This paper presents the design, prototype engineering, and initial testing of a relatively low-voltage, high aspect ratio electron gun for versatile pulsed power applications. The sheet beam, diode electron gun is designed for (23–25) kV voltage and 6 µA/V1.5, as is required for x-band klystron, as a part of a compact Klylac delivering electron beam energy in the MeV range. The gun prototype was engineered, built, and tested using a Scandinova M1-0.5 modulator. Beam loss on the anode aperture is evaluated from transient waveforms and equivalent circuit modeling.

Sheet or ribbon electron beams are of interest for numerous applications, ranging from high-power microwave tubes, emerging THz and sub-THz sources1 to gas laser excitation, and plasma chemistry reactors.2 

Sheet beam, centimeter- to millimeter-wave klystron amplifiers and slow-wave devices offered the prospect of considerably higher output power specific power than can be achieved with pencil beam devices operating at the same voltage. Output power per unit weight or volume is an important parameter for traveling wave tubes (TWT), twystrons, or backward wave oscillators (BWO) also. This is because a significantly higher current can be transported through the sheet beam device circuit at a given voltage, avoiding many engineering and design problems imposed by higher voltages at pulsed power generation, transportation, and feeding into the device. Sheet beam, high aspect ratio topologies are also proved to be effective at millimeter to sub-millimeter wavelengths1,3,25 because the very small planar features are amenable to microfabrication techniques and advanced bandgap structures. Operation of such devices at voltages below 25 kV offers many practical and operational benefits such as compactness, reliability, and low cost. The key challenges in implementing such devices are related to beam forming, injection, confinement, and transport. Sheet beam devices can be employed at substantially higher current and peak power compared to pencil beams4 at similar or lower current densities in the beam–wave interaction circuit. This is achievable by combining a high total perveance of the electron source and significant aspect ratio of the beam, defined as the ratio of beam width to beam height at the beam waist or in the beam channel. The equivalent pencil beam perveance, or “square” partial perveance, can be estimated as the total perveance divided by the aspect ratio.5 The “square” perveance is analogous to the “beamlet” perveance of a multi-beam klystron (MBK) and drives the electronic efficiency of the sheet beam klystron (SBK). The specific power of an SBK system may exceed that of an MBK system due to denser packaging and more effective usage of permanent magnets for alternating periodic or wiggler type6 focusing.

Among the novel applications of sheet beam electron guns are Klylac7 and Klynac.6,8 These terms are reserved for integrated devices combining a klystron and a linear accelerator (linac) into a single electronic device.9,10 In these devices, the klystron and linac share the same beam and vacuum enclosure. Elimination of RF windows, circulators, and electron injectors for the linac and SF6-filled waveguide system could make either the Klynac or Klylac6,11 a much more compact and less expensive alternative compared to a conventional resonant linac fed by a magnetron through a circulator.

The Klylac, supplied, e.g., by a tungsten target, can become a crucial element in the replacing of radioisotopes employed in well logging tools for densitometry in oil field exploration. These circulator-free devices could have an application also in medicine, for medical radiotherapy and imaging. Many of these systems, such as the MRIdian™,12 use rotating gantry, in which a low form factor is critical. Such compact, low energy (<8 MeV) applications are usually extremely limited in maximum high voltage and, hence, the e-beam energy by the available space. A high perveance driver would make these devices competitive with magnetron-based commercial systems in terms of efficiency and compactness. The development of an inexpensive, portable, easy-to manufacture linac technique such as the Klylac, capable of replacement of a radioisotope source, is especially important to improve public security and prevent the diversion of radioactive material for Radiation Dispersion Devices.13 

An X-band SBK can be designed11 to approach the efficiency of a magnetron at comparable power and voltage with the benefit of bandwidth, frequency, and phase control. These controls are required by a Klylac feedback system operating in self-oscillating mode. Note that the bi-resonant Klynac or Klylac system is planned to operate in harsh environments with extreme temperatures and vibrations related to well logging. Therefore, for such applications, this feedback presents game changing technology, enabling automatic self-regulation14 of frequency and amplitude. The Klylac prototype design for borehole lodging11 includes a sub-MW power X-band SBK operating at ∼24 kV and ∼23 A beam current. This requires a corresponding electron gun with perveance as high as 6 µA/V3/2 (μPerv). The expected efficiency of the SBK11 is 34% vs the 31%–35% efficiency of PM-220 or PM-450X magnetrons from L3 Communications. Such emerging applications require the development of compact and robust, MW and sub-MW power electron guns with total perveances similar to magnetrons. The following magnetrons also have comparable microwave power, wavelength, and voltage: the L0327X11 L0155X,11 and L0325X.15 This requirement presents a significant challenge: To our knowledge, there are no klystrons designed to compete with these magnetrons in terms of voltage and perveance (of the order >3 μPerv).

