kA Electron Beam Research Articles

TEM and X-ray diffraction studies of structure of TiNi SMA treated by low-energy high-current electron beam

Advanced scientific and technological achievements in the surface engineering of TiNi shape memory alloys are closely related with their mechanical performance. Electron-beam treatments could be applied in order to modify surface properties of smart materials and promote stress-induced martensitic transformations. The fundamental issue of electron-beam processing is residual elastic stresses arising from the ultrafast quenching (∼108 K/s) of thin (∼2–4 μm) surface layers. We report the experimental results on phase composition, structure and residual elastic stresses formed in the binary TiNi SMA irradiated by a low-energy (< 25 keV) high-current (∼ 25 kA) electron beam. Microstructure, fracture mechanisms and phase analysis of the as-cast and deformed tensile specimens have been studied by X-ray diffraction and electron microscopy. After electron-beam treatment, both martensitic B19′ and R phases are found in the near surface layer. The X-ray diffraction studies indicate that compressive stresses in the B2 phase of the irradiated TiNi alloy reach –460 MPa. In the fractured specimen (after tensile test) the residual stresses decrease to –160 MPa and release through the B2→B19′→B2 martensitic transformation. In-depth gradient distribution of the defect substructures responsible for the mechanical behavior of TiNi alloys was revealed.

Characterization of Pseudospark Discharge-Based Multigap Plasma Cathode Electron Source for the Generation of Short Pulsed Energetic Electron Beam

Pseudospark (PS) discharge-based devices are known as excellent source for the generation of high current density and energetic self-focused electron beam in the hollow cathode (HC) phase. In this article, short pulsed ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\!\!\!&lt; 100$ </tex-math></inline-formula> ns) and high energetic (~20 keV) electron beam has been generated and propagated (up to ~60 mm) in the drift region without using any external guiding magnetic field from the four-gap configuration of PS discharge-based plasma cathode electron (PD-PCE) source. The particle-in-cell (PIC) simulation code OOPIC Pro has been employed, whose results have shown the strong dependence of beam propagation into the drift region on the penetration and distribution of potential lines. The combined experimental and simulation investigations have also been carried for the multigap (four-gap) with wide range of external storage capacitor (40 pF–18 nF) and operating voltages (5–35 kV). The circuit parameter controls the appearance of HC phase for the energetic (50%–70% applied voltage) electron beam and conductive phase for the high current (~10 A to ~0.5 kA) electron beam. The potential distribution has clearly indicated that the electron beam with higher applied voltages can propagate more focused in the drift region. The investigations have evidently shown the generation of low energy and high current to high energy and low current electron beams suitable to cover the potential applications in the field of extreme ultraviolet (EUV)/soft X-ray radiation generation, surface modification, and microwave-terahertz radiation generation.

Highly spin-polarized multi-GeV electron beams generated by single-species plasma photocathodes

High-gradient and high-efficiency acceleration in plasma-based accelerators has been demonstrated, showing its potential as the building block for a future collider operating at the energy frontier of particle physics. However, generating and accelerating the required spin-polarized beams in such a collider using plasma-based accelerators have been a long-standing challenge. Here we show that the passage of a highly relativistic, high-current electron beam through a single-species (ytterbium) vapor excites a nonlinear plasma wake by primarily ionizing the two outer $6s$ electrons. Further photoionization of the resultant ${\mathrm{Yb}}^{2+}$ ions by a circularly polarized laser injects the $4{f}^{14}$ electrons into this wake, generating a highly spin-polarized beam. Combining time-dependent Schr\"odinger equation simulations with particle-in-cell simulations, we show that a subfemtosecond, high-current (4 kA) electron beam with up to 56% net spin polarization can be generated and accelerated to 15 GeV in just 41 cm. This relatively simple scheme solves the perplexing problem of producing spin-polarized relativistic electrons in plasma-based accelerators.

