Annular Cathode Research Articles

Preliminary study of dual annular multiple-beam cathode for V-band coaxial transit-time oscillator

Since the research toward high-power millimeter-wave generator becomes a tendency in high-power microwave, overmoded structure with the high-order mode has been a considerable interest because of its potential to increase power handling capacity (PHC). To expand the PHC of V-band transit-time oscillator and excite higher mode TM03, a dual annular multiple-beam cathode has been proposed. In the geometric structure of the dual annular multiple-beam diode, cathode rods around two concentric circles are uniformly placed on cathode base, and each circle has several graphite rods. Because of space charge shielding effect and fluctuation of electron beam, explosive emission current of inner and outer cathode circles is difficult to be balanced, and the electron beam transmission rate is not very high. To solve those two problems, the relative length (ΔL) and the relative radius (Δr) between the inner and outer cathode circles are optimized to obtain balanced currents of inner and outer beams and a good transmission rate. In this paper, the preliminary study of a dual annular multiple-beam cathode is carried out by optimizing the cathode structure. When ΔL and Δr are equal to 2.5 and 4 mm, respectively, the dual annular multiple-beams cathode can provide 2.71 kA uniform intense relativistic electron beams under 421 kV, and the magnetic field is 1.2T. As a simulation result, explosively emitted and the 95.2% of total beam current transmission rate can be reached.

Reltron Oscillator for Long Pulse and High-Power RF Generation

In this article, a new configuration of the reltron oscillator (i.e., gridless reltron configuration with annular explosive emissive cathode) was proposed for long pulse and high-power RF generation. To investigate the electromagnetic (EM) and RF behavior, the structure was modeled in the CST Studio Suite and then simulated with the help of different solver modules of the CST simulation tool. The effect of various electrical parameters of the proposed configuration on the generated pulsewidth and RF power was also studied. For the optimized structure, the proposed configuration produced an RF signal of peak microwave power of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt;$</tex-math> </inline-formula> 190 MW with a pulsewidth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> s. These simulation results were compared with the experimentally reported gridded and gridless reltron, and it was noted that: 1) the proposed configuration produced a peak RF power similar to a gridded reltron with a lower operating voltage; 2) the pulsewidth of the generated RF signal was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 10 times higher than the gridded reltron; 3) obtained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 10 times higher peak RF power than the gridless reltron; and 4) pulsewidth was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 0.2 times lesser than the gridless reltron. Eventually, the structure was benchmarked with the experimentally reported gridded and gridless reltron for its validation.

Characterizing the HERMES-III Extended Magnetically Insulated Transmission Line

Modeling and simulation of the HERMES-III extended magnetically insulated transmission line (MITL) terminated by a diode with an annular cathode and a planar anode was performed with an electromagnetic particle-in-cell code, EMPIRE-PIC. Self-limited MITL theory was developed and implemented in order to model select shots in EMPIRE by allowing one to transform an experimental current trace into a voltage waveform that is input into EMPIRE. Simulating shots 11 132, 11 133, 11 134, 11 135, 11 136, and 11 146 in EMPIRE shows good agreement with experimental current traces at several locations along the MITL. New calibrations for the anode current shunts are proposed based on low-power shots fired into a short-circuited diode configuration (short-shots). The new calibrations show improved agreement with simulation results.

Sub-Millimeter Hard X-Ray Source Based on Wire-Shorted Low-Impedance Rod Pinch Diode

Rod pinch diode (RPD) is an intense X-ray point source with a millimeter spot diameter, tens of nanosecond pulsewidth, and tens of keV to MeV photon energy, resulting in extraordinary flash radiography performance. Classical RPD is featured with a high-impedance vacuum gap between the annular cathode and the central anode rod, which limits the electron current and electrical power density, therefore is not conducive to the improvement of brightness. Plasma prefilling can effectively reduce the RPD impedance, thereby increasing the electron current and realizing an impedance match of RPD with low-impedance, high-power-density drivers. In this article, an experimental study on plasma-prefilled RPD is carried out based on a fast linear transformer driver (FLTD) with relatively low charging voltage; the RPD anode and cathode are initially connected by metal wires, which are transformed into plasma shortly after the onset of discharge current. A quasi-spherical X-ray spot is obtained with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 0.45 mm diameter, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 30 ns pulsewidth, 0.6 rad@1 m (LiF) dose, and 55–60 keV equivalent mono-energy photon energy. The radiography performance is further characterized by a tungsten rolled-edge and slit plate. The preliminary results proved the feasibility of low-impedance RPD as a flash radiography source within the relatively low photon energy range.