The main approach used to design most sheet beam electron guns is based on Pierce theory applied to correspondingly modified cylindrical surfaces of the cathode and the high-aspect ratio anode aperture. Similar to a classic electron gun with a circular beam, it may contain a grid and focusing electrode(s).

To our knowledge, there are only a few X-band SBKs that have been built. One of them is the VKX-8293 A from Communications & Power Industries (76 kV voltage and 144 A beam current16,17) and the SBK from the Key Laboratory of High-Power Microwave Sources and Technologies (150 kV voltage and 100 A beam current18). However, the perveance of these guns does not exceed 2 μPerv, and the voltages are too high for applications where dimensions and cost are critical. Voltage reduction below (25–30) kV would open a new range of applications, where the reliability and simplicity of the arcing-free HV pulse forming and delivery network are critical. For instance, for borehole logging tools, voltages exceeding 25 kV are prohibitively high for two reasons: the limit imposed by HV cables to deliver pulsed power with a sufficient safety factor (at least 2) and too large transverse dimensions imposed by HV insulation. Designing a high perveance gun for SBK is challenging, as the numerical means of design do not guarantee adequate performance. For example, one of the systems built revealed an as high as 37% beam loss16 that required extensive tweaking of the already fabricated gun by adjustment of the cathode position.

There are two alternative methods to form a high aspect ratio elliptical or flat sheet beam for O-type devices. In O-type devices an electron beam drifts in the same direction as the applied magnetic field. Among them, the O-type slow-wave devices use the axial slow-wave structure to achieve electron beam clustering and beam interaction. Both beam-forming methods are based on the conversion of a circular electron beam from a conventional type of electron gun. One method is applied to a W-band TWT at Los Alamos National Laboratory. It uses a magnetized beam, a scraper, solenoids, and a quadrupole doublet8 to implement that conversion. This method, adapted from charged particle accelerators,19 exploits the exchange between the transverse phase spaces of a relatively high voltage (∼120 kV) beam. Another method applied more recently to millimeter and sub-millimeter wave generation uses an alpha-magnet placed between quadrupole doublets.3 Both methods are limited by the relatively low space charge effect (beam perveance <0.5 μPerv).

In this paper, we present a prototype of a sheet beam gun for an X-band SBK, in which the beam dynamics is strongly dominated by the space charge effect. The design enables the operation at moderate voltage and high perveance, comparable to that for an X-band magnetron. However, it is to be noted that 2D theory does not give the solution for how to design and engineer a robust, high perveance sheet beam gun with practical tolerances. The feasibility of such a design is demonstrated in this paper with 3D simulations and tests. Added by the SBK simulations,11 these efforts may pave the way for a new generation of SBKs, having an efficiency comparable to or exceeding that for X-band magnetrons, as discussed above.

In this section, we present an SBK gun design, approaching the specification for the high-aspect ratio SBK driving the Klylac,11 which was the end goal of this work. To simplify operation and fabrication, a diode-type electron gun has been chosen. The maximum operating voltage of the gun design was limited to that in available modulators driving sub-MW power magnetrons. Simultaneously, that choice limits the gun size and breakdown probability. The gun construction employs an osmium-coated dispenser cathode, impregnated with barium oxide, calcium, and aluminum. That emitter provides an electron work function of ∼2 eV at an emitter optimum temperature range (1100–1230°C) and moderate beam loading, allowing a sufficient cathode lifetime. The expected gun parameters are given in Table I.

TABLE I.