Demonstration of a Ka-Band Oversized Coaxial Multi-Beam Relativistic Klystron Amplifier for High Power Millimeter-Wave Radiation

We proposed an oversized coaxial multi-beam relativistic klystron amplifier (OCMB-RKA) with a new input coupler of lesser complexity giving enhanced input microwave power absorption rate for generating high power millimeter-wave radiation in Ka-band. The high-frequency structure of the OCMB-RKA was designed, fabricated, and cold-tested for results that agreed well with the simulated results. Furthermore, using the 3D particle-in-cell (PIC) code, the beam-wave interaction of the OCMB-RKA was simulated, considering two different materials (stainless steel and copper) making the interaction structure. When a 500 kV, 3 kA electron beam, and a focusing magnetic flux intensity of 1 T were adopted in the PIC simulation, a five-cavity OCMB-RKA yielded the output power of 610 MW at 30.525 GHz with the electronic efficiency of 40.7% and the saturated gain of 55.3 dB. Experimentally, the OCMB-RKA demonstrated its capability of generating high power Ka-band radiation typically of 390 MW power with the pulse width of 27 ns.

Two-color interferometer for study of dense low-ionized plasma on the target in high-power pulse linear accelerator.

A two-color infrared interferometer has been developed for the investigation of high-density weakly ionized tantalum plasma in x-ray complexes based on linear induction accelerators (1.6 kA electron beam current, 4.6MeV energy, and 100ns pulse duration). Simultaneous probing at two different wavelengths makes it possible to independently measure the density of neutral and electron components. The interferometer uses wavelength values of 1.064 µm (Nd:YAG laser) and 10.6 µm (CO2 laser). To attenuate the effect of sample beam refraction in inhomogeneous plasma, the interferometer used a refraction suppression scheme composed of spherical mirrors focusing the object beam into the region occupied by the plasma. In addition, the power of the sample beam transmitted through the plasma was controlled in order to analyze whether there was a distortion of the interference pattern because of strong sample beam refraction and absorption in the plasma cloud. To calibrate the initial phase shift of the probe radiation, a movable mirror mounted on a piezoelectric element and oscillating according to a harmonic law with amplitude greater than the laser wavelengths was used. In initial experiments, the parameters of target plasma registered by this interferometer are as follows: the linear density of neutrals reached 1.5 · 1017cm-2, and the degree of ionization was of the order of 10-2. The target plasma expansion velocity is determined as ∼6 km/s.

Plasma Anode Formation in a High-Current Electron Gun with the Use of a Hybrid Discharge

The paper presents the results of a study of a glow spatial structure and temporal dynamics of a hybrid discharge combining a high-current reflective discharge with vacuum arcs at a voltage supplying the discharge up to 9 kV and working gas (argon) pressures of 0.1–1 mTorr. The possibility of forming a plasma anode with enhanced ion density at the periphery has been demonstrated with the use of this discharge. Preliminary results of measurements of the energy density distribution over the cross section of a low-energy (up to 30 keV) high-current (up to 25 kA) electron beam formed in an electron gun with explosive emission cathode and plasma anode based on the hybrid discharge are also presented. The promise of the proposed method for improving the beam homogeneity has been shown.

Multilayer Surface-Area Structure and the Influence of Residual Oxygen on its Formation During Irradiation of TiNi Alloy with a Low-Energy High-Current Electron Beam

The structure and phase composition of a subsurface layer of TiNi alloy modified in the mode of pulsed melting with a microsecond low-energy (to ~40 keV) high-current (to ~50 kA) electron beam is investigated. The influence of the number of pulses at a constant energy density on the characteristics of structure, phasecomposition changes, and behavior of the phase distribution near the surface is considered. The role of oxygen in the columnar structure stabilization of the recrystallized surface layer is discussed.

