Ion Beam Density Research Articles

A Conical Ion Diode with Self-Magnetic Insulation of Electrons

The results of studying the generation of a gigawatt-power pulsed ion beam formed by a diode in the mode of self-magnetic insulation of electrons are presented. Studies were carried out at the TEMP-4M accelerator in the mode of generating two pulses: the first is negative (500 ns, 150–200 kV) and the second is positive (150 ns, 250–300 kV). The formation of anode plasma occurs during the explosive emission of electrons during the first pulse. To improve the efficiency of generating an ion current, a conical geometry of the diode was proposed in which the electron-drift length exceeds that in the previous diode structures by factor of 2. It was found that the energy efficiency in the conical diode increased to 15–17%, while the ion-beam energy density at the focus increased to 2–3 J/cm2; the beam consisted of protons and carbon ions. The efficiency of suppression of the electron component of the total current in the diode was analyzed and the calculations of the electron-drift duration and ion acceleration were performed. It is shown that in the diode of the new design, efficient plasma formation occurs on the entire working surface of the graphite anode, while the plasma concentration may limit the ion current.

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Using an electrostatic double probe, profiles of the ion beam current density, electron density, and electron temperature were measured in the background plasma that appears by injecting a hydrogen ion beam into low-pressure hydrogen gas in a propagation chamber. The hydrogen ion beam is extracted using concave-shaped electrodes at extremely low energies (Eib = 60–120 eV). Focusing of the beam occurs when Eib exceeds a certain threshold. One probe electrode P3 is far from the beam source and is set in the shadow of another electrode in the beam trajectory. The ion saturation current in P3 is then estimated without considering the beam contribution. The measured electron densities are much larger than those of the ion beam density, the electron temperatures are very low (<1 eV), and the ion beam current densities exhibit fairly good agreement with those measured by Faraday cups. The profiles of the space potential are also estimated by measuring the potential of P3 with respect to ground with a voltage divider having an extremely high resistance. The space potentials obtained are quite low at <10 eV with focusing and ∼23 eV without focusing. The data with and without focusing are compared and conditions for focusing are examined. Focusing achieved through additional electron beam injection in the ground electrode is also examined. The results obtained indicate that a large electron source is required to balance the ion charges. Secondary electron emissions and/or small electron beam injection are effective sources.

The study of operation modes of the self-magnetically insulated ion diode.

The operation modes of a self-magnetically insulated ion diode were studied. The plasma formation at the anode surface was found to be accompanied by an electron leakage current due to the bipolar high voltage pulse. The emitting anode surface and ion beam density distribution were investigated at the ion diode output area. The characteristics of the electron leakage current are presented depending on the ion diode operation mode. The area of the emitting anode surface was approximately 25%-30% of the total anode area. The electron leakage current reaches up to 6.5 kA in the ion diode.

Charge dissipation and self focusing limit in high current density ion beam transport through a micro glass capillary

Charge dissipation and self focusing limit in high current density ion beam transport through micro glass capillaries are investigated. It is demonstrated that ion beams can be guided efficiently (capillary exit diameters; straight capillary (SC): 860 µm, tapered capillary (TC): 500 µm, 300 µm, 200 µm, 110 µm, 45 µm and 20 µm) with minimal loss of beam current, and thereby transport of ion beams (J ~ 600 Am−2) become possible, over a tilt angle of 5° from the incident beam axis. The investigation reveals the existence of a lower limit on the capillary size, below which Coulomb repulsion of the beam’s space charge dominates over the inward radial forces of the charges that get smeared on the capillary inner wall, and the beam can no longer self-focus, although the beam may be transported. Beam size compression at the capillary exit by as much as 81% could be achieved. A theoretical model validates the result and the self focusing factor tends to 1 for smaller capillary outlet. Charge dissipation is evaluated during a hysteresis cycle of beam current with ion energy, and it is found that maximum charge dissipation occurs for the 300 µm capillary outlet (~6 × 1013 electronic charge). A nonlinear behaviour of the beam spot diameter with the capillary outlet is observed. Particle in cell (PIC) simulations are performed to interpret results.