Electron gun specification for Klylac.11 

Cathode voltage U 24 kV 
Beam perveance Pb 3–6 µA/V3/2 
Cathode beam loading Jc <5 A/cm2 
Anode transmission ηA >93% 
Waist position downstream 
the internal anode end plane surface Zw >7mm 
Aspect ratio near the waist Ar >10 
Duty factor Df 0.4% 
Pressure P ≤10−7 mbar 
Cathode voltage U 24 kV 
Beam perveance Pb 3–6 µA/V3/2 
Cathode beam loading Jc <5 A/cm2 
Anode transmission ηA >93% 
Waist position downstream 
the internal anode end plane surface Zw >7mm 
Aspect ratio near the waist Ar >10 
Duty factor Df 0.4% 
Pressure P ≤10−7 mbar 

The SBK gun was designed with CST Studio Suite20 using Electro-Static and Particle Studio solvers and subsequent stepwise optimization. An important part of the design process was the development of a proper parametric 3D modeling. The model optimization was performed over eight geometric parameters, using a combination of various optimization algorithms. The Nelder Mead Simplex and rust Region Framework were used primarily.

The simulation started with a 2D model illustrated in Fig. 1 as the 1st-order approximation, which seeded the 3D modeling. The 3D design illustrated in Fig. 2 was performed at moderate cathode beam loading (2.6 A/cm2). The initial 3D simulations demonstrated a more than 25 A beam current and 93% beam transmission, occurring mostly within the tapered anode tunnel—mostly near the transition to the regular (non-tapered) anode aperture.

FIG. 1.

2D model of the electron gun.

FIG. 1.

2D model of the electron gun.

Close modal
FIG. 2.

Trajectories in the vertical (a), horizontal (b), and transverse (c) planes of the initial design. Beam aspect ratio near the waist is ∼13 at ∼27.2 mm distance from the cathode vertex.

FIG. 2.

Trajectories in the vertical (a), horizontal (b), and transverse (c) planes of the initial design. Beam aspect ratio near the waist is ∼13 at ∼27.2 mm distance from the cathode vertex.

Close modal

However, inclusion of thermal electrons at 1127 °C cathode temperature resulted in lossy transmission (∼67%). This was an indication of excess vertical beam emittance, which needed to be reduced. In addition, some gap between the beam and the anode aperture is desirable, to allow margin for manufactured geometry imperfections. This suggested reduction of the vertical size of the emitter.

The geometry of the initial design has been modified mostly in vertical cross-section, and as a result, compression was reduced. The modified design is shown in Fig. 3. Thus, compression was reduced by decreasing the height of the cathode. The higher beam loading of the cathode in that design (4.6 A/cm2 averaged) enabled a simplified shape of the adjacent focusing electrode, with a 0.5 mm gap separation from the cathode. The simulated trajectories of a 24 A beam emitted from the cathode at 1227 °C temperature are given in Fig. 4.

FIG. 3.

Modified design with reduced compression.

FIG. 3.

Modified design with reduced compression.

Close modal
FIG. 4.

Emitted beam 3D trajectories, considering thermal electron velocities, with racetrack shape of the cathode assembly (on the top) and magnified emission area (on the bottom).

FIG. 4.

Emitted beam 3D trajectories, considering thermal electron velocities, with racetrack shape of the cathode assembly (on the top) and magnified emission area (on the bottom).

Close modal

Optimization of the modified design resulted in a 100% transmission through the anode and 63% filling factor of the anode aperture.

In Table II, we summarized results of the two optimized variants simulated. The cathode beam loading remains less than 5 A/cm2, allowing sufficient lifetime of a dispenser cathode.

TABLE II.

Cathode current Ic, cathode beam loading Jc, transmission through the anode T, and anode aperture filling F for two variants of the diode sheet beam electron gun, simulated and optimized for 24 kV voltage.

VariantIc (A)Jc (A/cm2)T (%)F (%)
High compression 25.1 2.6 67 100 
Low compression 23.2 4.6 100 63 
VariantIc (A)Jc (A/cm2)T (%)F (%)
High compression 25.1 2.6 67 100 
Low compression 23.2 4.6 100 63 

All three main solid parts of the Klylac E-gun (focusing electrode of the cathode, the insulator, and the anode) have a racetrack shape to allow a compact assembling, to leave space for engineering feeds of the Klylac package. However, fabrication of racetrack shapes is more expensive than that for cylindrical parts (especially for ceramic insulators).