Theoretical research on TEM mode feedback for compact design of an X-band triaxial klystron amplifier

A relativistic triaxial klystron amplifier (TKA) is considered to be one of the most promising high power microwave (HPM) sources for coherent power combination in high frequency bands. In general investigations, the TEM mode leakage and asymmetric modes' competition are considered to play unfavorable roles in the normal operation of TKA. Therefore, mode reflectors with high reflection efficiency in both the TEM mode and the asymmetric modes are employed and expected to significantly suppress the transmissions of the microwaves between the cavities. So the cavities can be separated from each other, while the long drift length is necessary to achieve the deep modulation. Therefore, the size of TKA is noticeably larger compared to other HPM oscillators, which requires large magnetic coil and huge energy consumption. In our present research, it is found that the TEM mode leakage can be utilized to form positive feedback between the cavities, which is significant for electron beam modulation and furthermore to make the devices more compact. In this paper, the mechanism of TEM mode feedback between the cavities is discussed and the conditions of positive and negative feedback are achieved. Besides, an especially designed reflector with high reflection coefficients in asymmetric modes and low reflection coefficients in the TEM mode is designed to utilize the TEM mode and suppress the asymmetric modes. Based on the above research, a TKA adopting positive TEM mode feedback, whose total length is decreased by 23.6% compared to the previous design, is presented. A high power microwave with a power of 2.25 GW and a frequency of 8.4 GHz is generated with a 690 kV, 9.3 kA electron beam excitation and a 40 kW radio-frequency signal injection. Meanwhile, there is insignificant self-excitation of the parasitic mode in the proposed structure in three-dimensional particle-in-cell simulation, and the relative phase shifts between the injected signals and the output microwaves are kept locked after the amplifier becomes saturated.

Shielding of the azimuthal magnetic field by the anode plasma in a relativistic self-magnetic-pinch diode

In relativistic electron beam diodes, the self-generated magnetic field causes electron-beam focusing at the center of the anode. Generally, plasma is formed all over the anode surface during and after the process of the beam focusing. In this work, we use visible-light Zeeman-effect spectroscopy for the determination of the magnetic field in the anode plasma in the Sandia 10 MV, 200 kA (RITS-6) electron beam diode. The magnetic field is determined from the Zeeman-dominated shapes of the Al III 4s–4p and C IV 3s–3p doublet emissions from various radial positions. Near the anode surface, due to the high plasma density, the spectral line-shapes are Stark-dominated, and only an upper limit of the magnetic field can be determined. The line-shape analysis also yields the plasma density. The data yield quantitatively the magnetic-field shielding in the plasma. The magnetic-field distribution in the plasma is compared to the field-diffusion prediction and found to be consistent with the Spitzer resistivity, estimated using the electron temperature and charge-state distribution determined from line intensity ratios.

Quantifying spot size reduction of a 1.8 kA electron beam for flash radiography

The spot size of Axis-I at the Dual Axis Radiographic Hydrodynamic Test facility was reduced by 15.5% by including a small diameter drift tube that acts to aperture the outer diameter of the electron beam. Comparing the measured values to both analytic calculations and results from a particle-in-cell model shows that one-third to one-half of the spot size reduction is due to a drop in beam emittance. We infer that one-half to two-thirds of the spot-size reduction is due to a reduction in beam-target interactions. Sources of emittance growth and the scaling of the final focal spot size with emittance and solenoid aberrations are also presented.

A high-gain and high-efficiency X-band triaxial klystron amplifier with two-stage cascaded bunching cavities

To achieve GW-level amplification output radiation at the X-band, a relativistic triaxial klystron amplifier with two-stage cascaded double-gap bunching cavities is investigated. The input cavity is optimized to obtain a high absorption rate of the external injection microwave. The cascaded bunching cavities are optimized to achieve a high depth of the fundamental harmonic current. A double-gap standing wave extractor is designed to improve the beam wave conversion efficiency. Two reflectors with high reflection coefficients both to the asymmetric mode and the TEM mode are employed to suppress the asymmetric mode competition and TEM mode microwave leakage. Particle-in-cell simulation results show that a high power microwave with a power of 2.53 GW and a frequency of 8.4 GHz is generated with a 690 kV, 9.3 kA electron beam excitation and a 25 kW seed microwave injection. Particularly, the achieved power conversion efficiency is about 40%, and the gain is as high as 50 dB. Meanwhile, there is insignificant self-excitation of the parasitic mode in the proposed structure by adopting the reflectors. The relative phase difference between the injected signals and the output microwaves keeps locked after the amplifier becomes saturated.

