Magnetically Insulated Transmission Line Oscillator Research Articles

Experimental Study on the Frequency Tunability of a Magnetically Insulated Transmission Line Oscillator

In the present study, the frequency tunability of a magnetically insulated transmission line oscillator (MILO) is investigated by simulation in conjunction with an experiment. In the simulation, the tunable MILO reaches a maximum output power of approximately 5.0 GW with an applied voltage of 590 kV and an input power of 29 GW. The corresponding power efficiency is 17.2%, and the frequency is 1.575 GHz. In addition, by adjusting the outer radius of the vane from 77 to 150 mm, the 3-dB tunable frequency of the tunable MILO ranges from 0.85 to 2.12 GHz, and the tuning bandwidth is 85.5%. The experimental results show that when the voltage and current are 405 kV and 42 kA, respectively, the peak microwave power of the tunable MILO is approximately 2.3 GW with a microwave frequency of approximately 1.900 GHz. The difference in the two frequencies at the peak microwave power is significant because of the tuning mechanism. When the outer radius of the vane varies in the range of 76.6–93.0 mm, the 3-dB tunable frequency range is between 1.552 and 2.168 GHz, corresponding to a 3-dB tuning bandwidth of 33.1%. The experimental measurements verify the simulation results.

Evaluation of Explosive Emission Carbon Fiber Cathodes for High-Power Microwave Devices

Most high-power microwave (HPM) sources, such as the magnetically insulated transmission line oscillator (MILO) being developed at Texas Tech, utilize cold cathodes that generate electrons via explosive emission. Highly emissive cathodes such as the presented can generate current densities and currents greater than 1 kA/cm 2 and 10 kA, respectively, which are required for devices that can output radio frequency (RF) power greater than 100 MW. Typically, these cathodes are made of materials such as metal, silk or synthetic velvet, carbon fiber, and cesium iodine (CsI)-coated carbon fiber. In order to optimize the MILO performance, we fabricated carbon fiber velvet cathodes and compare their performance with other commercially available carbon fiber cathodes. Fabrication was done on a manual, mechanical loom using commercially available carbon fiber thread. Four carbon fiber cathodes were tested: in-house fabricated monomodal carbon fiber velvet, in-house fabricated bimodal carbon fiber velvet, in-house fabricated carbon fiber plain weave cloth, and bimodal carbon fiber velvet manufactured by ESLI Inc. Testing was performed in a vacuum chamber with variable AK gap in the high vacuum range (10−7 torr). High-speed optical imaging was performed in order to determine the uniformity of the generated plasma as well as the e-beam. Voltage and current measurements were performed to determine diode impedance and perveance.

Electromagnetic Analysis of an Axially Periodic Disk-Loaded Slow-Wave Structure With Low-Loss Dielectric Filling Between Disks for a Magnetically Insulated Line Oscillator

The partially dielectrically filled periodic disk-loaded co-axial structure as a slow wave interaction structure for magnetically insulated transmission line oscillator (MILO) has been electromagnetically analyzed using field matching technique. The slow wave structure (SWS) includes a periodic disk-loaded coaxial structure in which the low-loss dielectric material is partially filled between successive disks. In the analysis, the dispersion relationship and interaction impedance have been calculated taking into account all the harmonics generated inside the structure. The effect of the dielectric constant of the material on the dispersion characteristics and phase velocity has been investigated. Also, the effect of different structural parameters on the dispersion characteristics and the interaction impedance has been obtained. To validate the performed theoretical analysis, the analytically obtained dispersion results have been compared with previously known dispersion results and then also confirmed by the commercial code “Computer Simulation Tool (CST) Studio Suite.” The agreement between theoretical analysis and simulation with respect to the dispersion characteristics is within 5%.

A Repetitive Low Impedance High Power Microwave Driver

A low impedance high power microwave (HPM) driver is designed, which can be used in studying multi-gigawatt HPM devices such as the magnetically insulated transmission line oscillator (MILO), based on a helical pulse forming line (PFL) and the Tesla pulse transformer technology. The co-axial PFL is insulated by ethanol–water mixture, whose dielectric constant can be adjusted; and the helical line increases the output pulse width as well as the impedance to make a better match with the load. By the optimal combination of PFL charging voltage and output switch working voltage, the reliability of the PFL can be improved. The Tesla transformer has partial magnetic cores to increase the coupling coefficient and is connected like an autotransformer to increase the voltage step-up ratio. The primary capacitor of the transformer is charged by a high voltage constant current power supply and discharged by a triggered switch. A transmission line is installed between the PFL and the HPM load, to further increase the load voltage. A ceramic disk vacuum interface is used for improving the vacuum of the HPM tube. The experiments show that the driver can operate at 30 GW peak power, 75 ns pulse width and 5 Hz repetition rate.

