Beam Current Increases Research Articles

Effect of beam current on microstructure, tensile properties and corrosion behaviour of electron beam welded zircaloy-4 joints

ABSTRACT Butt joints of zircaloy-4 were made using electron beam (EB) welding. The effect of beam current (15–20 mA) on weld zone size, microstructure, porosity, mechanical and corrosion properties has been investigated. Local temperature and cooling rate were estimated, and the average cooling rate increased from 2.4 to 6.3°C ms–1 with a decrease in beam current in the said range. Joints prepared at lower beam current possessed smoother weld surface, narrow weld bead, and fine lamellar type widmanstätten structure in the fusion zone (FZ). The length of the α-Zr lamella (from 105 to 37 µm) and interlamellar space (from 0.8 to 0.3 µm) decreased with a decrease in beam current. The XCT result revealed that porosity increased with an increase in beam current, and large-size micro pores of 180 µm were present in joints prepared at high beam current. The hardness of the welded samples increased by 13–20% than the base metal. The joint prepared with a low beam current possessed higher ductility (29%) and a minimum corrosion rate (0.0085 mmpy in 3.5 wt-% of NaCl solution) than the joint made with a higher beam current.

Electron heating mode in magnetic field diffusion region of microwave discharge ion thruster

In magnetic field design principle of microwave discharge ion thruster, it is universally received that enlarging the magnetic mirror region can confine more electrons to acquire better energy utilization rate, while reducing the magnetic field diffusion region can prevent electrons from losing at wall to reduce the discharge loss. However, recently the integrated simulation proposes a hypothesis that electrons can also be heated in the magnetic field diffusion region when the Child-Langmuir sheath is considered as a constraint condition for electrons. Therefore, herein a magnetic field structure for the magnet array microwave discharge ion thruster is designed to verify the hypothesis, in which the magnetic field diffusion region is located near the screen grid. Then, an integrated simulation is conducted for studying the initial discharge and ion beam extraction stages of the thruster. The simulation results show that in the magnetic field diffusion region, the electron temperature is 4–8 eV when the grid system voltage is not applied, while the electron temperature is 4–12 eV when the the grid system voltage is applied. And the plasma density in the latter case has one order of magnitude higher than that in the former case. It means that electrons are obviously heated in the magnetic field diffusion region when they are confined among the Child-Langmuir sheath, the plasma sheath at antenna surface, and magnetic mirror. This electron heating mode produces more high-energy electrons outside the magnetic mirror region to generate plasma in front of the grid system, which can significantly increase the plasma density and ion beam current density. The result shows that under the conditions of 0.3 sccm (1 sccm = 1 mL/min) xenon gas flow, 1 W input microwave power, 300 V screen grid voltage and –50 V acceleration grid voltage, the ion beam current and its density are 0.47 mA and 0.60 mA/cm<sup>2</sup> for the magnet array microwave discharge ion thruster, while the ion beam current and its density are 1.2 mA and 0.38 mA/cm<sup>2</sup> for the 2-cm microwave discharge ion thruster. The ion beam current density increases by 57.9%. Through the integrated simulation, a new electron heating mode in the magnetic field diffusion region is proved theoretically, which provides a theoretical basis for the magnetic field structure optimization of microwave discharge ion thruster.

Electron beam welding of 4J36 invar alloy to TC4 titanium alloy using Cu-Nb composite diffused metal interlayer

Electron beam welding of 4J36 invar alloy and TC4 titanium alloy was carried out with Cu-Nb composite diffused metal interlayer. The results showed that the depth to width ratio and the tensile strength of welded joints increased with the increase of electron beam current. Under the designed process parameters, two melting zones (MZ) in welded joints formed solid solutions (ss), the welded joint tensile strength reached maximum value of 244.1 MPa, and the fracture location was mainly located at 4J36/Cu interface.

