A method for controlling the negative ion density of a radio frequency negative ion source by feedback

Abstract

This invention provides a feedback control method for negative ion density of a radio frequency (RF) negative ion source, belonging to the field of plasma. The method includes: using the branch ratio method and non-invasive optical emission spectroscopy diagnostic technology to measure the relative negative ion density online in real time; establishing an optical observation path to collect the emission spectral signal; processing the signal through a branched fiber and narrowband filter to extract Balmer characteristic spectral lines and calculate the relative negative ion density; analyzing interference sources and employing electromagnetic shielding methods combined with band-stop filter design principles to suppress interference in experimental and control signals, improving the accuracy of feedback and control signals; and constructing a negative ion density feedback control algorithm based on closed-loop control theory, using the RF power source as the controlled object and referencing closed-loop control theory based on plasma load characteristics to achieve closed-loop feedback control of the negative ion density. This invention can maintain the stability of the negative ion density under long pulse width operation.

Description

Technical Field

[0001] This invention belongs to the field of plasma, specifically relating to a method for feedback control of negative ion density in a radio frequency negative ion source. Background Technology

[0002] Neutral beam injection (NBI) is an auxiliary heating method with a relatively clear physical mechanism and highly effective heating method, widely used in plasma heating and current-driven processes in fusion devices. With the future development of nuclear fusion devices, higher demands are placed on plasma heating and current-driven processes, requiring NBI systems to possess long pulse widths and high power operation capabilities. Radio frequency (RF) negative ion sources are the optimal choice for future NBI systems. Achieving stable operation of RF negative ion sources hinges on negative ion density feedback control. Currently, negative ion generation and maintenance are achieved through two methods: increasing yield through cesium vapor feeding and reducing negative ion loss through filtering magnetic fields. However, neither of these methods can be dynamically adjusted in real time, leading to instability in negative ion density during long-pulse operation. Optimizing the negative ion discharge parameters of negative ion sources is one of the key and challenging research areas for major neutral beam injection devices both domestically and internationally.

[0003] Reliable measurement of plasma parameters is a prerequisite for achieving steady-state operation of negative ion density. Contact-based electrical methods are susceptible to the influence of factors such as the size and materials of the external beam source system. During radio frequency discharge, the plasma potential changes with the coupling of radio frequency power. Even with radio frequency compensation, significant errors still occur in plasma parameter measurement. Optical Emission Spectroscopy (OES), a non-invasive technique for in-situ measurement of negative ion density in high-voltage and radio frequency environments, has been developed and applied in major fusion devices both domestically and internationally.

[0004] Internationally, in the development of radio frequency negative ion sources, to achieve the research and development tasks of ITER-NBI, the neutral beam injection research group at the IPP (Max Planck Institute for Plasma Physics) in Germany established various test platforms with different design specifications to complete the preliminary research work on the ITER-NBI ​​negative ion source. To achieve comprehensive analysis of plasma discharge characteristics, all test platforms are equipped with an OES diagnostic system. Leveraging its non-contact characteristics and a comprehensive collision radiation model, each test platform at the German IPP obtained plasma temperature and density, as well as density measurements of different types and excited-state particles in the source plasma. The neutral beam injection system of the LHD (Large-Scale Nuclear Fusion Experiment) in Japan uses a hot cathode negative ion source and is currently the longest-operating experimental device with the highest negative ion extraction current density. The LHD-NBI is equipped with a wealth of diagnostic methods, including an OES diagnostic system and a cesium emission spectroscopy monitoring system for cesium monitoring and recycling studies; an emission spectroscopy imaging system is used to guide the optimization of the negative ion generation region of the plasma electrodes. Current literature shows that international research has already conducted studies using OES for plasma parameter analysis, achieving relatively comprehensive plasma parameter diagnosis and using this as a basis to optimize negative ion generation. In China, radio frequency negative ion sources are the preferred ion source for the CFETR (Crystal Fusion Experimental Reactor) - NNBI, and related physics and engineering research has already begun.