To simplify fabrication of the prototype, we designed a cylindrical variant of the gun. The beam trajectories are simulated in Fig. 5(a). The prototype geometry is shown in Figs. 68. In the prototype, both the cathode and anode have a cylindrical exterior, simplifying fabrication of ceramic and metal parts.

FIG. 5.

Beam trajectories in the vertical plane for the prototype of SBK diode gun with cylindrical (a) and racetrack (b) gun exteriors. Thermal electron velocities are considered.

FIG. 5.

Beam trajectories in the vertical plane for the prototype of SBK diode gun with cylindrical (a) and racetrack (b) gun exteriors. Thermal electron velocities are considered.

Close modal
FIG. 6.

Solid CAD design of the cathode part of the gun prototype. The cathode assembly dimensions are 152 mm (diameter) × 177 mm (height).

FIG. 6.

Solid CAD design of the cathode part of the gun prototype. The cathode assembly dimensions are 152 mm (diameter) × 177 mm (height).

Close modal
FIG. 7.

Emitter unit (a) and cathode sub-assembly with heater (b).

FIG. 7.

Emitter unit (a) and cathode sub-assembly with heater (b).

Close modal
FIG. 8.

Prototype of a sheet beam, high-perveance electron gun, fabricated and brazed (a), with anode removed (b). The anode is made within a form factor of the standard DN100CF vacuum flange.

FIG. 8.

Prototype of a sheet beam, high-perveance electron gun, fabricated and brazed (a), with anode removed (b). The anode is made within a form factor of the standard DN100CF vacuum flange.

Close modal

The beam trajectories for the cylindrical (prototype) gun geometry can be compared to that for the original (racetrack) geometry given in Figs. 5(a) and 5(b) (see also last line in Table II). The comparison shows that this modification affected current distribution, but did not change much the main beam parameters given in Table II (the last line). Both variants provide 100% transmission at (23–25) kV beam voltage. For the cylindrical variant, the beam waist and output vertical sizes are (2.4, 4.7) mm vs the (2.3, 4.2) mm, respectively, in the racetrack variant, whereas the output beam envelope divergence is 74 vs 77 mrad, respectively, in racetrack geometry. The variant with the cylindrical enclosure has been adopted for engineering of the gun prototype.

The gun was fabricated with average tolerances 50 μm (down to 20 μm for some parts).

The CAD design of the gun prototype shown in Fig. 6 consists of an emitter unit comprising a cathode assembly integrated with a heater, a cathode focusing electrode, an alumina ceramic isolator, and an electric connection plate. This is paired to a removable anode plate by means of a conventional conflate DN100CF vacuum flange, with a copper gasket.

The cathode assembly of the electron gun shown in Fig. 7 is fabricated using high-temperature brazing. The gun assembly with the anode removed is shown in Fig. 8.

Testing the gun prototype includes ampere–voltage and temperature characterization of the cathode assembly with heater (presented in Figs. 9 and 10), evaluation of transmission loss, and gun perveance measurement.

FIG. 9.

Cathode temperature measured vs filament power (top curve with circular marks) of the cathode unit and filament resistance (bottom curve with green squares). The temperature was measured with a calibrated, emission- (brightness-) based pyrometer, with estimated accuracy better than 4% (in Kelvin temperature scale).

FIG. 9.

Cathode temperature measured vs filament power (top curve with circular marks) of the cathode unit and filament resistance (bottom curve with green squares). The temperature was measured with a calibrated, emission- (brightness-) based pyrometer, with estimated accuracy better than 4% (in Kelvin temperature scale).

Close modal
FIG. 10.

Thermal image of the cathode (a) placed into the test chamber and its histogram (b) representing the number of pixels in an IR image, plotted as a function of intensity. The inset in the plot shows the infrared image of the cathode, post-processed with standard Gaussian deconvolution and filtering, to mitigate artifacts caused by camera tilt and internal reflections within the test chamber.

FIG. 10.

Thermal image of the cathode (a) placed into the test chamber and its histogram (b) representing the number of pixels in an IR image, plotted as a function of intensity. The inset in the plot shows the infrared image of the cathode, post-processed with standard Gaussian deconvolution and filtering, to mitigate artifacts caused by camera tilt and internal reflections within the test chamber.