Investigating emission characteristics and plasma properties in a long-pulse magnetically insulated coaxial diode with different cathode materials

Magnetically insulated coaxial diodes are widely used in O-type high power microwave device for intensive annular electron beam generation. This study reports on a series of electrical and optical measurements on a long-pulse magnetically insulated coaxial diodes in which the emission characteristics and the plasma properties were observed. The impact of cathode material, blade angle, diode voltage and guiding magnetic field on the emission characteristics and the plasma properties of six types of annular cathodes were studied. Two configurations of these cathodes have been used, one with a sharp edge for aluminum, bronze, graphite, stainless-steel, silicon-aluminum alloy and another metal–dielectric. The impressing diode performance was achieved with the metal-dielectric cathode characterized on the excellent emission characteristics and plasma properties. Moreover, high voltage and small blade angle are beneficial to the emission capability and emission uniformity. For the cathode with smaller blade angle, its axial plasma expansion velocity is smaller during the pulse plateau. The strong magnetic field or the large diode voltage is confirmed to reduce the axial and radial expansion velocities.

The impact of accelerating diode resonances on L-band high-power long-pulse relativistic backward wave oscillator operation

An L-band high-power relativistic backward wave oscillator is designed. In the simulation, microwaves centered at 1.6 GHz are generated, with the power of 3.6 GW and the efficiency of 40%. In the preliminary experiment, the pulse duration of the device was only 45 ns, presenting a pulse-shortening phenomenon. Through the 3D particle-in-cell simulation analysis, it was found that the accelerating diode resonances significantly impact the operation of the L-band high power relativistic backward wave oscillator, and the resonance of a TE11 mode in the accelerating diode played the primary role in the pulse shortening. Moreover, we found that choosing the appropriate distance between the cathode baffle and the end of the annular cathode is beneficial to effectively suppress the starting oscillation of the parasitic TE11 mode. In the improved experiment, we changed the distance between the cathode baffle and the end of the annular cathode from previous 5.4 to 4.6 cm. Eventually, when the diode voltage is 650 kV and the diode current is 14 kA, microwaves centered at 1.58 GHz are generated with the power of 3.3 GW, the efficiency of 36%, and the pulse duration above 104 ns.

Quantitative evaluation of emission uniformity of the annular explosive emission cathode

Explosive emission cathodes (EECs) are widely used in the field requiring intense electron beams. The uniform emission of EECs is vital for applications. To effectively evaluate the emission uniformity of annular EECs, this paper proposes a quantitative evaluation model based on the analysis of cathode plasma images and constructs a parameter which is named as the non-uniformity value to describe the extent of emission uniformity. The experiment designed to investigate the cathode emission uniformity under different circumstances demonstrates that the model is valid. The experimental results give quantitative information of uniformity change in explosive emission and show that the non-uniformity values of graphite and aluminum cathodes can be significantly decreased by increasing the guiding magnetic field and the diode voltage.

Investigation of an Axial Virtual Cathode Oscillator with an Open-Ended Coaxial Cathode

A cathode with an open-ended coaxial structure is experimentally investigated using an axial virtual cathode oscillator (vircator). To enhance the microwave power output, an open-ended coaxial cathode is installed in the axial vircator. The proposed cathode is designed based on the reciprocating frequency of the vircator. The operation features of an axial vircator with a solid cathode, an annular cathode, and an open-ended coaxial cathode are comparatively analyzed through simulations and experiments. Three cathodes are machined using graphite. A stainless steel mesh with a transparency of 70% is used as an anode. The anode-to-cathode gap is fixed to 6 mm. The vircator is driven using a 10-stage PFN-Marx generator with a characteristic impedance of 31 Ω. The PFN-Marx generator applies -150 kV voltage pulses with a 170–200 ns pulse width into the vircator. The microwave power from the solid and annular cathodes is 11.22 MW and 11.27 MW, respectively. The proposed cathode generates a microwave with a power of 12.65 MW while enhancing the microwave power by 13% compared with the solid and annular cathodes. The proposed cathode shows a frequency shift to 3.4 GHz, which is a much lower frequency than that of the solid cathode at 6.34 GHz.