[0005] The existing technology, "Research on Feedback Control of Negative Ion Source Beam Based on Radio Frequency Power Adjustment (Authors: Shu Xianlai et al.)," discloses that by detecting the current signal of the negative ion flow, a closed-loop feedback method is used to adjust the power output of the radio frequency power source, thereby achieving a stable negative ion flow. However, it is important to note that: First, many factors affect the negative ion flow, such as bias current, magnetic field current, and negative ion density, all of which can have an impact. Second, while increasing the radio frequency power source can change the extracted beam current, it does not fundamentally detect whether the problem is caused by the negative ion density; it only controls the beam current by increasing or decreasing the power, without knowing the influence of the negative ion density. This method also ignores the influence of bias and magnetic field, failing to identify the root cause of the problem. Third, the extracted beam current is measured using a current sensor, which includes both the negative ion flow and a small amount of electron flow. However, the accuracy of the negative ion density measured by this method is very poor.

[0006] In the field of emission spectroscopy diagnostic systems, many research institutions simply treat optical emission spectroscopy as a routine monitoring method, measuring the characteristic spectral lines of impurities and hydrogen within the plasma to monitor the operating status of the ion source, without applying it to experimental studies of the steady-state operation of negative ion sources. Furthermore, current methods for controlling negative ion density cannot achieve real-time feedback control of the negative ion density. Summary of the Invention

[0007] This paper focuses on current diagnostic methods for negative ion density in radio frequency (RF) negative ion sources. Research has revealed that contact-based electrical methods are susceptible to the influence of factors such as the size and materials of the external beam source system. Furthermore, during RF discharge, the plasma potential changes with the coupling of RF power. Even with RF compensation for plasma parameter measurement, significant errors still occur. To address these technical problems, this invention provides a feedback control method for negative ion density in RF negative ion sources. By analyzing the formation process of excited-state hydrogen atoms, the paper elucidates the effects of different types of particles on the generation of excited-state hydrogen atom density, deriving the contribution of the main excitation pathways to the characteristic spectral line radiation of the Balmer series. This invention employs the branch ratio method to achieve online monitoring of negative ion density. Considering the specific characteristics of the plasma load and referencing closed-loop control theory, a closed-loop control algorithm for negative ion density is developed, using the RF power source as the controlled object. This research completes the key technology for steady-state operation of negative ion density.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for feedback control of negative ion density in a radio frequency negative ion source includes the following steps:

[0010] Step 1: Using the branch ratio method, an optical observation path is established to collect the spectral signal emitted by the plasma through non-invasive optical emission spectroscopy diagnostic technology. After processing by branched optical fiber and narrowband filter, the characteristic spectral lines of the Balmer series are extracted, and the relative negative ion density is calculated to obtain the real-time negative ion density parameter.

[0011] Step 2: Analyze the interference sources and use electromagnetic shielding methods combined with the design principle of band-stop filters to suppress interference in experimental and control signals, thereby improving the accuracy of feedback and control signals.

[0012] Step 3: Construct a negative ion density feedback control algorithm based on closed-loop control theory. Taking the radio frequency power source as the controlled object, and referring to the closed-loop control theory based on the plasma load characteristics, realize closed-loop feedback control of negative ion density to maintain the stability of negative ion density under long pulse width operation.

[0013] Beneficial effects:

[0014] 1. By using the branch ratio algorithm, a real-time online monitoring method for negative ion density was established, which solved the problem of radio frequency interference of experimental parameters in the radio frequency environment and realized the accurate acquisition of load-side parameters.

[0015] 2. This invention enables the characterization of negative ion density in the form of analog quantities and designs a closed-loop feedback algorithm for plasma density, thereby realizing feedback control of the negative ion density of the radio frequency negative ion source.

[0016] 3. Conduct source plasma characteristic analysis, develop a closed-loop control algorithm for operating parameters with negative ion density as the target quantity, and maintain the stability of negative ion density under long pulse width operation. Attached Figure Description

[0017] Figure 1 A schematic diagram of the main formation process of excited-state hydrogen atoms;

[0018] Figure 2 A schematic diagram of the negative ion density conversion circuit for a negative ion density feedback control system;

[0019] Figure 3 Flowchart of the design method for a fifth-order Butterworth band-stop filter;

[0020] Figure 4 This is a schematic diagram of a negative ion density feedback control system. Detailed Implementation

[0021] 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.

