade sequence optimizes impedance matching at the filter interface, effectively suppressing signal echo reflection and reducing standing waves.

[0035] 4. The filter interfaces of this invention all adopt standard rectangular waveguide connections and add a thermal barrier layer to effectively block the high temperature generated by the absorption of megawatt-level leakage energy during filter operation, prevent heat transfer to the sensitive millimeter-wave receiver at the back end, ensure the performance stability of the filter during long pulse operation of the tokamak, and extend the life of the diagnostic system.

[0036] 5. Each filter in this invention adopts a modular structure with a resonant cavity fabricated inside the waveguide. The notch filter is aligned with the reference frequency by adjusting the cavity size, the bandpass filter achieves phase compensation by adjusting the cavity spacing, and the low-pass filter adjusts the resonant frequency by reducing the cavity size. The debugging method is simple and has good consistency. Compared with the need for precise adjustment of discrete component parameters, the waveguide cavity structure of this invention is more suitable for mass production and field maintenance in the millimeter-wave band.

[0037] 6. This invention employs a single-band multi-section waveguide structure, with the three-stage filter compactly cascaded through short waveguides. Compared to multiple independent filters installed separately, the volume is significantly reduced, meeting the space requirements of the tokamak device near the diagnostic window.

[0038] In summary, this invention effectively solves the core problems of existing technologies, such as incomplete mid-frequency coverage, insufficient signal fidelity, lack of thermal management, and insufficient standing wave suppression, through a three-stage cascaded waveguide filter architecture, a specific cascading sequence, passive transmission in the waveguide cavity, and integrated thermal isolation design. It provides a highly reliable pre-protection scheme for millimeter-wave diagnostic systems for tokamak fusion reactors. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the expected loss of a millimeter-wave diagnostic pre-protection filter across the entire wavelength band.

[0040] Figure 2 This is a schematic diagram of a pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to the present invention;

[0041] Figure 3 This is a graph showing the spectral loss characteristics of a notch filter.

[0042] Figure 4 This is a curve showing the spectral loss characteristics of a bandpass filter.

[0043] Figure 5 This is a graph showing the spectral loss characteristics of a low-pass filter.

[0044] Figure 6 This is a schematic diagram of a bandpass filter;

[0045] Figure 7 This is a schematic diagram of a notch filter;

[0046] Figure 8 This is a schematic diagram of a low-pass filter;

[0047] Figure 9 This is a schematic diagram of the thermal barrier layer layout.

[0048] The attached figures are labeled as follows: 1. Notch filter; 2. Bandpass filter; 3. Low-pass filter; 4. Sensitive millimeter-wave receiver; 5. Thermal barrier layer. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0050] like Figure 2As shown, the pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to the present invention is suitable for a superconducting tokamak millimeter-wave diagnostic system, and includes, in sequence, a notch filter 1, a bandpass filter 2, and a low-pass filter 3. The low-pass filter 3 is connected to a sensitive millimeter-wave receiver 4.

[0051] like Figure 3 As shown, the notch filter uses the characteristic frequency of the interference signal in the diagnostic band as the reference frequency. By processing a resonant cavity in the waveguide, the resonant frequencies of multiple cavities are aligned with the reference frequency to achieve overlap, thereby realizing notch filtering and removing interference signals in the diagnostic working frequency band.

[0052] like Figure 7 As shown, the notch filter includes a notch filter housing, an input port, and an output port. The notch filter housing contains an internal hollow structure and employs a fourth-order rectangular resonant cavity structure to selectively resonate with interference signals in the diagnostic band, thereby achieving damping filtering. Figure 7 Taking the 105 GHz notch filter as an example, a fourth-order resonant structure is connected in series to achieve a damping depth of over 60 dB. In order to effectively accommodate the requirements of narrowband high-damping filtering and broadband low-loss passband, the connection between the four resonant cavities retains the original standard rectangular waveguide, and the waveguide length is determined according to the passband frequency range requirements.

[0053] like Figure 4 As shown, the bandpass filter adopts the multi-cavity working mode, but distributes the resonant frequency within the diagnostic frequency band, adjusts the cavity spacing to achieve phase compensation, thereby realizing bandpass filtering and removing interference signals in the low-frequency band. Figure 4 It includes "difference loss characteristic curve" and "echo reflection characteristic curve". The bandpass filter exhibits high damping characteristics in frequency bands below the passband (100 – 110 GHz), especially below 96 GHz, where the damping depth exceeds 40 dB. In the passband, the filter has a difference loss of less than 1.5 dB, exhibiting high transmittance characteristics.