Compact E-Band Sheet Beam Folded Waveguide Traveling Wave Tube for High Data Rate Communication

The design of a compact <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{E}$</tex-math> </inline-formula> -band (71–76 GHz) sheet beam (SB) folded waveguide (FWG) traveling wave tube (TWT) for high data rate communications is presented in this article. The round beam tunnel for a pencil FWG slow wave structure (SWS) has been redesigned with a rectangular beam tunnel to be suitable for the SB with increased beam current. A novel periodic permanent magnet (PPM)–periodic quadrupolar magnet (PQM) focusing system has been developed to reduce the beam ripple and improve the transmission ratio. A multistep side-out coupler is designed to import and extract the RF power from the SWSs and be compatible with the planar periodic magnetic focusing system. Particle-in-cell (PIC) simulations are employed to evaluate the performance of the high-frequency circuit. Driven by a 1 W signal, the 30-period circuit is estimated to produce more than 100 W power and achieve a 21 dB gain with a less than 0.4 dB variation over 71–76 GHz. Moreover, phase and linearity analysis show a less than 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> in-band phase distortion and <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 dB third-order intermodulation distortion (C/3IM) at 30 W of output power, meeting the requirements of a communications-focused TWT. Over 5% circuit efficiency and 40% overall efficiency when equipped with a single-stage collector can be achieved by the compact TWT over the target band.

Influence of electron‑beam surfacing modes on thickness and uniformity of single‑layer coating when restoring parts made of titanium alloys

The influence of the beam current, the distance between the beads of the clad material and the wire feeding intensity during electron‑beam (EB) surfacing of titanium alloy VT9 on the thickness and uniformity of the resulting restoring coating was investigated. As the beam current strength increases, the width of the deposited layer increases and the height decreases. The optimal distance between the beads should be considered an offset that does not exceed the width of the bead. It is possible to control the height of the deposited layer in a wide range by precise dosing of the wire feed intensity. The uniformity of surfacing is ensured by a balanced energy impact. Chemical composition of the clad layer meets the requirements of normative documents on the content of impurity gases for products from medical materials, obtained by additive technologies. The results of the study are used in the restoration of the surface of the landing gear element of aviation equipment.

EFFECT OF HEAT TREATMENT ON THE STRUCTURE AND PROPERTIES OF TITANIUM ALUMINIDE ALLOY TI-AL-V-NB-CR-GD PRODUCED BY SELECTIVE ELECTRON BEAM MELTING

This study examines the influence of hot isostatic pressing and heat treatment on the microstructure and mechanical properties of samples manufactured by selective electron beam melting (SEBM) of metal powder composition (MPC fraction 40-100 pm) of a new six-component intermetallic beta-solidifying TiAl alloy Ti-44.5Al-2V-1Nb-2Cr-0.1Gd, at % (Ti-31.0Al-2.5V-2.5Nb-2.5Cr-0.4Gd, wt %). It is shown that SEBM with a high line energy input (El = 285 J/m) produces a fine-grained microstructure in the as-built material with a grain size of 5-14 pm and residual porosity of less than 0.5 vol %. An increase in the electron beam current (I) from 9.5 to 19.0 mA causes Al evaporation, and as a result the fraction of large columnar grains (d = 30-100 pm in width, h = 150-400 pm in height) formed mainly in Al-depleted regions (layers) increases. Heat treatment of the as-built SEBM samples by two-stage annealing in the (a + y) and (a2 + у + в) phase fields or by thermal cycling in the (a + y) phase field leads to complete or partial fragmentation of columnar grains. For the samples produced at lower values of I, a combined post-processing treatment by hot isostatic pressing in the а phase field and two-stage annealing completely eliminates residual porosity and transforms the columnar structure into a fine-grained one with a grain size of less than 150 pm. As a result, the achieved short-term mechanical characteristics at 20 °С (аВ = 525 ± 5 MPa, 5 = 1.1%) and 750 °С (аВ = 405 ± 10 MPa, 5 = 3.8%) are comparable to those of the studied TiAl alloy in the as-cast state.

