CN110896162A

CN110896162A - Method for realizing adjustable terahertz gyrotron frequency ultra wide band by adopting multimode cascade

Info

Publication number: CN110896162A
Application number: CN201911016617.1A
Authority: CN
Inventors: 刘頔威; 黄杰; 王维; 宋韬; 胡巧
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2019-10-24
Filing date: 2019-10-24
Publication date: 2020-03-20

Abstract

The invention provides a method for realizing frequency ultra-wideband adjustment of a terahertz gyrotron by adopting multimode cascade, which is characterized in that a mode of cascade operation of a plurality of working modes is adopted, the working modes are sequentially excited by adjusting energy of a working magnetic field or an electron beam, and compared with the terahertz gyrotron with continuously adjustable frequency of the traditional single-mode operation, the tunable range of the frequency can be greatly improved. Under the condition that the main frequency is 500GHz, four working modes TE are adopted_5,7,q,TE_10,5,q,TE_3,8,q,TE_1,9,qAnd the cascade operation finally obtains a frequency tuning bandwidth of about 10GHz, and the output power can be stabilized in a certain range. The ultra-wideband frequency continuously adjustable terahertz gyrotron has good application prospects in DNP-NMR systems and other fields.

Description

Method for realizing adjustable terahertz gyrotron frequency ultra wide band by adopting multimode cascade

Technical Field

The invention provides a method for realizing adjustable frequency ultra wide band of a terahertz gyrotron by adopting multimode cascade connection, which adopts four working modes TE_5,7,q,TE_10,5,q,TE_3,8,q,TE_1,9,qThe (q-1, 2,3 … …) cascade work enables the output bandwidth of the gyrotron to reach more than 10GHz, and the output power can be stabilized in a certain rangeIn addition, the device belongs to the field of vacuum terahertz radiation sources.

Background

Nuclear Magnetic Resonance (NMR) is a physical process in which a Magnetic moment of a Nuclear atom is not zero, a spin energy level undergoes zeeman splitting under the action of an external Magnetic field, and Resonance absorbs electromagnetic radiation of a specific frequency. Due to the high resolution of nuclear magnetic resonance spectroscopy, nuclear magnetic resonance has been widely used in the fields of physics, chemistry, material science, and biomedicine as a method of spectral analysis. However, the low sensitivity of conventional Nuclear magnetic resonance spectroscopy limits its further application, and Dynamic Nuclear Polarization (DNP) driven by electromagnetic waves is an effective method for improving the sensitivity of Nuclear magnetic resonance spectroscopy. With the development of superconducting magnet technology, modern nmr spectroscopy is developing towards high fields in order to further improve sensitivity of nmr spectroscopy. For a 300-1000MHz high-field nuclear magnetic resonance spectrum analysis system, the electron paramagnetic resonance frequency range is 200-650GHz, and the range of terahertz is reached. In order to saturate the electron spin polarization and maximize sensitivity, the power of the radiation source is required to be 20-100W. Among various high-power terahertz radiation sources developed at present, the gyrotron is the only terahertz radiation source capable of meeting the requirements, and the maximum output power and efficiency of the solid-state source developed at present are far from the vacuum source. Traditional vacuum electronic devices, such as traveling wave tubes and klystrons, are limited in average power capacity and application in terahertz wave bands due to the fact that the structures of the traditional vacuum electronic devices are more and more complex, and electron beams revolve in a longitudinal magnetic field and interact with a high-frequency angular electric field to generate stimulated radiation.

DNP-NMR requires a gyrotron to have a wider frequency adjusting range and output power and frequency stability, so how to enable the gyrotron to obtain a wider bandwidth and output power stability is a research hotspot in the field at present, the traditional gyrotron changes the longitudinal index of a working mode by adjusting the energy of a working magnetic field or an electron beam so as to achieve the purpose of adjusting the frequency, and the working current is required for exciting the working modeGreater than the starting oscillation current, starting oscillation current I of different longitudinal modes in the gyrotron_stIs approximately equal to

In a direct proportion to the total weight of the composition,

Q_d，Q_ohmrespectively, a diffractive quality factor and an ohmic quality factor. In the traditional three-section cavity, the diffraction quality factor and the effective length of the cavity are approximately satisfied:

this means that for higher order longitudinal modes the onset current will be large, so it is almost impossible to achieve a wider frequency tuning range by tuning the longitudinal index using only one mode of operation, typically only around 2GHz if only one mode of operation is excited.

Disclosure of Invention

The invention provides a novel method, namely a working mode of cascade connection of a plurality of modes is adopted to realize adjustable frequency ultra wide band of a terahertz gyrotron_5,7,q,TE_10,5,q,TE_3,8,q,TE_1,9,qAnd (q ═ 1,2 and 3 … …) is excited sequentially, and finally, the tuning bandwidth of about 10GHz is realized, and compared with the traditional gyrotron, the tuning bandwidth is greatly improved. To obtain a wider frequency tuning range, more modes can be adopted for cascade operation, but more competition modes are encountered, and it is difficult to ensure that all the operation modes can be strongly coupled with the field, so the invention also emphasizes on how to reasonably select the operation modes, ensure that all the selected operation modes can start oscillation and have strong interaction with the field. Meanwhile, the improved three-section cavity is adopted, compared with the common three-section cavity, a slightly upward conical waveguide section is introduced between the input section of the gyrotron and the main cavity, and the novel interaction structure can effectively reduce the oscillation starting current of a high-order longitudinal mode.