Experimental and numerical study of a split cathode fed relativistic magnetron

The relativistic magnetron is one of the most efficient high power microwave (HPM) sources but pulse shortening, the result of explosive cathode plasma's radial expansion toward the anode, makes it impractical because the HPM pulse terminates much earlier than the applied voltage. We present experimental results of the operation of a relativistic magnetron fed by a split cathode. A split cathode [Leopold et al., Phys. Plasmas 27, 103102 (2020)] consists of a cathode placed upstream and outside the anode, connected by an axial rod to a reflector (a transverse conducting circular plate) placed downstream from the anode. The electron charge, emitted by an annular explosive cathode emitter, accumulates in the space between the cathode and the reflector and at the same time, screens the rod from explosive plasma formation. This accumulated space charge serves as the electron source for the magnetron. The explosive plasma developing on the emitter remains outside the magnetron and does not propagate into the anode while it operates. We compare the performance of the magnetron operating with a standard explosive emitting solid carbon cathode to that with a split cathode. The experiments demonstrate that whereas for the solid cathode, the microwave pulse developing in the magnetron suffers from pulse shortening, with a split cathode, the pulse survives as long as the amplitude of the applied voltage is sufficient for the magnetron's operation. We support the experiment by particle-in-cell simulations.

Investigation of Annular Explosive-Emission Cathodes of the Conductor–Insulator Structure

Research is devoted to investigation of properties of pulsed explosive emission cathodes for X-ray tubes. Three types of annular explosive emission cathodes were studied: metal ceramic with slits (of a comb-shaped kind), ring metal ceramic, and carbon ceramic. A pulsed high-voltage generator used in experiments is characterized with the following parameters: voltage pulse full-width at half-maximum is about 20 ns, pulse voltage amplitude is up to 150 kV, and pulse repetition rate is up to 200 Hz. The installation makes it possible to study demountable models of pulsed X-ray tubes of different designs. Current and voltage waveforms corresponding to different operation modes and rates of rise of X-ray tube anode voltage were analyzed. The obtained characteristics of explosive cathodes depend on the cathode’s material, the shape, and the supply voltage of the primary energy storage. The current pulse duration for the ring carbon-ceramic cathode is considerably lower than that for the other cathodes, although it generates a higher radiation dose rate.

Suppression of backward current in a low-magnetic-field foilless diode

An annular cathode is generally fixed on a cylindrical cathode holder in a magnetically insulated coaxial foilless diode. Electrons emitted from cathode plasmas or the cathode holder may easily move backward under the action of the quasi-static electric field and axial magnetic field, leading to backward current loss, which will seriously reduce the efficiency of high power microwave sources, especially under the condition of a low guiding magnetic field. A novel storage electrode immersed in the uniform magnetic field is proposed to suppress the backward current. Reverse electrons will be decelerated and blocked by the storage electrode. Under the guiding magnetic field of 0.66 T, over 66% of the backward current is suppressed (from 3.6 kA to 1.2 kA) when the storage electrode is used, and furthermore, the waveforms of the diode voltage and current are improved significantly.

Investigation on explosive emission and attenuation processes of an intense current electron beam diode with annular graphite cathode via 24 framing camera

An experimental study of an intense current electron beam diode with a foil-free annular graphite cathode is carried out. The explosive emission images of a graphite cathode at discharging voltages of 240 kV and 780 kV and the attenuation images of cathode carbon ions at different magnetic fields are recorded using a 24-framing camera for the first time. The experimental results show that when the discharging voltage is 240 kV, the light spots of the plasma generated by the explosive emission process are randomly distributed over the annular cathode and the size and brightness of the light spots of the plasma are also randomly distributed; when the discharging voltage is 780 kV, if the background light generated by Čerenkov radiation is not taken into account, the light spots of the plasma generated by the explosion emission is more uniform than that when the discharging voltage is 280 kV and the brightness of the light spots of the plasma is higher, too. Meanwhile, when the axial magnetic field strength is less than 0.77 T, the plasma spots appear in the gap between the cathode and the anode. The smaller the axial magnetic field strength is, the higher the brightness of the plasma spots will be. In addition, when the discharging voltage is 780 kV/200 ns and the axial magnetic field strength is 0.77 T, a rough calculation indicates that the axial diffusion velocity of the cathode carbon ions ranges from 1.77 cm/μs to 2.1 cm/μs.