A Real-Time Electron Beam Uniformity Measurement System for Irradiation Electron Accelerator: Design and Test

Electron accelerators are becoming increasingly significant in the irradiation industry. Beam uniformity is a critical accelerator parameter that influences the quality of irradiation processing technology. As a result, precise measurement of beam uniformity is necessary. In actual processing and production, the accelerator often needs to operate continuously under different beam current conditions. The aim of this work is to offer a new measurement system that can determine the beam uniformity of electron accelerators in real-time under various beam currents. The measurement system is implemented based on the intercept measurement method. The electron collection device is designed using the Monte Carlo simulation code FLUKA, and the electron beam collection efficiency is effectively calculated. To realize the measurement under large beam power conditions, the thermal–mechanical analysis of the collection device was carried out using the finite element simulation software ANSYS, and a cooling device was designed. A measurement device platform was created using the design parameters and tested under an industrial irradiation electron accelerator with a beam energy of 1.2 MeV and beam current of 0–18 mA. The test results showed that the device could measure the beam scanning distribution in real-time under different beam current conditions. In addition, the uniformity measurement results showed that under the same beam energy condition, as the beam current increases, the uniformity gradually decreases and tends to be stable, but the difference between the beam current at the two ends and the middle position increases. The effective design and successful test of the device make it possible to use the device to measure the scanning uniformity of electron accelerators in the future, and the measurement results will provide an essential reference value in the production and application of accelerators.

Initial Results From the SPIDER Beamlet Current Diagnostic

Source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER) is the full-scale prototype for the ITER heating neutral beam (HNB) source. Beam operation in SPIDER, with the eventual aim of achieving 100-keV, 46-A hydrogen negative ion beam for 1 h, has mainly been performed in volume operation without cesium. Recently, with the introduction of cesium, SPIDER has moved to the surface operational regime, with a noted increase in beam current. Electrical and calorimetric measurements of the beam current have previously shown discrepancies between the two, possibly due to background gas interactions in the accelerator and upstream of the STRIKE calorimeter. Taking advantage of the reduced number of open apertures due to the plasma grid (PG) mask, a series of fluxgate current transducers have been installed in SPIDER to measure the current of individual beamlets. These beamlet current monitors (BCMs), positioned downstream of the grounded grid (GG) around individual grid apertures, have provided direct measurement of the current of up to five individual beamlets during the recent SPIDER campaigns with and without cesium. This contribution describes the installation of the BCM dc sensors, analysis of the dc signal, and initial characterization of the beamlet direct current component in volume and surface operational regimes.

Experimental Investigations on Effects of Operation Parameters on a 263-GHz Gyrotron

In this article, the experimental investigations on a 263-GHz gyrotron are presented. With the change of the operating magnetic field and the beam energy, the operating frequency is adjusted and the output power is also varied. It is found that when the operating magnetic field changes from 9.59 to 9.82 T at a fixed operating voltage of 20 kV and a beam current of 0.8 A, the operating frequency increases from 263.39 to 264.84 GHz, the frequency-tuning range is about 1.45 GHz, and the output power is from 26 to 463 W. When the operating voltage increases from 14.4 to 22.2 kV at a fixed operating magnetic field of 9.65 T, the operating frequency decreases from 263.68 to 263.40 GHz, the frequency-tuning range is 0.28 GHz, and the output power changes from 52 to 424 W. When the operating magnetic field is fixed at 9.65 T and the operating voltage is fixed at 20 kV, but the beam current increases from 120 to 880 mA, the output power increases from 21 to 277 W, the operating frequency increases from 263.396 to 263.425 GHz, and the frequency-tunable range is 0.029 GHz. The effects of the axial position of the gyrotron in the superconducting magnet system on the operating frequency, the frequency-tuning range, and the output power are also investigated experimentally. When the beam current is increased, the nonstationary phenomenon is observed.