An efficient X-band relativistic backward wave oscillator combining single-mode structure with overmoded structure

A combined modes operation mechanism in a sectioned relativistic backward wave oscillator (RBWO) is presented. The RBWO consists of single-mode slow wave structure (SWS) and overmoded SWS. In the single-mode SWS, the electron beam interacts with TM01 mode synchronously; in the overmoded SWS, the operation mode is mainly TM02 mode. The microwave frequency is determined by the single-mode SWS, which alleviates the mode competition in the overmoded SWS. Moreover, the power capacity of the RBWO can be improved due to the employment of overmoded structure and TM02 mode operation. Particle-in-cell simulation results demonstrate that a microwave with a power of 2.5 GW and a frequency of 9.18 GHz is generated with a 680 kV, 7.3 kA electron beam, corresponding to a beam-wave power conversion efficiency of 50%.

Direct measurement of the image displacement instability in a linear induction accelerator

The image displacement instability (IDI) has been measured on the 20 MeV Axis I of the dual axis radiographic hydrodynamic test facility and compared to theory. A 0.23 kA electron beam was accelerated across 64 gaps in a low solenoid focusing field, and the position of the beam centroid was measured to 34.3 meters downstream from the cathode. One beam dynamics code was used to model the IDI from first principles, while another code characterized the effects of the resistive wall instability and the beam break-up (BBU) instability. Although the BBU instability was not found to influence the IDI, it appears that the IDI influences the BBU. Because the BBU theory does not fully account for the dependence on beam position for coupling to cavity transverse magnetic modes, the effect of the IDI is missing from the BBU theory. This becomes of particular concern to users of linear induction accelerators operating in or near low magnetic guide fields tunes.

Highly efficient X-band relativistic twistron

AbstractThe paper proposes a new scheme of high-power microwave oscillator of twistron type using a moderately relativistic high-current electron beam. In numerical experiment using axisymmetric version of the completely electromagnetic PiC code KARAT, a 56% conversion efficiency of electron beam power to electromagnetic radiation was demonstrated. With 340 kV accelerating voltage, 3.3 kA electron beam current, and 2.2 T guiding magnetic field strength, the simulated microwave power was 630 MW at 9.7 GHz. The “electronic efficiency” of the source reaches 66%.

Pulsed Electron Beam Propagation in Oxygen

The paper presents the results of the experimental investigation of the pulsed electron beam propagation propagated in a drift tube filled with oxygen. The pressure was 50, 300, and 760 Torr in the drift tube. The experiments were carried out using a TEA-500 pulsed electron accelerator (450 kV accelerating voltage; 10 kA electron beam current; 80±1 ns pulse duration to the base; up to 200 J pulse energy; and 5 cm diameter beam). The electron beam was propagated in the drift tube (40 cm long), 14 cm in diameter composed of two sections equipped with two reverse current shunts. The experiments were carried out which fixed the absorbed dose registered on the walls of the drift chamber.

A high efficiency Ku-band radial line relativistic klystron amplifier

To achieve the gigawatt-level microwave amplification output at Ku-band, a radial-line relativistic klystron amplifier is proposed and investigated in this paper. Different from the annular electron beam in conventional axial relativistic klystron amplifiers, a radial-radiated electron beam is employed in this proposed klystron. Owing to its radially spreading speciality, the electron density and space charge effect are markedly weakened during the propagation in the radial line drift tube. Additionally, the power capacity, especially in the output cavity, is enhanced significantly because of its large volume, which is profitable for the long pulse operation. Particle-in-cell simulation results demonstrate that a high power microwave with the power of 3 GW and the frequency of 14.25 GHz is generated with a 500 kV, 12 kA electron beam excitation and the 30 kW radio-frequency signal injection. The power conversion efficiency is 50%, and the gain is about 50 dB. Meanwhile, there is insignificant electron beam self-excitation in the proposed structure by the adoption of two transverse electromagnetic reflectors. The relative phase difference between the injected signals and output microwaves keeps stable after the amplifier saturates.