[0022] To achieve steady-state operation of negative ion density, this invention provides a feedback control method for negative ion density of a radio frequency negative ion source, comprising the following steps:

[0023] Step 1: Real-time online measurement of relative negative ion density using the branch ratio method: Conduct research on non-invasive OES (Optical Emission Spectroscopy) diagnostic technology to solve the problem of real-time online monitoring and analog characterization of negative ion density of radio frequency negative ion sources, improve small signal analog quantity extraction technology, and improve the sampling accuracy of feedback signals;

[0024] Step 2: Solve signal interference problems in the radio frequency environment and improve the accuracy of feedback and control signals: Based on the working characteristics of the radio frequency power source and the design principle of band-stop filter, solve the interference problems of experimental signals and control signals in the radio frequency environment.

[0025] Step 3: Construct a negative ion density feedback control algorithm: Based on closed-loop control theory, develop a feedback control algorithm for plasma loads, thereby realizing closed-loop feedback control of negative ion density and laying the foundation for improving the steady-state operation capability of high-current negative ion sources.

[0026] Specifically, step 1 includes:

[0027] To reduce negative ion stripping loss, all radio frequency negative ion sources employ a low-pressure discharge mode. Within the possible parameter range of radio frequency plasma, different types of particles, H0 and H1, are used. + H2, H2 + and H - The contributions to the generation of excited-state hydrogen atom density vary. By investigating its emission cross-section and rate coefficient, and evaluating the dependence of the Balmer series characteristic spectral line intensity on the selected plasma parameters, it can be found that the Balmer series characteristic spectral line ratio is highly sensitive to the negative ion density. To simplify the particle processes and plasma parameters with weak dependencies in the negative ion density feedback control system, a basic expression for the hydrogen negative particle density depending on the characteristic spectral line ratio is established. Different characteristic spectral line ratios are used to reduce the dependence of the characteristic spectral line intensity on plasma parameters.

[0028] The basic expression is as follows:

[0029] ;

[0030] in, Represents the concentration of ground-state hydrogen atoms. Represents electron number density. Represents the coupling strength of hydrogen atoms. Represents the number density of hydrogen atoms. Represents the coupling strength of hydrogen ions. Represents the hydrogen ion number density. The coupling strength represents the second energy level of the hydrogen ion (the subscript 2 represents the second energy level). Represents the hydrogen ion number density (subscript 2 represents the second energy level). Represents the coupling strength of hydrogen ions (subscript 3 indicates the third energy level). Represents the hydrogen ion number density (the subscript 3 represents the third energy level).

[0031] Hydrogen Balmer series spectral lines compared to those obtained by combining characteristic spectral lines The ratio of these two values ​​can directly characterize the density of negative ions, and is expressed as:

[0032] ;

[0033] in, Represents the intensity of radiation (the brightness of red light emitted when an electron in a hydrogen atom transitions from n=3 to n=2). Represents radiation intensity (the brightness of blue-green light emitted when electrons in a hydrogen atom transition from n=4 to n=2). Let p represent the spectral intensity from energy level p to energy level k, where p=4 and k=2. Let n(4) represent the spontaneous emission transition probability from energy level p to energy level k, and n(5) represent the fourth energy level and the fifth energy level, respectively. Represents the concentration of hydrogen atoms. Represents the concentration of electrons. This represents the coupling strength of hydrogen atoms.

[0034] Based on this, the present invention combines characteristic spectral lines / Based on the ratio of negative ions, a real-time online monitoring method for negative ion density is established using the branch ratio algorithm.

[0035] Furthermore, such as Figure 1 As shown, the five processes of effective excitation, dissociation-recombination, collisional recombination, dissociation excitation, and mutual neutralization excitation constitute the formation process of excited-state hydrogen atoms. Converting the contribution of each excitation pathway to the line radiation into the ratio of particle number density to hydrogen atom density provides a good understanding of the contribution of these five excitation pathways to the characteristic spectral line radiation of the Balmer series. From the mutual neutralization process, it can be seen that: when the electron temperature is greater than 1 eV, collisional recombination can be ignored; when the electron density is less than 10... 19 m -3 At this point, dissociation and recombination can be ignored; due to the low discharge pressure (0.3 Pa), the absolute density of hydrogen atoms is very low, and the self-absorption of the Raman lines can be disregarded. Therefore, the main components are H and H2. - The particle determines the radiation intensity H of the characteristic spectral lines of the Balmer series. α H β H γ By combining characteristic spectral lines H α / H β The ratio of can directly characterize the density of negative ions.