Collimator irradiation studies in the Argonne Advanced Photon Source at energy densities expected in next-generation storage ring light sources

The Advanced Photon Source (APS) team is building a fourth-generation storage ring (4GSR), replacing the present double-bend achromat lattice with a multibend achromat system thereby allowing the production of ultrabright x-ray beams. The new lattice enables a 2-order-of-magnitude reduction in horizontal beam emittance and a factor of two increase in beam current. The result is an electron beam of very high energy and power densities. Initial predictions suggest many common ultrahigh-vacuum-compatible materials struck by the full-intensity electron beam will be damaged. Two experimental beam abort studies have been conducted on collimator test pieces in the present APS SR to inform the design of a fully-functional machine protection system for APS 4GSR operations at 200 mA. A comprehensive suite of diagnostics was utilized during the studies. The diagnostics used in these experiments are not new, but employed in different ways to obtain unique data sets. With these sets now in hand, we are developing new numerical tools to guide collimator design using pelegant [M. Borland, elegant: A flexible SDDS-compliant code for accelerator simulation, Technical Report No. LS-287, Advanced Photon Source, 2000; Y. Wang and M. Borland, Implementation and performance of parallelized elegant, in Proceedings of the 2007 Particle Accelerator Conference, http://cern.ch/AccelConf/p07/PAPERS/THPAN095.PDF, pp. 3444--3446], mars [N. V. Mokhov and S. I. Striganov, Fermilab-conf-07/008-ad, AIP Conf. Proc. 896, 50 (2007)], and flash [B. Fryxell et al., flash: An adaptive mesh hydrodynamics code for modeling astrophysical thermonuclear flashes, Astrophys. J. Suppl. Ser. 131, 273 (2000); P. Tzeferacos et al., Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma, Nat. Commun. 9, 591 (2018)].

Effect of MeV ion induced heating on secondary electron emission from Si surface and estimation of local temperature

We report the variation of secondary-electron yield and target temperature with the variation of ion beam intensity during 100 MeV N+5 bombardment on Si surfaces. Theoretical understanding of secondary-electron emission during ion bombardment predicts no variation of yield with ion beam intensity. We discovered that the variation of secondary-electron yield arises due to the rise of local temperature at ion impact sites of the target. The secondary-electron yield first decreases with the increase of ion beam current but subsequently, it increases with the further rise of beam current. The initial decrease is due to the temperature-dependent change of electrochemical potential of Si. The considerable increase of the secondary-electron yield with a further increase of ion beam current is caused by local surface heating and subsequent additional thermionic electron emission. We have estimated the surface temperature of ion-impact sites by separating the thermionic electron current from the collisional secondary electron current.

Optimization of 2.45 GHz microwave coupling depending on axial magnetic field profiles and impedance transformer in electron cyclotron resonance ion source

The optimization of 2.45 GHz microwave coupling in an electron cyclotron resonance (ECR) ion source is investigated through experiments and full wave simulation studies. The axial magnetic field profile and an impedance transformer are the two major factors affecting the optimum power coupling. Experimental observations have shown that the beam current density increases when the two ECR zones are located in the plasma chamber and when the first ECR zone is close to the RF entrance. A coupling study with various magnetic field profiles finds that the gap distance between the RF window and first ECR zone and the effective ECR zone width determines the coupling efficiency. Optimum power coupling is obtained under the condition of an appropriately slight gap distance comparable to the half width of the effective ECR zone. The extracted beam current density increases when the second ECR zone becomes closer to the plasma grid. With the optimized axial magnetic field profile, impedance transformers are designed in consideration of reflection coefficient with respect to the plasma density. The performance of the impedance transformers is experimentally evaluated in terms of the efficiency (mA/W) with the findings consistent with the results of a simulation. The highest beam current density (∼165 mA/cm2) in these experiments is achieved with the optimized axial magnetic profile and the impedance transformer.

Analysis of liquation and solidification cracks in the electron beam welding of GTD-111 nickel-base superalloy joint

The weldability of GTD-111 superalloy by electron beam welding was studied, and the cracking evolution of solidification and liquation, as well as microstructural characteristics, were also investigated. The weld metal microstructure consisted mainly of the inter-dendritic phases of MC, γ-γ′ and γ′ nanoparticles, primarily responsible for the solidification crack in the weld metal. In addition to solidification cracks, the formation of liquation cracks in HAZ was an important challenge with electron beam welding. It was found that M5B3, γ-γ′, MC and γ′ phases leave formation during the increase of non-equilibrium temperature in HAZ, with the formation of partial liquation in the phase-matrix interface and absorption of shrinkage stresses due to welding cooling. Both solidification and liquation cracks decreased as the heat input increased due to the increase in beam current. This was attributed to the reduction of cooling stress and the increase of backfilling capacity.