Close modal

Pyrometric data taken in Fig. 9(b) and infra-red (IR) image in Fig. 10 show relatively uniform heating of the cathode, indicating good quality of the emitter: The temperature non-uniformity was within the measurement error, i.e., < 1%–2%. That was concluded from the collimated scan of the pyrometer along the length of the cathode (∼30° maximum temperature reduction closer to edges), as well as from the IR image analysis, with histogram validated by deconvolution and filtering [see Fig. 10(b)].

For the next stage of the gun testing, we have designed a special setup, with schematics shown in Fig. 11(a). The manual actuator enables switching to perform three types of measurements: temperature, beam current on the Faraday cup, and beam profile. That was provided by switching the in-vacuum construction to interchange the Faraday cup with the scintillator screen or to allow optical imaging of the cathode via a vacuum window, optical filter, and with the inset set in “open” position (when the HV modulator is off). The vacuum chamber attached to the anode uses a flanged vacuum cube with three ports for the camera, vacuum pump, and vacuum feedthrough of the manual linear actuator. The setup schematic shown in Fig. 11(a) employs the M1-0.5 Scandinova modulator, having limited filament power (presumably <200 W21). To avoid overloading of the modulator filament supply and pulse transformer, we used an external battery as the filament power source at the floating HV potential.

FIG. 11.

(a) Block-scheme of the SBK gun test setup. (b) Movable part of the assembly with a copper Faraday cup mounted and scintillator screen unmounted (white strip) with two stainless steel protecting meshes (two Gy strips) and holder (frame on the bottom).

FIG. 11.

(a) Block-scheme of the SBK gun test setup. (b) Movable part of the assembly with a copper Faraday cup mounted and scintillator screen unmounted (white strip) with two stainless steel protecting meshes (two Gy strips) and holder (frame on the bottom).

Close modal

The mechanical design of the vacuum chamber provides three positions: (a) open position with the cathode seen via the view port for temperature measurement of the cathode; (b) in-vacuum insertable Faraday cup to measure beam current; and (c) in-vacuum insertable scintillator screen to take an image of the beam with a triggered camera through the view port and a 450-nm filter.

The mechanical design enables interchange between the in-vacuum insert, the Faraday cup, and the scintillator, as shown in Fig. 11(b). There is a sub-milllimeter gap between the motionless grounded anode outlet and the movable insertable scintillator assembly. The assembly contains two stainless steel meshes, mounted together with the Faraday cup, having finite potential (determined by beam loss) with respect to the ground. The gap enables to insulate electrically the Faraday cup from the grounded anode and mechanical movement, to allow the movement for IR or beam imaging as close as possible to the beam waist. Significant difficulties are related to the scintillator screen: it must be vacuum-compatible, sustain a high density of the ∼0.2 MW/cm2 peak and ∼0.25 W/cm2 average beam power density, have sufficient dynamic range, and have sustainability to intense infrared radiation from the cathode. The challenge is to image a beam of such high peak power density at such a low beam energy. Initially, we used Mylar coated by ZnS:Ag and protected by two layers of stainless wire meshes [see Fig. 11(b)].

Prior to starting operation at high voltage, we have executed a multistep (tens of hours long) procedure for the cathode initiation (conditioning) to achieve 2 mbar vacuum pressure. Gun operation demonstrated lack of any breakdowns or arcing up to the maximum voltage 25 kV. The top view of the SBK gun test setup is shown in Fig. 13(a).

The modulator interface reading at normal operation is shown in Fig. 12. The reading suggests an internal perveance as high as 5.4 µPerv.

FIG. 12.

M1-0.5 modulator interface reading during normal operation. Average beam power is ∼1.5 W, resulting in the estimated beam power density deposited on the scintillator fluoroscreen assembly as ∼0.25 W/cm2.

FIG. 12.

M1-0.5 modulator interface reading during normal operation. Average beam power is ∼1.5 W, resulting in the estimated beam power density deposited on the scintillator fluoroscreen assembly as ∼0.25 W/cm2.

Close modal

The beam profile was taken only for the beam edge [see Fig. 13(b)]. Among the reasons for low quality of the image are scattering, non-flatness, and secondary emission from the two layers of stainless steel meshes. Nevertheless, we were able to image the edge part of the beam in a single shot mode. The beam image shown in Fig. 13(b) was obtained with the screen inserted about half-way. One can see that one edge is rather uniform, whereas another one is not. The sharp edge corresponds to the beam core, whereas the diffused edge is distorted by the beam halo scattering on the edge, the defects of the scintillator coating on the edge, and the uneven gap between the protecting stainless steel meshes and the mesh-to- scintillator screen gap.