[0036] Based on the analysis of the mutual neutralization process, this invention provides a design basis for the engineering implementation of the branch ratio method. First, an optical observation path is established according to the location where plasma diagnostics are required to collect emission spectral signals. Second, the emission spectral signals are processed into three parts through a branched optical fiber, and Balmer characteristic spectral lines are extracted using a narrowband filter. Third, a branch ratio calculation method for negative ion density is established, and a calculation expression for negative ion density is given. Finally, the electronic design for Balmer characteristic spectral lines is implemented, converting the spectral signal into an electrical signal. Based on the branch ratio calculation method, the corresponding algorithm circuit is designed and completed, thereby realizing the analog representation of relative negative ion density. Therefore, this invention uses relative negative ion density as the feedback object, greatly improving the timeliness and feasibility of negative ion density feedback control.

[0037] like Figure 2The diagram shows a schematic of the negative ion density conversion circuit in a negative ion density feedback control system. The negative ion density feedback control system performs the following functions: ① Photoelectric conversion of the spectral signals from three characteristic spectral lines via a photomultiplier tube; ② Voltage output of the signal via a current-to-voltage conversion circuit; ③ Amplification of the converted characteristic spectral line signal (i.e.,...). Figure 2 (Small signal amplification); ④ Calculation of negative ion density and electron density through algorithm circuit. Since the signal after the characteristic spectral lines are converted by the photomultiplier tube is a weak signal on the order of milliamperes, small signal amplification is required to improve the calculation accuracy of the algorithm circuit.

[0038] Specifically, step 2 includes:

[0039] Precise acquisition of experimental parameters for the ion source is a prerequisite for achieving precise control of negative ion density. To achieve precise acquisition of experimental parameters under radio frequency (RF) conditions, it is necessary to analyze the interference sources of the RF negative ion source test platform. Based on the characteristics of the interference sources and in conjunction with electromagnetic shielding methods, research on electromagnetic shielding of analog signals under RF conditions is conducted.

[0040] The radio frequency (RF) negative ion source testing platform of this invention uses a 1MHz RF power source as the driving source. During the ion source discharge process, RF interference affects the experimental signal through both spatial coupling and wire propagation. The experimental parameters are affected by RF interference, coupling a large amount of 1MHz noise, which cannot be used as feedback signals for negative ion density control, directly affecting the accuracy of the entire control algorithm. Therefore, to achieve negative ion density control in an RF environment, it is first necessary to solve the RF interference problem of experimental parameters and improve the accuracy of the feedback signal of the negative ion density control algorithm. This invention employs both active and passive defense methods to suppress the influence of RF interference. By analyzing the signal wavelength of the 1MHz RF power source, a corresponding shielded enclosure is designed to cut off spatial RF interference. A 1MHz band-stop filter is used for the analog output of the on-site signal acquisition and control circuits, effectively increasing the attenuation of the interference signal.

[0041] This paper focuses on four relatively mature filter design methods: Butterworth, Bessel, Chebyshev, and Caul filters. First, the amplitude characteristic curves of these four filters are analyzed. Based on the comparison of the amplitude characteristic curves of various fifth-order filters, it can be seen that: at the same order, the Butterworth (Bu) filter has the flattest passband and a gentle transition band decrease; the Bessel (Be) filter has a flat passband, the slowest transition band decrease, and the worst frequency selectivity in its amplitude-frequency response; the Chebyshev (Ch) filter has ripple in its passband and a relatively fast transition band decrease; and the Caul (Ca) filter has ripple in both the passband and stopband, and the fastest transition band decrease.

[0042] The filter required by this invention needs to first possess good flatness, and secondly, a steep amplitude attenuation characteristic in the transition band. Based on the comparison results of four types of filters, it can be seen that Butterworth and Bessel filters have good flatness within the passband, but the Bessel filter has a slow transition band drop and poor frequency selectivity. Therefore, the Butterworth filter is most suitable for the needs of this invention.