Multi-scale simulation of solidification behavior and microstructure evolution during vacuum electron beam welding of Al-Cu alloy

A multi-scale model is established to study the complex melting and solidification behaviors during the electron beam welding of an Al-Cu alloy. The model combines a macroscopic thermal-fluid algorithm for molten pool behavior and a microscopic cellular automaton algorithm for the microstructure evolution. The solidification parameters, including the thermal gradient and cooling rate, build a bridge between the macroscopic and microscopic simulations. The model is validated by comparing the experimental fusion profile results, microstructure, morphology, grain size and element distribution that result from different welding parameters. On the macroscopic scale, with an increase in beam current, the molten pool fluctuation intensity increases, the thermal gradient increases and the cooling rate decreases. On the microscopic scale, the grain growth process includes nucleation, growth and competitive growth. After solidification, the concentration of Cu on the grain boundaries is larger than that inside the grains. With increasing beam current, the grain size tends to increase, and the segregation ratio decreases. The power-law function between the grain size and cooling rate can be summarized as d∝CR−0.55, where d is the grain size and CR is the cooling rate. Moreover, the segregation ratio first increases and then decreases slowly with decreasing solidification time. We expect this study to provide a deeper understanding of how to control weld formation and microstructure and improve confidence in optimizing the actual welding process.

Experimental Study on Microwave Generation due to Merged Instability in F-Band Surface Wave Oscillator

A surface wave oscillator (SWO) is driven by an electron beam to generate intense microwaves. The electron beam possesses slow space-charge (SSC) and slow cyclotron (SC) modes that interact with the surface wave leading to microwave generation. The beam current and external magnetic field affect the relationship between SSC and SC modes. The SSC mode gradually approaches the SC mode when the beam current increases. Meanwhile the SC mode gradually approaches the SSC mode when the magnetic field decreases. The two modes merge in a low magnetic field and high beam current. In this work, we experimentally examine the operation of an F-band SWO in the low magnetic field region. The output power decreases with low beam current when magnetic field decreases. Meanwhile, the SWO maintains its power level with high beam current even though the magnetic field decreases to around 0.4 T. The merged instability enables a sustained power level in the low magnetic field region.

The structures and thermoelectric properties of Zn-Sb alloy films fabricated by electron beam evaporation through an ion beam assisted deposition

Zn-Sb alloys are potential low-cost and non-toxic p-type thermoelectric materials for applications in the temperature range between 300 and 700 K. In this experiment, Zn-Sb alloy films were prepared by electron beam evaporation through an ion beam assisted deposition (IBAD). Our studies have confirmed that the structural phase, chemical composition, chemical binding, carrier concentration and microstructures of the film can indeed be effectively controlled by the voltage and current of the ion beam. Particularly, the carrier concentration of the film will rise along with the increase of the argon ion beam current. When the ion beam currents are set at 0.2–0.6 A, the carrier concentrations of the films can be controlled at around 1019–1020 cm−3, which fall within the optimal carrier concentration range for Zn-Sb based thermoelectric materials. The temperature dependence of Seebeck coefficient and the electrical conductivity of the films were measured to evaluate their thermoelectric performance. The results indicate that the film with Zn4Sb3 + ZnSb mixed phase will have better thermoelectric properties. A high power factor value of ~1280 μW/m-K2 is obtained in the films assisted by the ion beam current of 0.6 A. Our results demonstrate that the IBAD technique is extraordinary promising to fabricate Zn-Sb films with excellent thermoelectric performance and can be used to produce other potential thermoelectric materials.