FIG. 13.

Top view of the Klylac gun test setup (a) and image of the beam edge of the half-open scintillator (b). The dimensions of the brightest rectangular region are given for about full width (W) and presumably half-height (H/2) of the beam.

FIG. 13.

Top view of the Klylac gun test setup (a) and image of the beam edge of the half-open scintillator (b). The dimensions of the brightest rectangular region are given for about full width (W) and presumably half-height (H/2) of the beam.

Close modal

Being exposed even partially to the beam, the Mylar scintillator intensively desorbs molecules, resulting in rapid growth of pressure. That prevented the imaging at full opening of the beam even with single shots. An alternative scintillator could be a stainless mesh (by Eljen Technology) coated with ZnS:Ag using soda silicate as a binder (with subsequent baking procedure developed and standardized by NASA for high vacuum). However, the binder was too fragile for our application.

Beam losses on the anode aperture can be determined from oscilloscope traces shown in Fig. 14. The waveforms are distorted (i.e., are not trapezoidal) because a conventional lead battery with related circuitry was used for filament supply at floating HV potential. The resistance of the Faraday cup circuit is 530 mΩ.

FIG. 14.

Oscilloscope traces of the anode gun current from Pearson coil related to the cathode current (yellow), Faraday cup (magenta), and high- voltage probe (blue).

FIG. 14.

Oscilloscope traces of the anode gun current from Pearson coil related to the cathode current (yellow), Faraday cup (magenta), and high- voltage probe (blue).

Close modal

To understand the waveform overshoots, we assessed the parasitic inductances and capacitors related to the gun circuit. An approximate equivalent circuit is shown in Fig. 15(a). The electron gun is modeled as two non-linear resistances in parallel, having constant perveance: one corresponds to the beam transmitted to the Faraday cup, whereas another one, to the beam loss on the anode aperture. Waveforms shown in Fig. 15(b) are simulated using the LTSpice VII.22 Note that the voltage peak is delayed by about 150 ns with respect to the beam current peak similar to that observed in the Fig. 14 traces. We analyzed the circuit performance at variable L1, C1 + C2, and the Loss parameters given in Fig. 15(a). The transient circuit analysis shows that the transmission (which equals 1-Loss) can be found as a ratio of I(R3)/I(R1) at the very end of the plateau of the modulator voltage [i.e., at t = 1.35 µs for Fig. 15(b)]. Note that this point corresponds to a local minimum of the I(R3) and/or I(R1) derivative as a function of time (omitting the first minimum at overshoot). Also note that the peak magnitude (i.e., at the 1st zero of the derivative) is not indicative in terms of the beam loss: The corresponding overshoot is related to the transient caused by the front rise, whereas the magnitude at the 2nd minimum of the derivative is directly related to the steady state part of the incident voltage.

FIG. 15.

Approximate high voltage electric scheme of the gun test circuit with fixed perveance 5.4 µA/V^3/2% and 6.4% beam loss on the anode aperture (a). Waveforms simulated for gun voltage [V(vgun), green], Pearson coil (cathode) current [I(L1), blue], modulator voltage [V(vgn), purple], and transmitted beam current [I(R3), red] (b).

FIG. 15.

Approximate high voltage electric scheme of the gun test circuit with fixed perveance 5.4 µA/V^3/2% and 6.4% beam loss on the anode aperture (a). Waveforms simulated for gun voltage [V(vgun), green], Pearson coil (cathode) current [I(L1), blue], modulator voltage [V(vgn), purple], and transmitted beam current [I(R3), red] (b).

Close modal

Next, we apply this approach to the experimental waveforms of Fig. 14. The waveforms for the currents from Fig. 14 are depicted in Fig. 16(a), with eliminated time shift. We found the position of the 2nd local minimum of the derivative, which corresponds to t = 2.295 µs. And finally, from Fig. 16(b), we determined the 0.935 ratio of the currents at this point, which corresponds to 6.5% loss at the anode aperture.

FIG. 16.