[0043] Furthermore, this invention uses a Butterworth filter as the design prototype for a band-stop filter. Based on the currently mature and excellent amplitude characteristics of the "normalized" filter design method, a fifth-order Butterworth band-stop filter with a center frequency of 1MHz, a bandwidth of 200kHz, a characteristic impedance of 50Ω, and an attenuation of ≥-20dB is completed. The "normalized" designation in this invention refers to a filter with a characteristic impedance of 1Ω and a cutoff frequency of 1 / 2πHz.

[0044] like Figure 3 The diagram illustrates the design method for a fifth-order Butterworth band-stop filter (BRF):

[0045] First, the signal is filtered through a normalized low-pass filter and a normalized high-pass filter. The normalized high-pass filter is then transformed into a high-pass filter with a cutoff frequency equal to that of the fifth-order Butterworth band-stop filter to be designed. Next, the high-pass filter is transformed into a high-pass filter with a characteristic impedance equal to that of the fifth-order Butterworth band-stop filter to be designed. The circuit is modified according to the correspondence between the high-pass filter and the basic unit of the fifth-order Butterworth band-stop filter (types I to IV). The component values ​​of the transformed circuit are calculated according to the rules to obtain the fifth-order Butterworth band-stop filter.

[0046] The fifth-order Butterworth band-stop filter designed according to the "normalization" principle meets the requirements of this invention in terms of flatness in the passband region and attenuation in the transition region. However, in the actual circuit fabrication process, due to errors in electronic components and the special nature of the environment, it is still necessary to fine-tune the filter using attenuation characteristic curves and echo reflection curves obtained from a frequency response analyzer to obtain the filter with optimal performance.

[0047] Specifically, step 3 includes:

[0048] Using an operational radio frequency (RF) negative ion source experimental platform, this study designs and implements a negative ion density control system. Due to the unique characteristics of the plasma load, traditional feedback control algorithms cannot meet the requirements for steady-state operation of the negative ion density. The load characteristics of the plasma during discharge are not always resistive or capacitive, but change with the discharge power and plasma configuration. If a traditional feedback control algorithm is used, it is easy to cause an increase in the reflected power of the RF power source, preventing the driving power from being coupled to the plasma to the maximum extent, thus affecting the feedback control of the negative ion density. Therefore, based on the unique characteristics of the plasma load and combined with the reflected power of the RF power source during discharge, a new control method needs to be developed to achieve feedback control of the negative ion density.

[0049] To achieve feedback control of negative ion density, the target value for negative ion density must first be selected. Based on the relative negative ion density curve during the operation of the ion source, the optimal relative negative ion density value is selected as the setpoint. According to the radio frequency negative ion source plasma control method, combined with the negative ion density signal, the radio frequency power output value is calculated. Simultaneously, taking into account the reflection of the radio frequency power source, the output power of the radio frequency power source is adjusted in a timely manner, thereby achieving closed-loop control of the negative ion density.

[0050] Furthermore, based on the unique characteristics of the plasma load in the radio frequency negative ion source and combined with closed-loop control theory, a negative ion density control algorithm was designed, and the corresponding field control system electronics were designed. The principle is as follows: Figure 4 As shown, the main control system, through 0-10V voltage regulation, and with the action of integrated circuits (RF power source, impedance matching unit, isolation transformer) and other components, converts the electrical signal into an optical signal through the conversion unit and optical fiber, which is then transmitted to the negative ion density conversion circuit. After processing by the negative ion density conversion circuit, the negative ion density signal is transmitted to the band-stop filter. At the same time, the conversion unit also transmits experimental parameters such as RF reflection power to the band-stop filter. Through 0-10V voltage regulation, the negative ion density feedback control circuit feeds the signal back to the main control system.

[0051] To improve the timeliness of negative ion density control, this invention implements feedback control of negative ion density through a field control circuit. Without altering the logic of the entire radio frequency ion source control system, the addition of an external control circuit effectively expands the functionality of the overall control system while simultaneously improving the fault tolerance of the entire system.