Beam background study for the Belle II Silicon Vertex Detector

The Belle II experiment aims to accumulate 50ab−1 of e+e− collision data at the SuperKEKB asymmetric energy collider (Tsukuba, Japan). The first physics data using all Belle II detectors were taken in spring 2019.In the vast physics program of the Belle II experiment, the vertex detector plays a crucial role for the determination of the B-meson decay vertices. It consists of two inner layers of pixelated silicon detectors and four outer layers of double-sided silicon strip detectors (SVD).To achieve a design luminosity of 8×1035cm−2s−1, 40 times higher than the recorded luminosity of its predecessor, the SuperKEKB collider squeezes the beams to a vertical size of 50 nm (“nano-beam scheme”) and doubles the beam currents.Therefore, the detectors are required to tolerate intense beam induced background due to the very high luminosity. During the 2019 spring run we measured the occupancy rate in the SVD to estimate the level of the beam induced background. With the low initial luminosity, the observed beam induced background mostly originated from Touschek processes and beam-gas scattering within individual beams. Since these different background contributions depend differently on accelerator conditions, such as the beam current, beam size and pressure, they can be disentangled. We estimate the background rate of each contribution and compare them with simulated ones. The results enable us to predict the background levels at increased beam currents and luminosity in the coming years. They also hint at background mitigation measures for running at higher luminosity. In this proceeding we present the results of our study of the beam induced background in the SVD and the prospects for future operation.

Development of an ion beam measurement instrument for divertor simulation experiments in radio-frequency plasma

A retarding field analyzer (RFA) that consists of three grids and a collector was developed, and the measurement of an ion beam that passes through plasma was demonstrated. First, a suitable grid potential structure to allow the measurement of an ion beam in plasma was investigated. After this investigation, a helium ion beam was measured without the production of plasma. It was found that the helium ion beam current was significantly overestimated when an unoptimized potential structure was utilized. One probable reason for the overestimation is secondary electron emission. Next, ion beam measurement in low density helium ionizing plasma was conducted. Accompanying the onset of the beam extraction, the collector current clearly increased, which implies that the beam ions penetrated through the plasma and reached the RFA. Subsequently, similar measurements were conducted after the electron density of the helium plasma was changed. Since a nearly identical beam extraction condition was retained, the ion beam current obtained after plasma production was almost constant. However, the ion beam current obtained during plasma production increased as the electron density increased. A calculation of the ion beam envelope indicated that space charge neutralization by bulk electrons could account for the increase in the ion beam current.

Nonlinear excitation of parasitic modes in harmonic gyrotrons

The investigation of interaction between modes resonant with electrons at different cyclotron harmonics is presented. The study is focused on mode interaction in the region of currents lower than the start current of the parasitic mode at the fundamental. In this region, the excitation of such parasitic modes is possible due to nonlinear excitation of them by the harmonic modes only. It is shown that, when the beam current increases, various oscillation regimes replace each other: when the current exceeds the start current of the harmonic mode only slightly, the single-frequency oscillations of this mode are stable. At higher currents, due to nonlinear excitation, the oscillations of both modes with constant amplitudes are possible. Then, this regime is replaced by the auto-modulation of two modes with periodically varying mode amplitudes. Finally, at even higher currents, the parasitic mode at the fundamental suppresses the parental second harmonic mode. The role of two saturation mechanisms (the orbital phase bunching and the depletion of the free rotational energy in the process of beam-wave interaction) in the nonlinear excitation of parasitic modes at the fundamental is analyzed. The parallels between this set of events and events that take place in the prey–predator theory are discussed.

Selective Electron Beam Melting of TiAl Alloy: Metallurgical Defects, Tensile Property, and Determination of Process Window

Selective electron beam melting (SEBM) is a promising method to manufacture complex titanium aluminide (TiAl) alloy parts. Herein, the effects of beam current, scanning speed and energy density on surface appearance, porosity, and Al loss of Ti–47Al–2Cr–2Nb alloy samples fabricated by SEBM are investigated systematically. The porosity decreased initially and then increased, and Al loss increased gradually with an increase in beam current. The porosity increased continuously and the Al loss decreased gradually with an increase in scanning velocity. The tensile strength decreased when the porosity of SEBM‐produced TiAl alloy increased. The processing window for Ti–47Al–2Cr–2Nb alloy fabricated by SEBM is determined from the energy density and scanning speed, and the formation mechanisms of small pores and B2 phase precipitation around partially melted powder are explained in detail. The findings provide a valuable reference for fabricating Ti–47Al–2Cr–2Nb alloy by SEBM with a desired surface appearance, porosity and Al loss.