Experimental waveforms for currents re-plotted without mutual time shift (a). Ratio of the currents plotted as a function of time (b). The cross point at 2.3 microseconds corresponds to the local minimum of the derivative of the currents (the next minimum to that at the overshoot peak).

FIG. 16.

Experimental waveforms for currents re-plotted without mutual time shift (a). Ratio of the currents plotted as a function of time (b). The cross point at 2.3 microseconds corresponds to the local minimum of the derivative of the currents (the next minimum to that at the overshoot peak).

Close modal

A flat beam, high-perveance, high aspect ratio, relatively low voltage, diode gun has been designed as a trade-off between cathode beam loading and transportation efficiency. The gun prototype has been built and tested. It demonstrated a significant potential for emerging low-voltage applications such as SBKs and sheet beam TWTs. The estimated outcoupled beam perveance is 5.4 0.935 = 5.05 µPerv at a beam aspect ratio about 12.

The measured beam loss of 6.5% is about the same as that for the electron gun of the carefully “tweaked” SBK VKX-8293A,16 having comparable perveance, but much higher voltage (76 kV). Such a relatively lossy electron gun prototype can be used in systems where efficiency and total dissipation power are not critical, and/or anode cooling is easily accessible. Among those are gas laser excitation, plasma chemistry, e-beam material processing (e.g., used in plasma reactors in semiconductor industry), and borehole logging in the SBK-Klylac system.11 Klylacs and Klynacs combine a klystron and an electron accelerator, sharing the same electron beam source and vacuum envelope. Note that the emerging devices require perveances approaching that in magnetrons having comparable microwave power, wavelength, and voltage. Their application to borehole logging does not require high rep and dose rates: the densitometry tool movement speed is low (unlike, e.g., cargo inspection during transportation). Klylacs and Klynacs present significant potential as medical linacs, for replacement of radioactive sources, in geology, and in portable, non-destructive diagnostics. Another potential application of the high perveance gun could be electron cooling at low energies, where an increase of beam current at a fixed energy increases the cooling rate.23 

The beam loss in the prototype tested can be caused by geometrical inaccuracies of the assembly, including thermal deformations of the emitter, not considered in the model presented. The gun fabrication and assembly also need tighter geometrical verification and control to correct mechanical errors, to make sure that the assembly is qualified for tests. These standard engineering procedures were not fully implemented in manufacturing of the prototype. The gun performance can be improved further with fine-tuning of the cathode to anode distance, self-consistent designing to include more accurate thermal stress, and thermal and mechanical deformations, tolerance analysis to improve agreement with measurements, and deeper optimization, including compression ratio, and with probably additional focusing electrodes—especially on the edges along the wider dimension (which was in the initial design).

The test revealed difficulties related to (i) beam imaging and (ii) transient of the gun circuitry having substantial parasitic inductances and capacitances. The first issue was mitigated in part with beam collimation and minimized exposition. More robust fluoroscreens (e.g., inorganic scintillator crystals24) and/or substrates are required for beam imaging. The diagnostic distortion by transients is addressed here with equivalent circuit modeling.

This work was supported by the U.S. Department of Energy (DOE) (Award No. DE-SC0015721). The authors are grateful to Dr. Bruce E. Carlsten, Dr. Vladislav Tsarev, Dr. Sergey Kurennoy, Dr. Ahmed Badruzzaman, as well as Salime Boucher and Dr. Alexander Murokh, for the deep interest in this work, discussions, and support. The author (A.V.S.) is indebted to Robert Berry for the great teamwork in the experiment setup and Stephen Kemp for the manuscript corrections that were made.

The authors have no conflicts to disclose.

A. V. Smirnov: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Resources (lead); Supervision (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead). R. Agustsson: Investigation (supporting); Methodology (supporting); Supervision (supporting); Validation (supporting). D. Chao: Investigation (supporting); Software (supporting); Visualization (supporting). D. Gavryushkin: Methodology (supporting); Validation (equal); Visualization (equal). K. J. Hoyt: Data curation (equal); Formal analysis (equal); Investigation (equal); Validation (equal); Writing – review & editing (equal). D. Shchegolkov: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal). A. Zavadtsev: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Validation (lead); Visualization (equal); Writing – original draft (supporting); Writing – review & editing (supporting).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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A. V.
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F. H.
O’Shea
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Spranza
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Pasky
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Ruelas
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Zholents
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