Influence of annular electron beam uniformities on the microwave characteristics in an S-band relativistic klystron amplifier

The non-uniform annular electron beam models are established based on the annular cathode explosive emission luminescence images. The influence of electron beam uniformities on the microwave characteristics of an annular structure S-band relativistic klystron amplifier output cavity is investigated with particle-in-cell simulations. The electron beam non-uniformities are simulated using four different electron emission models: (1) continuous area without emission, (2) spaced emission, (3) enhanced emission, and (4) current density variation emission. The simulation results with the first emission model show that the output power decreases as the continuous area without emission increases, while the continuous area without emission has little effect on the frequency and pulse width. The simulation results with the second emission model show that the output power of the spaced emission is related to the distribution of non-emission areas. The more evenly the areas without emission are distributed on the annular cathode, the greater the output power is. The simulation results with the third emission model show that the higher the current density in the local area is, the smaller the output power is. The simulation results with the fourth emission model show that the fluctuation of current density causes the fluctuation of frequency and output power. The larger the change area of the current density is, the greater the fluctuation amplitudes of the output power and frequency are. The larger the fluctuation amplitude of current density is, the greater the fluctuation amplitudes of the output power and frequency are.

Efficient generation of multi-gigawatt power by an X-band dual-mode relativistic backward wave oscillator operating at low magnetic field

An X-band dual-mode relativistic backward wave oscillator (RBWO) operating at low magnetic field is presented in this paper. Three new design principles are introduced in the device. First, the electron beam interacts with TM01 mode and TM02 mode simultaneously, rather than with a fixed single mode. Second, the device outputs with mixed modes, rather than with a pure mode. Third, an internal reflector inserted into the annular cathode, rather than a long resonant reflector before the slow-wave structure, is adopted to reflect part of the backward wave. Accordingly, the beam–wave interaction efficiency is increased significantly and the whole device is very compact. The particle in cell simulation results reveal that at a magnetic field of 0.64 T, the output microwave power is 4.8 GW and the conversion efficiency is up to 44%. In the experiment, at a guiding magnetic field of 0.66 T, a microwave pulse with power of 4.6 GW, frequency of 9.96 GHz, pulse duration of 42 ns, and efficiency of 42% was generated when the diode voltage was 880 kV and beam current was 12.5 kA, which agree well with the simulation results. Furthermore, as the diode voltage was 1.17 MV, a highest microwave power of 7.6 GW was achieved. This is a record of high efficiency and high power of microwave generation in an X-band RBWO operating at low magnetic field.

Emission Uniformity of an Annular Graphite Cathode With a Focusing Electrode in a High Power Vacuum Diode

Emission Uniformity of an Annular Graphite Cathode With a Focusing Electrode in a High Power Vacuum Diode

A novel high power vacuum diode with a focusing electrode for effective operation in a low guiding magnetic field.

A novel diode featuring an annular cathode and a ring-shaped focusing electrode and operating in a low guiding magnetic field (GMF) has been developed. It was found that the breakdown threshold for a focusing electrode made of titanium was 140 kV/cm higher than a stainless steel electrode for the same operating conditions. Comparison of the diode current measured by a Rogowski coil and the beam current measured by a Faraday cup at the collector indicated that an intense relativistic electron beam was transmitted effectively even in a GMF of as low as 0.21 T. In addition, a photo of the cathode plasma and the targeting results indicated that the emission uniformity of the graphite cathode was satisfactory.