[0054] like Figure 6 As shown, the bandpass filter includes a bandpass filter housing, an input port, and an output port. An internal hollow structure, a sixth-order symmetrical cavity structure, is set within the bandpass filter housing to construct six low-quality-factor resonant structures. These six resonant frequencies are evenly distributed within the passband of the bandpass filter. By enhancing the coupling between adjacent resonant cavities, low reflection and high transmittance characteristics are achieved within the passband frequency range, thus realizing bandpass filtering.

[0055] like Figure 5As shown, this includes the "difference loss characteristic curve" and the "echo reflection characteristic curve." The low-pass filter exhibits high damping characteristics in frequency bands above 120 GHz, especially above 122 GHz, where the damping depth exceeds 40 dB. In the frequency band below 115 GHz, the filter has a difference loss of less than 3 dB, exhibiting high throughput characteristics. The low-pass filter adopts a multi-cavity operating mode, but by reducing the size of the resonant cavity and adjusting the resonant frequency to a higher frequency band, damping is achieved, thereby realizing low-pass filtering and removing interference signals in the high-frequency band.

[0056] like Figure 8 As shown, the low-pass filter consists of an input port, a ridged low-pass filter structure, a matching structure, and an output port. The low-pass filtering function is achieved using a series-connected ridged low-pass filter structure. Its periodically arranged ridged structure effectively suppresses high-frequency signals while allowing low-frequency components to pass through with low loss. Following the ridged low-pass filter structure, a precise matching structure adjusts the high-frequency damping of the input and output ports. This design significantly optimizes the voltage standing wave ratio (VSWR), reduces reflection loss at the passband edge, and thus improves the overall impedance matching performance of the system. To further improve the frequency response, the matching structure integrates a phase coupling structure, enabling precise control of the signal phase within the passband. This effectively improves the smoothness of the passband amplitude-frequency response and, through coherent cancellation of internal multipath reflections, suppresses in-band ripple and fluctuations caused by structural discontinuities or impedance mismatches. Ultimately, this achieves low-pass filtering performance that combines good out-of-band suppression with a flat passband.

[0057] like Figure 6 , Figure 7 , Figure 8 As shown, all third-order filters use standard rectangular waveguide interfaces. Preferably, the three types of waveguide filters are connected by short waveguides, and all filter interfaces use waveguide connections.

[0058] like Figure 9 As shown, a thermal barrier layer is added between the output port of the notch filter and the input port of the bandpass filter to effectively block the high temperature generated by the filter during operation from being transmitted to the sensitive millimeter-wave receiver 4 at the back end.

[0059] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pre-protection filter for millimeter-wave diagnostics in superconducting tokamak, characterized in that, Along the signal transmission direction, the filter includes a notch filter, a bandpass filter, and a low-pass filter. The notch filter, bandpass filter, and low-pass filter are connected by a short waveguide. The notch filter is used to remove interference signals in the diagnostic working frequency band, the bandpass filter is used to remove low-frequency band interference signals, and the low-pass filter is used to remove high-frequency band interference signals. The notch filter, bandpass filter, and low-pass filter all adopt a modular structure with resonant cavities processed inside the waveguide.

2. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 1, characterized in that, The notch filter includes a notch filter housing and a fourth-order rectangular resonant cavity structure disposed inside the notch filter housing. The fourth-order rectangular resonant cavity structures are arranged in series and connected to each other by a standard rectangular waveguide.

3. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 2, characterized in that, The notch filter has an input port and an output port.

4. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 1, characterized in that, The bandpass filter includes a bandpass filter housing and a sixth-order symmetrical cavity structure disposed inside the bandpass filter housing. The resonant frequencies of the six resonant cavities are evenly distributed within the passband frequency band of the bandpass filter.

5. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 4, characterized in that, Low-reflection transmission in the passband frequency range is achieved by enhancing coupling between adjacent resonant cavities. The bandpass filter is equipped with input and output ports.

6. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 1, characterized in that, The low-pass filter includes, in sequence along the signal transmission direction, an input port, a ridge-shaped low-pass filter structure, a matching structure, and an output port. The ridge-shaped low-pass filter structure adopts a periodically arranged ridge structure.

7. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 6, characterized in that, The matching structure integrates a phase coupling structure.

8. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 1, characterized in that, The input and output ports of the notch filter, bandpass filter, and lowpass filter all use standard rectangular waveguide interfaces.

9. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to claim 1, characterized in that, A thermal barrier layer is provided between the output port of the notch filter and the input port of the bandpass filter.

10. A pre-protection filter for millimeter-wave diagnostics of a superconducting tokamak according to any one of claims 1 to 9, characterized in that, The operating frequency band covers the 50-170 GHz millimeter wave band.

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