Knife-edge Cathode Research Articles

Design and investigation of a capacitance-coupling pre-ionized sharpening switch.

The sharpening switch is one of the most important elements in the trigger generator, which is used to sharpen the front of the trigger pulse. The breakdown performance of the sharpening switch has an important influence on the output and stability of the trigger generator. In this paper, a novel 200kV capacitance-coupling pre-ionized sharpening switch, which can realize pre-ionization by dividing voltage through its own structural capacitances, is proposed and investigated. In order to obtain the optimal parameters of the pre-ionized sharpening switch, the influences of the electrode structure, electrode material, main gap distance, and pre-ionized gap distance on the breakdown performance are studied experimentally. The experimental results show that the electrode structure with a circular knife-edged cathode and a plate-shaped anode has the smallest breakdown jitter, followed by the needle-plate structure and the tapered ball-head structure. The stainless steel electrode has the smallest jitter, followed by graphite, aluminum, and brass electrodes. When the gap distances of the main gap and the pre-ionized gap are 13 and 1.5mm, respectively, the breakdown stability of the pre-ionized sharpening switch is the best. Under an input voltage pulse with a rise time of about one microsecond, the jitter of the capacitance-coupling pre-ionized sharpening switch with the optimal parameters is 6.08ns, which is about 0.6% of the rise time of the input pulse. The jitter decreases by 44.5% compared to the switch without pre-ionization. The rise time of the output pulse is sharpened to 17ns, corresponding to a voltage rise rate of more than 11 kV/ns.

Comparison of round and knife-edge-like cathodes on gas-puff implosions.

Preionization is believed to play an important role on the implosion of gas-puff Z pinches. Some experiments have used an external preionization source, e.g., UV light or electron beam. In contrast, other experiments rely completely on over voltage breakdown by the own generator's voltage pulse. However, this approach lacks shot-to-shot reproducibility since self-breakdown is mainly a stochastic process. In this work, we performed a systematic study on self-breakdown using two different cathode geometries: (i) a smooth, round cathode to provide a homogeneous electric field, (ii) a sharp, knife-edge-like geometry to enhance the electric field locally and eventually electron emission. The experiments were carried out on the Llampudken current generator, which provides a current pulse of ∼400kA amplitude and 200ns rise time (10%-90%). We implemented gated XUV imaging, filtered diodes and time-integrated x-ray imaging to obtain information about the implosion as well as the stagnation phase for the two cathode geometries. We found that erosion of the knife-edge cathode might be a serious problem, and we had to replace it every 15 shots. On the other hand, the round cathode lasted for the whole series of experiments. We also measured a more reproducible and larger peak current for the knife cathode. From the photo-conductive detectors we observed that even if the round cathode might present shots with higher x-ray yield compared to the knife cathode, dispersion is almost twice as large. Moreover, after a statistic analysis, it is demonstrated that the dispersion in the yield is due solely to differences imposed by the cathodes and not to variations in the driver, as no correlation was found between them. We found that in order to fit the experimental data with the snowplow model, only ∼60% of the total mass is compressed in the knife cathode while ∼20% for the round one, highlighting the importance of the cathode and preionization. Therefore, we conclude that the use of the knife cathode increases the reproducibility of the experiment in comparison with the round cathode.

Brief Introduction and Recent Applications of a Large-Scale Parallel Three-Dimensional PIC Code Named NEPTUNE3D

This paper introduces a large-scale parallel 3-D fully electromagnetic particle-in-cell code named Nonlinear Evolution of Plasma-interaction Technology Under Nonneutral Environment-Three Dimensional (NEPTUNE3D) developed with the support of a parallel adaptive structured mesh application infrastructure JASMIN. NEPTUNE3D can be used to simulate beam-wave interactions in high-power microwave (HPM) generators. Some novel parallelization techniques are adopted in the NEPTUNE3D code for high parallel efficiency and good scalability. Some real-world HPM devices have been successfully simulated by NEPTUNE3D, such as the relativistic backward wave oscillator (RBWO), the magnetically insulated line oscillator (MILO), the virtual cathode oscillator, and the relativistic klystron amplifier. In this paper, two recent simulation examples are presented. The first is simulations of a knife-edge cathode RBWO, using 128 processors very efficiently. The second example is simulations of a MILO with a plate-inserted mode-transducing antenna, modeled using 27 million cells, 3 million particles, and 1024 processors with relatively high parallel efficiency.