Experimental Demonstration of a Ridged Magnetically Insulated Transmission Line Oscillator

In this study, we investigate the effect of including ridged disk vanes on the operation of a magnetically insulated transmission line oscillator (MILO) in an experiment using a comparison of a ridged MILO (RMILO) with a conventional MILO (CMILO). The only technical difference between the two models is the presence of ridges in the RMILO. The experimental results show that when the voltage is 486 kV and the current is 51 kA, the RMILO’s peak microwave power is approximately 2.9 GW with a pulse duration of more than 35 ns and a microwave frequency of approximately 1.48 GHz. In contrast, the CMILO has the highest power microwave with a peak power of 2.1 GW corresponding to a pulse duration of more than 40 ns and a microwave frequency of approximately 1.53 GHz. The RMILO’s power efficiency increases to 11.7%, a 25% increase compared to that of the CMILO under the same conditions. The experimental measurements exactly coincide with the simulation results. Thus, including ridged disk vanes produces a superior performance.

Study on Load-Side Optimization in Bifrequency MILO Source

In this article, we present an in-depth study of bifrequency magnetically insulated transmission line oscillator (MILO) generating RF output power in S-band with azimuthal partition. It is proposed to optimize the parameters across the load side, inserting gold plate in the collector and tuning the collector cavity depth. The proposal aims at enhancing the power conversion efficiency by employing the concept of a tapered extraction region and implementing four stubs. The stubs help to achieve proper impedance matching, subsequently improving the output power. Further, loss-free FR-4 dielectric is used in extractor vanes which helps to reduce the RF power distribution in other modes. The use of gold plate stimulates space charge near the surface and helps in deflecting the electrons toward the interaction cavities. The presence of FR-4 extractor vane helps to generate spoke current near the collector. The proposed system provides improvised RF coupling of the electron beam to the interaction cavities. The peak power efficiency of ~12.7% is achieved through the device. The beam–wave interaction process is illustrated using space charge instability mechanism. Taking into account the Lagrangian of electrons relationship between angular momentum and torque due to angular force is also derived. This in-depth study helps in the deep understanding of magnetic insulation process inside the device toward developing the proficient MILO source.

Improvement of vacuum maintenance capability and output pulse limit in a hard-tube MILO with a carbon fiber array cathode

This paper reports a great improvement of vacuum maintenance capability in a hard-tube magnetically insulated transmission line oscillator (MILO) by replacing traditional polymer velvet cathode with a low-outgassing carbon fiber array cathode. Based on this improvement, we tested and compared the output pulse limits of this hard-tube MILO with the two cathodes under the same conditions, without an external pumping system after base vacuum pressure was reached. Experimental results reveal that the acceptable pressure for MILO operation is about 1.7 Pa, which is about 100 times higher than the conventional vacuum level (~10−2 Pa). Moreover, the output pulse limit of this hard-tube MILO has been increased from 7 pluses with polymer velvet cathode to about 127 pulses with carbon fiber array cathode. This promotion in the output pulse limit will speed up the application process of MILO.

A High-Efficiency Ridged Magnetically Insulated Transmission Line Oscillator

In this study, we investigate the effect of the inclusion of ridged disk vanes on the operation of a magnetically insulated transmission line oscillator (MILO), using a combination of “cold” (without electron beam) and “hot” (with electron beam) simulations. Cold simulation shows that the inclusion of ridged-disks in a ridged MILO (RMILO) enhances its slow wave structure’s (SWS’s) coupling impedance, thus improving its power efficiency. Moreover, since the ridges’ effect on frequency is weak, the MILO’s inherent tunability is maintained. In hot simulation, the RMILO obtains a 3-dB tunable frequency range of 0.96–2.18 GHz with an applied voltage of 603 kV, in contrast to the 1.04–2.01-GHz range obtained with a conventional MILO (CMILO), owing to the former’s improved power efficiency. With this voltage, the device generates the high power microwave (HPM) with a maximum output power of 7.1 GW at a frequency of 1.40 GHz, corresponding to a power efficiency of 22.6%. The maximum power efficiency of 24.5%, corresponding to a 13.5-GW output, was obtained by increasing the applied voltage to 807 kV, an increase of 20.2% compared to the power efficiency obtained with a CMILO in the same condition.

A compact dual-band radiation system* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61671457 and 61871390).