[0052] Due to the coupling of numerous radio frequency (RF) interferences with the experimental parameters, the interference signals need to be attenuated by a band-stop filter before being applied to the negative ion density feedback control circuit for acquisition. After acquiring the experimental parameters, the negative ion density feedback control circuit collects data according to the negative ion density sampling time preset by the negative ion density control system, using the acquired negative ion density signal as the target quantity for the feedback control algorithm. The negative ion density feedback control circuit inputs the negative ion density signal and the target quantity into the closed-loop control algorithm to calculate the control correction quantity for the RF power source. This correction quantity is a digital quantity, requiring analog-to-digital conversion based on the relationship between the RF power source's output power and the control signal, thus completing real-time closed-loop feedback control with negative ion density as the feedback quantity and the RF power source as the controlled quantity. Due to the special nature of the plasma load, the reflection of the RF power source needs to be analyzed during RF power source output control, and the RF power output needs to be controlled in a timely manner. Simultaneously, at the high-voltage input moment, the extraction of a large number of negative ions will cause a significant decrease in the branching ratio at the observation point. Therefore, the control algorithm at the high-voltage input moment needs to be comprehensively considered to avoid erroneous feedback in the system.

[0053] The negative ion density control system uses a fixed sampling window combined with soft switching to shield against transients caused by a sudden drop in local density due to a large outflow of negative ions, thus avoiding algorithmic misjudgments. Soft switching refers to allowing the effective gain of the feedback loop to smoothly climb from 0 to its normal value along a programmable curve, avoiding secondary disturbances caused by "sudden cut-in". The high-voltage input is usually selected after the plasma linear density has entered the target range and maintained a stable value for a certain period of time.

[0054] In other words, based on the relative negative ion density curve during the operation of the ion source, the relative negative ion density value at the optimal moment is selected as the set value, and the radio frequency power output value is calculated in combination with the negative ion density signal. The optimal moment refers to the period when the negative ion density reaches a stable high value during the operation of the radio frequency negative ion source. The optimal moment is selected by real-time monitoring of the relative negative ion density curve, and its peak area is selected as the set value, which serves as the target input for closed-loop control.

[0055] During feedback control, the system must be interlocked with the interlocking protection system in the field. In the event of a fault, the control signal output must be cut off within microseconds to protect the entire system. This is achieved by controlling high-speed power switches via field nodes through programmable logic devices, compressing the fault response time and achieving microsecond-level cutoff of control signal output. Field nodes refer to signals such as emergency stop, door switch, over-temperature, over-voltage, and arc detection. High-speed power switches are semiconductor devices specifically designed to completely turn off (or turn on) under high current / high voltage conditions at the microsecond or even nanosecond level.

[0056] Conventional diagnostic methods for ion sources involve electrical measurements of the extracted ion and electron currents, such as measuring ion current density using a power measurement target. However, these parameters depend not only on the plasma negative ion and electron densities but also on external parameters such as the geometry of the grid system, beam optics, and extraction voltage. OES, as a simple and non-invasive technique for in-situ measurement of negative ion density in high-voltage and radio-frequency environments, has been developed and applied in major fusion devices internationally.

[0057] Regarding research on negative ion density measurement, the neutral beam research group at IPP in Germany has made substantial progress, confirming the feasibility of using OES for negative ion density measurement. Measurement results from the emission spectrum diagnostic system of the MANITU device (one of the radio frequency negative ion source experimental testing platforms built by IPP in Germany) under experimental conditions of 8kV extraction voltage, 80kW radio frequency power, and 0.45Pa discharge pressure show that H... - Ion current and branching ratio H, which characterizes negative ion density α / H β It exhibits strong dependency. The OES converts the signal characterizing negative ion density into a real-time analog output, providing the necessary feedback signal for negative ion density control and making the successful implementation of this invention feasible. Currently, Germany only uses OES to measure negative ion density and has not yet implemented feedback control of negative ion density using OES signals.

[0058] In summary, this invention analyzes the formation process of excited-state hydrogen atoms, elucidates the effects of different types of particles on the generation of excited-state hydrogen atom density, and derives the contribution of the main excitation pathways to the characteristic spectral line radiation of the Balmer series. Utilizing a radio frequency (RF) negative ion source testing platform, the branch ratio method is employed to achieve real-time online measurement of negative ion density, providing necessary feedback parameters for feedback control during the RF discharge process. Simultaneously, by combining plasma load characteristics, the traditional closed-loop feedback control algorithm is improved to achieve real-time feedback control of negative ion density. The operating status is analyzed, and corresponding control strategies are adopted accordingly. Finally, a radio frequency power source is used to achieve real-time and efficient control of the negative ion density.