Effect of sputtered lanthanum hexaboride film thickness on field emission from metallic knife edge cathodes

We report experiments and analysis of field emission from metallic knife-edge cathodes, which are sputter-coated with thin films of lanthanum hexaboride (LaB6), a low-work function material. The emission current is found to depend sensitively on the thickness of the LaB6 layer. We find that films thinner than 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode. This result is unexpected due to the higher work function of the bare metal cathode. We show, based on numerical calculation of the electrostatic potential throughout the structure, that the external (LaB6/vacuum) barrier is reduced with respect to uncoated samples for both thin and thick coatings. However, this behavior is not exhibited at the internal (metal/LaB6) barrier. In thinly coated samples, electrons tunnel efficiently through both the internal and external barrier, resulting in current enhancement with respect to the uncoated case. In contrast, the thick internal barrier in thickly coated samples suppresses current below the value for uncoated samples in spite of the lowered external barrier. We argue that this coating thickness variation stems from a relatively low (no higher than 1018 cm−3) free carrier density in the sputtered polycrystalline LaB6.

Analysis of electric field screening by the proximity of two knife-edge field emitters

The electric field of two semi-infinitely wide knife-edge cathodes with arbitrary separation is calculated by using a Schwarz–Christoffel transformation. This geometry could also represent a trench (or scratch) on a flat surface. It is found that the magnitude of the electric field on the knife-edge cathodes depends strongly on the ratio h/a, where h is the height of the knife-edge cathodes and 2a is the distance between the cathodes. When h/a increases, the magnitude of the electric field on the cathode’s surface decreases. This shows the screening of one cathode by another cathode; for example, keeping the height fixed and decreasing the distance between the cathodes, the field enhancement on the corner decreases. Analytic approximations for the divergent electric field in the immediate vicinity of the sharp edge are derived for the cases where h/a>>1, and h/a≪1. These results lead to insight on the relationship of the density of field emitter in field emitting arrays and field emission from rough surfaces.

Observation of Plasma at the Quartz Rod Inside Annular Electron Beam Produced From a Knife-Edge Cathode in a Magnetic Field

Time-resolved light emission imaging was used to observe the plasma formation at the surface of a dielectric rod serving as a slow-wave supporting structure in an antenna-amplifier Cherenkov maser configuration. Experiments were performed using a quartz rod inserted into the hollow knife-edge cathode of a magnetically insulated foilless diode generating an annular electron beam of 0.9-1.7-kA current. The accelerating voltage pulse was delivered to the diode by a linear induction accelerator; the voltage amplitude ranged from 260 to 380 kV at the full pulse duration of ~ 200 ns. It was shown that the plasma at the rod surface is produced in the vicinity of the cathode edge plane corresponding to the location of the strong tangential electric field component. This plasma appears later than the explosive emission plasma at the cathode edge, and the intensity of light from this plasma increases, while the voltage rises. At voltages below 300 kV, the measured side-view light intensity drastically decreases, which indicates a significant decrease in the plasma density. It has been found that the surface plasma does not propagate along the rod; yet, the presence of plasma at a distance of 5-10 cm from the cathode was registered sporadically at high voltages. The electron emission from the surface plasma was observed as well.