Complex magnetically insulated transmission line oscillator (MILO), as an important development direction, can enhance the power efficiency and generate dual-band high power microwaves (HPMs). A complex MILO and a preliminary dual-band radiation system have been proposed in our previous studies. However, the axial length of the dual-band radiation system is too long to meet the compact requirements. In this paper, a compact dual-band radiation system is presented and investigated numerically. The compact dual-band radiation system comprises a dual-band cross-shaped mode converter and a dual-band coaxial conical horn antenna. It can convert two coaxial TEM mode microwaves (1.717 GHz and 4.167 GHz) generated by the complex MILO into the coaxial TE11 mode microwaves, and then radiate them into the air. At 1.717 GHz, the gain of the antenna is 17.9 dB, and the total return loss and diffraction loss are 1.50% and 0, respectively. At 4.167 GHz, the gain is 19.4 dB, and the total return loss and diffraction loss are 1.17% and 0.78%, respectively. The power handling capacity of the antenna is 5.1 GW at 1.717 GHz and 2.0 GW at 4.167 GHz. Comparing with the original structure, the length of the dual-band radiation system is reduced by 45.2%.

A Low Standing-Wave-Ratio Wideband Mode-Transducing Antenna for High-Power Microwaves

A novel $L$ -band wideband coaxial mode-transducing antenna, which converts the coaxial TEM mode into circular TM01 mode, is realized and tested in this article. The mode-transducing antenna is composed of a coaxial wideband multiple-reflection mode converter with the three groups of supporting rods and a compact coaxial conical horn with a dielectric lens. First, the schematic and operating principle of the mode-transducing antenna are analyzed theoretically. Then, the structure design and optimization with simulated results are performed for demonstration. Finally, a prototype of the mode-transducing antenna is fabricated, measured, and verified in terms of reflection characteristics, radiation patterns, and far-field waveforms. The measured results reveal that the mode-transducing antenna achieves the bandwidth wider than 31% with the transmission efficiency higher than 98.2%. The high-power test with the magnetically insulated transmission-line oscillator (MILO) show that the power handing capacity of the mode-transducing antenna is greater than 4.0 gigawatts (GW). The measured results are in good agreement with the simulated results. Therefore, the mode-transducing antenna has a good application prospect in the field of high-power microwave (HPM).

Design and Simulation of a Novel High-Efficiency Magnetically Insulated Transmission Line Oscillator

A novel high-efficiency magnetically insulated transmission line oscillator (MILO) is proposed and investigated in this article. It has three features. First of all, the widths of vanes are nonuniform while the width of resonant cavities in slow wave structure (SWS) remains unchanged. Second, the width of the first extract cavity shrinks and an extra extractor vane is added to enhance the power extraction. In addition, the emission surface of the cathode decreases, reducing the energy waste. The simulation results show that the novel high-efficiency MILO can generate high power microwave (HPM) with the output power of 6.3 GW and the frequency of 1.42 GHz when the applied voltage and current are 586 kV and 49 kA, respectively. The corresponding power efficiency is 22%. Given the same input, the conventional MILO generates the HPM with the same frequency but a lower power of 5.0 GW. The corresponding power efficiency is 17% smaller than that of our proposed MILO. Additionally, the maximum axial electric field in our proposed structure is 0.64 MV/cm after chamfering, which is lower than the breakdown threshold (1 MV/cm).