Investigation of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment

The mechanism and realization of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment are presented. The device adopts coaxial dual-annular cathode to emit the inner and the outer electron beams, independently. When the inner annular cathode is extended and the outer annular cathode is retracted, the electron beams are emitted from the inner cathode and interact with the internal electromagnetic structure to generate Ka-band microwaves. On the contrary, the outer electron beams interact with the external electromagnetic structure to generate X-band microwaves. Thus, the cross-band microwave generation can be achieved only by adjusting the protrusion and retraction of dual-annular cathode. By the particle-in-cell (PIC) method, the physical process of beam-wave interaction in the device is studied, and the high efficiency internal-external double electromagnetic structures are designed. Simulation results show that the X-band microwave with a power of 1.50 GW and a frequency of 8.55 GHz is generated, corresponding to an efficiency of 32%. Meanwhile, the Ka-band microwave with power of 650 MW and frequency of 31.80 GHz is generated, corresponding to efficiency of 28%.

Anode ablation and performance improvement of microcathode arc thruster

This article presents the results of a study into the new concept of the microcathode arc thruster; a thruster with an ablative anode (AA-μCAT). Such a thruster uses a triggerless discharge initiation, and has a coaxial electrode configuration where the central miniature anode is made of an ablative material (brass, solder, copper, etc), and an outer annular cathode, with optional magnetic coil. We show that the use of an ablative material (brass) increases the ion-to-arc current ratio from ∼2 to ∼4% without any focusing magnetic field, and up to 7% with a magnetic field of 0.33 T, compared with the case where the central anode is made of nonablative stainless steel, for the same pulsing power circuitry and parameters. For the first time, we demonstrate that even without a magnetic field, the AA-μCAT has a higher thrust (7 μN) and a higher thrust-to-power ratio (5.5 μN W−1) with respect to the thruster with a nonablative refractory stainless steel anode (5.5 μN and 4.8 μN W−1, respectively). This fact is direct evidence of possible improvements to the microcathode arc thruster thrust-to-power ratio due to anode ablation.

Plasma flows generated by an annular thermionic cathode in a large magnetized plasma

A LaB6 thermionic emitter of annular shape is used in the Large Plasma Device at the University of California, Los Angeles to create off-axis heating conditions for various transport studies. Since the emitter is biased relative to a distant anode, which is many collision lengths away, the entire magnetized plasma develops a self-consistent, potential structure that simultaneously generates transverse and axial flows with shear. This study uses swept Langmuir probe techniques and Mach probes to map the flow patterns and their dependence on bias and plasma parameters. By implementing additional biasing configurations, it is possible to control the magnitude of the flows and their shear strength. The experimental measurements, including the self-consistent currents, are compared to predictions of a model that incorporates the boundary conditions associated with thermionic injection, combined with a Braginskii transport code for the electron temperature.

Influence of pressure on the morphology and structure of surfaces sintered in pulsed DC annular hollow cathode discharge

Modification of the morphology and structure of surfaces sintered in pulsed DC annular hollow cathode discharge (AHCD) at high temperature, subjected to continuous sputtering, was investigated for different gas pressures. The studied AHCD cylindrical surfaces concentrically placed each other comprised the surface of a pressed (green) iron powder sample (bcc, α-Fe structure), at the central cathode, and the internal surface of an austenitic stainless steel tubular part (fcc, γ-Fe structure), at the external cathode. In this case, metal atoms sputtered from the central cathode are condensed, clustered and sintered (in solid state) at the external cathode internal surface, and atoms sputtered from the external cathode tend to undergo the same events in the opposite direction. Sintering was performed at 1150 °C, for 60 min, at 1, 3, 6, and 9 Torr (133, 400, 800 and 1200 Pa, respectively). Results show that the pressure is the key parameter to control the modification of the AHCD processed surface characteristics subjected to the sintering treatment. Sputtering is strongly decreased at high pressures (6–9 Torr) keeping the original-surface structural and morphological characteristics of both the cathodes practically unaltered. On the other hand, significant changes on the surface characteristics of both the cathodes were observed for smaller pressures (1–3 Torr). The most important is that the austenite (γ-Fe) phase initially present in the AISI 310 austenitic stainless steel gives place to a deposited growing layer, mainly constituted by Fe atoms, and thus by the ferrite (α-Fe) phase, in the steel cathode internal surface, associated with a morphological modification from an apparently plasma etched (austenitic) to a rough (ferritic) surface, as the pressure is changed from 9 to 1 Torr.