Analytical expression for sheath edge around corner cathodes

A simple analytical expression for the position of the sheath edge around a two-dimensional corner cathode with included angle θc has been discovered. This expression is valid for weakly collisional sheaths in the Child–Langmuir regime ϕc ≫ kTe/e, where −ϕc < 0 is the cathode bias and Te is the electron temperature. In polar coordinates (r, θ), the sheath edge is given by (r/s0)sin[πθ/(2π − θc)] = [π/(2π − θc)] where s0 is the planar sheath width far from the vertex of the corner. This result is verified by comparison with previous numerical solutions (Watterson P A 1989 J. Phys. D: Appl. Phys. 22 1300) for the knife edge (θc = 0) and convex square corner (θc = π/2). The observed agreement suggests that this expression gives the sheath edge for all corner angles, both concave and convex. The utility of this result is demonstrated by computing the full sheath solution for a knife-edge cathode with ϕc = 100kTe/e.

One-dimensional combined field and thermionic emission model and comparison with experimental results

An integrated theoretical model has been developed to predict the entire range of emission from thermionic to field emission, including the mixed emission regime. The model assumes a Sommerfeld free electron model supply function, for which the Fermi-Dirac distribution applies with a nonzero temperature. The electron transmission coefficient is calculated in one dimension using a transfer matrix method (TMM) to solve the steady-state Schrödinger equation. Emission current densities have been measured for a periodic copper knife-edge cathode to compare with the TMM model result. It is shown that the computational result utilizing this model provides good agreement with the experimental data. Unambiguous and reliable estimates of the effective field enhancement factor βeff (βeff=Es∕Eg, where Es is the cathode surface electric field and Eg is the gap electric field between the cathode and anode) and the effective work function ϕeff are obtained from experimental measurements using this model by simultaneously fitting thermionic and field emission data for the cathode. Comparing the experimental and theoretical results reveals that finite temperature thermal contributions to the current emission can be significant in the operation of many field emission cathodes.

Modeling of cold emission cathode by inclusion of combined field and thermionic emission processes

Emission currents have been measured at elevated temperatures for a periodic copper knife-edge cathode. To model the emission process of the cathode, we have combined thermionic and field emission processes. Electron tunneling is calculated using a transfer matrix method. Using this model, we accurately reproduced the experimental deviation and minimum in the ln(J∕Eg2) vs 1∕Eg plot, where Eg is the gap electric field and J is the emitted current density. This phenomenon has been widely observed but no comprehensive explanation has been put forth. Unambiguous and realistic estimates of the cathode effective field enhancement factor beta averaged (βeff=Esurface∕Egap) and effective work function Φ are obtained from experimental measurements using this model.

Electric field distribution on knife-edge field emitters

Conformal mapping is used to calculate the electric field on a knife-edge cathode, modeled as a rectangular ridge on a flat surface. It is found that the field enhancement factor scales approximately as the square root of the height-to-width ratio of the knife edge. A simple analytic approximation for the divergent electric field in the immediate vicinity of the sharp edge is derived. When a smaller knife edge is placed on top of a larger one, both assumed to have large height-to-width ratios, the composite field enhancement factor is shown to be approximately equal to the product of the field enhancement factor of the individual knife edges, thereby proving the conjecture on multiplication of field enhancement factors for one special case.

Measurements of pulse modulation in an ECM

We report on experiments which have recently been conducted at the University of Strathclyde investigating rapid amplitude modulations occurring in the microwave output radiation of an electron cyclotron maser (ECM). The experiment used an electron beam injected from a co-axial diode with knife-edged graphite cathode in the fringing field of an adjustable magnet system producing a beam of up to 175 kV and 140 A. The time evolution of the electron beam was measured as the cathode plasma expanded using a Faraday cup in conjunction with upstream beam interceptors as a function of the magnetic compression ratio. The ECM cavity was configured so that its length and the length of the interaction space could be readily adjusted. The microwave output signal was studied using special fast rectifying diode detectors, a high performance deep memory oscilloscope and cut-off filters. Steady-state output was observed at high magnetic compression ratios (16:1) at a frequency of 16 GHz corresponding to cyclotron resonant maser (CRM) coupling between the beam and the radiation in the expected TE 1,2 mode. At lower compression ratios modulation was observed after an initial steady-state period and shown by antenna pattern measurements to be associated with transverse mode competition in the microwave cavity.