Competitions among modes in magnetically insulated transmission line oscillator

<sec>As a compact and high power microwave source, the competitions among various modes are prone to appear in the initial stage of the development of the radiated electromagnetic field in a magnetically insulated transmission line oscillator (MILO). If the mode competitions are not controlled effectively, the output characteristics of the MILO may decline in the end. </sec><sec>As is well known, the operating mode of MILO is generally designed on the π mode of the TM<sub>00</sub> mode and the coaxial disk-loaded waveguide is usually adopted as a slow-wave structure for beam-wave interaction in MILO. Therefore from the dispersion relations between the electron beam and the lower order electromagnetic modes(including TM<sub>00</sub>, TM<sub>01</sub> and HEM<sub>11</sub> modes) in the slow-wave structure, the characteristics and possible suppression methods of the three kinds of mode competitions are analyzed simply. The first kind mode competition is between the different axial modes of the fundamental TM<sub>00</sub> mode. In this case, the electromagnetic field of the competition mode is also axially symmetric and its frequency is slightly lower than that of the π mode. The second is the competition between the TM<sub>00</sub> and higher order TM<sub>01</sub> mode. In this case, the competition frequency is rather higher than that of the π mode (TM<sub>00</sub>). The third is the competitions between the TM<sub>00</sub> and low order asymmetric HEM<sub>11</sub> modes. In this case, the competition frequency is slightly higher than that of the main mode. Appropriately choosing the radii of the anode vanes, the number of the anode cavity and the load length of the cathode, the corresponding mode competition intensity can be weakened. </sec><sec>Based on the obtained results above and the existing model of the MILO, a compact high output power L-band MILO is proposed. Numerical studies of the mode competitions and output characteristics are carried by using the three dimensional particle-in-cell code. Cold-cavity test shows that in the low frequency range, the easily stimulated electromagnetic modes are the π mode of TM<sub>00</sub> and HEM<sub>11</sub> modes with frequencies of 1.61 GHz and 1.77 GHz, respectively. The numerical results of hot-cavity verify that the competition in the initial stage comes mainly from the asymmetric HEM<sub>11</sub> mode due to the fact that there exists the strut in the output region, the Cartesian coordinates are adopted during the simulation, and totally symmetry cannot be guaranteed. In addition, electron beam emission from the cathode is not ideally even. But stable and high output microwave power is obtained in the end in the L-band MILO by being optimized. The output power and efficiency are 8.1 GW and 18% respectively, and the mode purity reaches about 97%. </sec>

A High-Efficiency Magnetically Insulated Transmission Line Oscillator With Ridged Disk-Loaded Vanes

In order to improve power efficiency, a high-efficiency magnetically insulated transmission line oscillator (MILO) with ridge-disk-loaded vanes is proposed in this paper. Theoretical analyses show that slow wave structure (SWS) with ridges can improve the coupling impedance of MILO markedly, which thus increases the power efficiency of MILO. The proposed MILO with ridges is optimized with CHICPIC particle-in-cell code, and the simulation results are consistent with the theoretical results. In simulations, when the applied voltage and input power are 566 kV and 28.4 GW, the output power and maximum electric field of MILO with ridges are 6.0 GW and about 0.61 MV/cm, respectively. Moreover, the corresponding power efficiency reaches up to 21.0%, and the microwave frequency is 1.48 GHz. However, for the MILO without ridges, the output power and the power efficiency are only 5.0 GW and 17.6%, respectively, with the same input voltage and power, and the microwave frequency is 1.53 GHz. Therefore, the power efficiency of MILO with ridges increases by 19% in comparison to MILO without ridges.

Simulation of an S-Band MILO with Adjustable Beam Dump

This paper details the design, simulation, and optimization of a low-impedance high repetition rate magnetically insulated transmission line oscillator (MILO) driven by a compact Marx generator. The project goals require the MILO to generate an radio frequency (RF) pulse within the S-band frequency range with a peak output power greater than 1 GW with greater than 10% efficiency. Its design is based on a set of base equation which provide critical component dimensions applied to a three-dimensional model constructed within CST studio suite used in a particle-in-cell (PIC) simulation. Additional to the geometric model, simulation of the MILO with non-ideal material properties and a lumped element modeling of the Marx generator were performed. The results of these simulations then informed changes to the model as to the optimizing performance of the device. Within the framework of the model, the final MILO design achieves the design goals with an approximate RF peak power of 4.5 GW at 2.5 GHz operating in the TM 01 mode when an input driving pulse with a peak voltage of 600 kV while providing 58 kA is applied.

Influence of voltage rise time on operation frequency in magnetically insulated transmission line oscillator.

In the experiments of a magnetically insulated transmission line oscillator (MILO), the initiatory microwave frequency is slightly higher than the stable microwave frequency. This paper investigates the influence of the front edge of the voltage on microwave frequency. The theoretical analysis shows that the MILO operates at the 2π mode in the initiatory stage; therefore, the microwave frequency is determined by the single slow wave structure (SWS) cavity. During the stable operation, the MILO works at the π mode and the microwave frequency is decided by the structure parameters of the resonant cavity chain. It is a fact that the resonant frequency of a single SWS cavity is higher than that of the resonant cavity chain, so the reason why the initiatory microwave frequency is slightly higher than the stable microwave frequency can be explained. The simulation results agree with the theoretical results.

Performance improvement of a magnetically insulated transmission line oscillator with a carbon fiber array cathode.