Effect of inclusions on cratering behavior in TiNi shape memory alloys irradiated with a low-energy, high-current electron beam

Abstract Irradiation of TiNi shape memory alloys with a microsecond low-energy (10–30 keV), high-current (10–25 kA) electron beam in surface melting modes induces microcrater formation, which limits the practical use of this method of surface modification. Potential centers of crater formation are inclusions, which are inherent to TiNi alloys. However, there is limited information about their role in crater formation and the nature of this phenomenon as applied to TiNi alloys. In this paper, the microstructural identification of inclusions was carried out in commercial (VIM) and precision TiNi alloys using TEM/SAED/EDS methods. The topographical features of microcraters formed by a single-pulse irradiation of TiNi samples near the surface melting threshold of the B2 matrix phase were identified. It was found by SEM/EDS-analysis, that microcraters are mainly nucleated on titanium oxycarbide TiC(O) (commercial alloy) and oxide Ti 4 Ni 2 O (precision alloy) inclusions; in both cases, the central region of microcraters is enriched with oxygen. The role of gradient capillary forces in the melt mass transfer (Marangoni effect), which determine the formation of microcraters, was analyzed. It was shown that thermocapillary forces play an insignificant role, while chemicapillary forces induced by decreasing a surface tension of the melt due to its oxygen enrichment can be sufficiently large to produce microcraters.

On the low-energy high-current electron beam profile controlled using ferromagnetic insertions

We report on the results of investigation of the energy density distribution over the cross section of a low-energy (10–30 keV) high-current (up to 25 kA) electron beam transported in the plasma channel in a guiding magnetic field. Beam focusing and energy density redistribution in the beam cross section are carried out using hollow cylindrical insertions prepared from a ferromagnetic steel (magnetic-field concentrators) and installed behind a foil collector (made of nonmagnetic stainless steel of thickness 200 μm). The energy density distribution is measured by an infrared imager; for this purpose, the concentrator is moved aside by an electromagnet during the pause (about 1 s) between a beam pulse and the instant of thermogram recording. The experiments confirm the possibility of improving the beam homogeneity in the cross section by using magnetic field concentrators. The results of measurements are in satisfactory agreement with the earlier results of calculations.

A tunable magnetically insulated transmission line oscillator**Project supported by the National Natural Science Foundation of China (Grant No. 11075210) and the Special Financial Grant from the China Postdoctoral Science Foundation (Grant No. 201104761).

A tunable magnetically insulated transmission line oscillator (MILO) is put forward and simulated. When the MILO is driven by a 430 kV, 40.6 kA electron beam, high-power microwave is generated with a peak output power of 3.0 GW and frequency of 1.51 GHz, and the relevant power conversion efficiency is 17.2%. The 3-dB tunable frequency range (the relative output power is above half of the peak output power) is 2.25–0.825 GHz when the outer radius of the slow-wave structure (SWS) vanes ranges from 77 mm to 155 mm, and the 3-dB tuning bandwidth is 92%, which is sufficient for the aim of large-scale tuning and high power output.

Vacuum Outgassing Behavior of Carbon Nanotube Cathode with High-Intensity Pulsed Electron Emission

Experimental investigations on the vacuum outgassing of a carbon nanotube (CNT) cathode with high-intensity pulsed electron emission on a 2 MeV linear induction accelerator injector are presented. Under the 1.60 MV diode voltage, the CNT cathode could provide 1.67 kA electron beam with the amount of outgassing of about 0.51 Pa·L. It is found that the amount of outgassing, which determines the cathode emission current, depends on the diode voltage and the vacuum.