[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 method for feedback control of negative ion density in a radio frequency negative ion source, characterized in that, Includes the following steps: Step 1: Using the branch ratio method, an optical observation path is established to collect the spectral signal emitted by the plasma through non-invasive optical emission spectroscopy diagnostic technology. After processing by branched optical fiber and narrowband filter, the characteristic spectral lines of the Balmer series are extracted, and the relative negative ion density is calculated to obtain the real-time negative ion density parameter. Step 2: Analyze the interference sources and use electromagnetic shielding methods combined with the design principle of band-stop filters to suppress interference in experimental and control signals, thereby improving the accuracy of feedback and control signals. Step 3: Construct a negative ion density feedback control algorithm based on closed-loop control theory. Taking the radio frequency power source as the controlled object, and referring to the closed-loop control theory based on the plasma load characteristics, realize closed-loop feedback control of negative ion density to maintain the stability of negative ion density under long pulse width operation.

2. The radio frequency negative ion source negative ion density feedback control method according to claim 1, characterized in that, In step 1, contributing particles are identified by analyzing the formation process of excited-state hydrogen atoms; the contributions of different contributing particles to the density of excited-state hydrogen atoms are sorted out, and a basic expression for the hydrogen negative particle density dependent on the ratio of characteristic spectral lines is established. By using different ratios of characteristic spectral lines, the dependence of characteristic spectral line radiation intensity on plasma parameters is reduced, thereby realizing real-time online measurement of relative negative ion density.

3. The method of claim 1, wherein the feedback control of the negative ion density is performed by controlling the power supplied to the RF antenna. In step 2, a Butterworth filter is used as the design prototype of the band-stop filter to complete a fifth-order Butterworth band-stop filter with a center frequency of 1MHz, a bandwidth of 200kHz, a characteristic impedance of 50Ω, and an attenuation of ≥-20dB, so as to effectively increase the attenuation of interference signals.

4. The method of claim 1, wherein the feedback control of the negative ion density is performed by a feedback controller. In step 3, based on the relative negative ion density curve during the operation of the ion source, the relative negative ion density value at the optimal moment is selected as the set value, and the radio frequency power output value is calculated in combination with the negative ion density signal. The optimal moment refers to the period during which the negative ion density of the radio frequency negative ion source reaches a stable high value during operation. The optimal moment is determined by real-time monitoring of the relative negative ion density curve, selecting its peak region as the set value, and using it as the target input for closed-loop control.

5. The method of claim 1, wherein the feedback control of the negative ion density is performed by using a radio frequency (RF) power source. In step 1, the photoelectric conversion of the spectral signal is achieved by a photomultiplier tube, the voltage output of the signal is completed by a current-to-voltage conversion circuit, the characteristic spectral line conversion signal after photoelectric conversion and amplification is amplified, and finally the negative ion density and electron density are calculated by an algorithm circuit.

6. The method of claim 1, wherein the feedback control of the negative ion density is performed by a controller. In step 2, the electromagnetic shielding method is a passive defense method to suppress the influence of radio frequency interference; passive defense is achieved by designing a corresponding shielded housing to cut off spatial radio frequency interference.

7. The method of claim 4, wherein the feedback control of the negative ion density is performed by controlling the power supplied to the RF antenna. In step 3, the output power of the radio frequency power source is adjusted with reference to the reflection of the radio frequency power source to achieve closed-loop control of the negative ion density; if the reflection power of the radio frequency power source is high, the output power of the radio frequency power source is reduced, and if the reflection power of the radio frequency power source is low, the output power of the radio frequency power source is increased.

8. The method of claim 1, wherein the method further comprises: During the feedback control process, it is interlocked with the interlock protection system on-site.

9. The method of claim 1, wherein the method further comprises: In step 1, the characteristic spectral lines H are combined. α / H β The ratio of H to H directly characterizes the density of negative ions. α H represents the spectral radiation intensity of the transition from energy level 3 to energy level 2. β The spectral radiation intensity of the transition from energy level 4 to energy level 2.

10. The radio frequency negative ion source negative ion density feedback control method according to claim 1, characterized in that, In step 2, the method of suppressing interference between experimental and control signals, based on the design principle of band-stop filters, is an active defense method. Active defense is achieved by using a 1MHz band-stop filter for the analog output of the field signal acquisition and control circuit.

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