A carbon fiber array cathode (CFAC) was presented and tested in a magnetically insulated transmission line oscillator (MILO) in earlier papers; however, the performance of this cathode was not as good as the traditional polymer velvet cathode, and the nonuniform fibers' distribution on the cathode surface was postulated as the culprit for this. In this paper, this CFAC has been carefully reprocessed, and the performance is improved (efficiency increases from ∼12% to ∼16%) and reaches a level that is as good as the velvet cathode, which means that this CFAC provides a promising substitute for the polymer velvet cathode in MILO.

A low-outgassing-rate carbon fiber array cathode**Project supported by the National Natural Science Foundation of China (Grant No. 61671457).

In this paper, a new carbon fiber based cathode — a low-outgassing-rate carbon fiber array cathode — is investigated experimentally, and the experimental results are compared with those of a polymer velvet cathode. The carbon fiber array cathode is constructed by inserting bunches of carbon fibers into the cylindrical surface of the cathode. In experiment, the diode base pressure is maintained at 1×10−2 Pa–2×10−2 Pa, and the diode is driven by a compact pulsed power system which can provide a diode voltage of about 100 kV and pulse duration of about 30 ns at a repetition rate of tens of Hz. Real-time pressure data are measured by a magnetron gauge. Under the similar conditions, the experimental results show that the outgassing rate of the carbon fiber array cathode is an order smaller than that of the velvet cathode and that this carbon fiber array cathode has better shot-to-shot stability than the velvet cathode. Hence, this carbon fiber array cathode is demonstrated to be a promising cathode for the radial diode, which can be used in magnetically insulated transmission line oscillator (MILO) and relativistic magnetron (RM).

High-power broadband plasma maser with magnetic self-insulation

Presented in this paper are the results of a particle-in-cell modelling of a novel high-power microwave (HPM) source which combines the properties of two devices. The first prototype is a magnetically insulated transmission line oscillator (MILO), an HPM self-oscillator which does not need an external magnetic field and irradiates a narrow spectrum depending on its iris-loaded slow-wave structure. The second prototype is a plasma maser, a Cherenkov HPM amplifier driven by a high-current relativistic electron beam propagating in a strong external magnetic field in plasma which acts as a slow-wave structure. The radiation frequency of plasma masers mainly depends on an easily variable plasma concentration; hence, their spectrum may overlap a few octaves. The plasma-based HPM device described in this paper operates without an external magnetic field: it looks like an MILO in which the iris-loaded slow-wave structure is substituted by a hollow plasma tube. The small pulse duration of ∼1.5 ns prevents a feedback rise in the 20-cm long generation section so that the device operates as a noise amplifier. Unlike conventional ultra wideband generators, the spectrum depends not only on the pulse duration but mainly on plasma, so the operation frequency of the device ranges within 12 GHz. For irradiated frequencies above 2 GHz, the total pulse energy efficiency of 7% is demonstrated at the HPM power level ∼1 GW.

Performance testing of a carbon fiber array cathode in a hard-tube MILO

In this paper, a carbon fiber array cathode was constructed and the application in a hard-tube magnetically insulated transmission line oscillator (MILO) was investigated. The performance was compared with a traditionally used polymer velvet cathode in the same conditions, where applied electric power ranged from 9 GW to 28 GW and microwave with power level of several GW was the output. The MILO worked on single shot mode or repetitive mode at 5 Hz. For both cathodes, the central frequencies of output microwaves are in the L-band, and the pulse durations (full width at half maximum) range from 25 to 50 ns. Experimental results show that the pulse duration of output microwave of the carbon fiber array cathode is shorter than the velvet cathode, and the power conversion efficiency is also somewhat lower than the velvet cathode. Although the performance testing results show that this carbon fiber array cathode is somewhat not as good as the traditional velvet cathode in the present state, the reasons for the disadvantages of the carbon fiber array cathode have been analyzed and the corresponding solutions are presented to improve the performance of this cathode. If expected solutions are put into effect, this carbon fiber array cathode may be able to replace the velvet cathode.

A High-Efficiency Magnetically Insulated Transmission Line Oscillator

A novel magnetically insulated transmission line oscillator (MILO) with high efficiency has been proposed, based on a kind of conventional MILO. The new MILO has two typical characteristics. First, the width of the anode vanes decreases gradually, and especially, the extraction vane decreases to be the minimal. Therefore, the proportion of the electrons, which have little or no contribution to the beam–wave interaction, is decreased. As a result, the efficiency is enhanced. In addition, there is an extra extraction cavity designed behind the extraction vane. So the beam–wave interaction process is lengthened, and the extraction efficiency is enhanced. The results of the particle-in-cell simulation show that the power efficiency of this MILO can be elevated to be 20.5%, while it is only 14.4% for the conventional MILO.