Measurements of pulse modulation in an ECM

We report on experiments which have recently been conducted at the University of Strathclyde investigating rapid amplitude modulations occurring in the microwave output radiation of an electron cyclotron maser (ECM). The experiment used an electron beam injected from a co-axial diode with knife-edged graphite cathode in the fringing field of an adjustable magnet system producing a beam of up to 175 kV and 140 A. The time evolution of the electron beam was measured as the cathode plasma expanded using a Faraday cup in conjunction with upstream beam interceptors as a function of the magnetic compression ratio. The ECM cavity was configured so that its length and the length of the interaction space could be readily adjusted. The microwave output signal was studied using special fast rectifying diode detectors, a high performance deep memory oscilloscope and cut-off filters. Steady-state output was observed at high magnetic compression ratios (16:1) at a frequency of 16 GHz corresponding to cyclotron resonant maser (CRM) coupling between the beam and the radiation in the expected TE 1,2 mode. At lower compression ratios modulation was observed after an initial steady-state period and shown by antenna pattern measurements to be associated with transverse mode competition in the microwave cavity.

Development of Repetitive Virtual Cathode Oscillator.

A new method of repetitive operation of a virtual cathode oscillator was performed using a carbon plate anode with a hole in the center, a knife-edged annular cathode and an external axial magnetic field. Repetitive microwave radiations more than 450 were observed. Almost all microwave frequency of each operation was contained in 11.8 to 12.8 GHz, while the microwave emission was varied widely. The maximum microwave peak power and pulse duration were 14 MW and 16 ns, respectively, for the diode voltage of 460 kV, the electron beam current of 12.8 kA at the upstream. The dominant radiation pattern of microwave was found to be TE01 mode.

X-ray emission from 30 J Blumlein operated compact diode

An x-ray emitting diode of impedance 2.3 Ω with a knife-edge cathode energized by a 90 kV, 30 J, solid dielectric Blumlein driver of pulse length 10 ns is studied. X-ray emission from titanium, copper, molybdenum, tin, tantalum, and lead anodes was investigated. The radiation yield from titanium and copper is low. Molybdenum and tin emit a significant part as Kα line radiation, whereas the emission with tantalum and lead anode is essentially continuum radiation. The ratio of line to continuum with molybdenum target is estimated 17%±10%.

Explosive cathode gyrotron experiments

Experiments have investigated the behavior of explosive emission cathode gyrotrons using cylindrically symmetric coaxial diodes with centrally located knife-edged cathode and conical anode. Voltage probes and current shunts were exploited to monitor the diode electrical behavior when driven by a 3 /spl mu/s 1/e time Marx power supply (up to 150 kV). Temporal reference signals permitted comparison of the diode's electrical evolution with the simultaneously measured microwave output signal from the gyrotron. The microwave output pulse duration was found to depend on the diode gap spacing (in the range 16-27 mm), the cavity magnetic field (range 1.3-3.3 T) and the cathode material, the termination being caused by the decay of the accelerating potential across the diode and the disruption of the diode geometry. The ratios of the cathode flare expansion velocities for graphite, copper and stainless steel cathodes were estimated as (1:(1.3/spl plusmn/0.2):(2.2/spl plusmn/0.5)) with a corresponding change in the duration of the microwave output signal [((590/spl plusmn/17):(414/spl plusmn/25):(325/spl plusmn/10)) ns at 2.6 T]. Preliminary results are reported of recent experiments which have compared the optical emissions from the diode region during pulsed gyrotron operation with the simultaneously recorded diode electrical behavior and the gyrotron microwave output.