The invention is mainly realized based on the following theoretical guidance:

the working modes are reasonably selected, the characteristic values of the adjacent working modes are required to be different by a certain value (the specific value is determined by the working frequency and the cavity structure), so that the working modes can continuously work, and the working modes and the competition modes are ensured to have certain isolation.

The coupling coefficient of the selected mode is analyzed, and a proper guiding center radius is selected to ensure that all the working modes have strong interaction with the field, namely all the working modes can start to vibrate, and the coupling of the surrounding competition mode and the field is small or can be ignored relative to the coupling strength of the working modes.

And analyzing the starting oscillation current of the selected working mode and a competition mode near the working mode, roughly determining the working parameters through the analysis of the starting oscillation current, and simultaneously determining whether the competition mode starts oscillation or not and whether the competition mode starts oscillation or not has good isolation from the working mode.

The cold cavity rough analysis is carried out, the ohm quality factor and the diffraction quality factor of each mode are calculated, and the resonant frequency of the hot cavity of the gyrotron is very close to the resonant frequency of the cold cavity, so that whether the working frequencies of the modes can be connected or not can be verified through the analysis of the cold cavity, and meanwhile, the result of the cold cavity has a good guiding effect on the analysis of the hot cavity.

The thermal cavity is accurately calculated, whether each mode can start oscillation or not is verified through numerical simulation, the specific working frequency, the electronic interaction efficiency and the output power of each mode are calculated, and whether each mode can be excited or not and whether frequency continuous adjustability can be achieved or not is finally verified.

Drawings

Figure 1 is a modified three-segment chamber structure.

Fig. 2 is a pattern distribution diagram.

Fig. 3 shows the coupling strength in the working mode and the contention mode.

FIG. 4 shows the oscillation starting current in the operation mode and the contention mode.

Fig. 5 shows the results of a numerical simulation of the thermal chamber.

Fig. 6 shows the frequency adjustment range corresponding to each operation mode.

Detailed Description

The present invention is described in detail below with reference to examples, it should be noted that the examples are only for illustrative purposes and should not be construed as limiting the scope of the present invention, and those skilled in the art can make insubstantial modifications and adaptations of the present invention based on the above descriptions.

The specific implementation mode is that firstly, in order to reduce the starting oscillation current of a high-order longitudinal mode, an improved three-cavity structure is adopted, as shown in fig. 1, compared with the traditional three-cavity structure, a slightly upward conical waveguide section is introduced between the input section of the gyrotron and the main cavity, the structure shown in fig. 1 is finally obtained through continuous optimization, and specific parameters of the cavity structure are not convenient to give. At least 4 working modes are required to be selected in order to enable the frequency tuning range to reach 10GHz, and the difference of characteristic values of adjacent working modes is about 0.15, so that the resonant frequencies of the adjacent working modes can be connected, and the continuous frequency tuning is realized. Final TE_5,7,q,TE_10,5,q,TE_3,8,q,TE_1,9,qAnd (q ═ 1,2,3 … …) is selected as the operation mode. FIG. 2 is a diagram of the distribution of the modes (including fundamental and second harmonics) around the operating mode, and it can be seen that the operating mode has good mode isolation from the competing mode, but there is also a little competing mode TE_16,3,TE_25,1,TE_13,4It is inevitable. Fortunately, by calculating the coupling strength of the four modes with the electron beam guiding center radius, it can be seen from FIG. 3 that the four operating modes have strong coupling with the electron beam when the electron beam guiding center radius is selected to be 0.805mm, while the three competing modes TE are described above_16,3,TE_25,1,TE_13,4Coupling to electrons is substantially negligible. The oscillation starting current of the working mode and the competing mode (including fundamental wave and second harmonic) is shown in fig. 4, wherein the dotted line represents the fundamental wave, and the solid line represents the second harmonic, and it is also obvious from the figure that the working mode and the competing mode have good isolationAnd the vibration starting of a competition mode can be effectively avoided. Finally, through numerical simulation, the four working modes can be sequentially excited by adjusting the energy of the working magnetic field or the electron beam, the working frequencies of the four modes can be well connected, the final frequency tuning range reaches 10GHz, the frequency can be continuously adjusted, meanwhile, the output power is stabilized at about 500W, and the change of the output power along with the working frequency is shown in fig. 5.

Claims

1. A method for realizing frequency ultra-wideband adjustability of a terahertz gyrotron by adopting multimode cascade connection sequentially excites four modes by adjusting energy of a working magnetic field or an electron beam, so that the four modes work in cascade connection, and the frequency ultra-wideband can be continuously adjusted.

2. The method for realizing terahertz gyrotron frequency ultra-wideband adjustability by adopting multimode cascade as claimed in claim 1, characterized in that the difference between the eigenvalues of adjacent working modes is about 0.15, thereby ensuring that the working frequencies of each mode can be linked together.

3. The method for achieving terahertz gyrotron frequency ultra wide band tuning through multimode cascade as claimed in claim 1, wherein a modified three-segment cavity is used, and compared with a traditional three-segment cavity, a slightly upward conical waveguide segment is introduced between the input segment and the main cavity of the gyrotron, so that the starting oscillation current of a high-order axial mode can be effectively reduced.