Investigation of sacrificial layer masking fabrication of dual-frequency quartz crystal microbalance

To solve temperature compensation problems and eliminate environmental impact factors for quartz crystal resonators, a study on one-chip inverted mesa multi-channel/frequency quartz crystal microbalance (MQCM) was presented. MQCM was fabricated by a simple, precision controlled and plasma-ion-beam etching technique. The research intention was devoted to etching nanometer-scale depth inverted mesa of a quartz wafer using aluminum (Al) as a sacrificial layer. The masking method was demonstrated with 0.497 mA/cm2 ion-beam density, 300 eV argon-ion energy and 1.0 μm/h etching rate. Both morphology of etched surface and its depth were characterized by laser scanning confocal microscope (LSCM). Resonant frequencies of a fabricated MQCM indicated that the thinning approach demonstrated will fine tune relevant QCM resonant frequency as proposed. The one-chip dual-QCM in the case of 3.28 μm depth etching would result in its fundamental resonance peak shifted at 0.5054 MHz. The inverted mesa etched at micron level significantly will provide fine-tunable frequencies on one-chip monolithic device fabricated. The simple adhesion-type Al sacrificial layer masking method and plasma etching technique do have great potential for precise micro-nano scale structure fabrications.

Characterization of the Ion Beam Current Density of the RF Ion Source with Flat and Convex Extraction Systems

The characterization of ion beam current density distribution and beam uniformity is crucial for improving broad-beam ion source technologies. The design of the broad ion beam extraction system directly affects these two parameters, therefore, depending on the application, the design and geometry of the source is changed. In this study, the effect of the presence or the absence of a neutralization process on the ion beam density was investigated. Also, the effect of the probe bias on the ion beam current measurement was evaluated. Eventually, using a flat probe, the ion beam profiles obtained from the extraction system of the ion source (Model RFIS 60, ACECR, Iran) were measured at energies from 300 eV to 1000 eV and compared in terms of current and uniformity of the ion beam in the absence of the neutralizer. In these experiments, screen and accelerator grids with different geometries (flat and convex grids) were used for comparison purposes. According to the obtained results, the flat grids generate a higher current density while the convex grids generate a more uniform beam. Therefore, each of these grid geometries can be suitable depending on the desired application.

Effect of Ion Beam on Dust-Acoustic Waves Under Transverse Perturbations in Dusty Plasma

The dynamics of dust-acoustic waves (DAWs) is studied in the present investigation under transverse perturbations. We have considered dusty plasma consisting of negatively charged dust fluid, non-Maxwellian electrons, and ions embedded with positive ion beam. The Kadomtsev–Petviashvili (KP) equation is derived using a reductive perturbation method. From the solution of KP equation, the characteristics of DAWs have been studied under the influence of various plasma parameters in the presence of beam. It is noticed that characteristics (amplitude, width, and energy) of DAWs are very sensitive to the variation of the values of plasma parameters, such as ion density, beam density, beam velocity, and superthermality of ions and electrons.

Effect of the helically-trapped energetic-ion-driven resistive interchange modes on energetic ion confinement in the Large Helical Device

The effect of the helically-trapped energetic-ion-driven resistive interchange modes (EICs) on energetic ion confinement is studied in the Large Helical Device deuterium plasmas. Neutron diagnostics such as the neutron flux monitor and the vertical neutron camera (VNC) are used in order to measure neutrons mainly created by beam-plasma reactions. The line-integrated neutron profiles are obtained by VNC in magnetohydrodynamic-quiet plasma with various neutral beam (NB) injection patterns. The profiles are consistent with that expected by the beam ion density calculated using orbit-following simulations. Significant decreases of the total neutron emission rate (Sn) and the neutron counting rate of the VNC (Cn) in central cords are observed to be synchronized with EIC bursts with perpendicular-NB injection. The drop rates of both Sn and Cn increase with EIC amplitude and reach around 50%. The line-integrated neutron profiles before and after EIC burst show that in the central cords, Cn decrease due to EIC burst whereas there is almost no change in the other cords. The experimental results suggests that the effect of EIC on helically-trapped beam ion is substantial, however the effect of passing beam ion is not significant.

Generation of forerunner electron beam during interaction of ion beam pulse with plasma

The long-time evolution of the two-stream instability of a cold tenuous ion beam pulse propagating through the background plasma with density much higher than the ion beam density is investigated using a large-scale one-dimensional electrostatic kinetic simulation. The three stages of the instability are investigated in detail. After the initial linear growth and saturation by the electron trapping, a portion of the initially trapped electrons becomes detrapped and moves ahead of the ion beam pulse forming a forerunner electron beam, which causes a secondary two-stream instability that preheats the upstream plasma electrons. Consequently, the self-consistent nonlinear-driven turbulent state is set up at the head of the ion beam pulse with the saturated plasma wave sustained by the influx of the cold electrons from upstream of the beam that lasts until the final stage when the beam ions become trapped by the plasma wave. The beam ion trapping leads to the nonlinear heating of the beam ions that eventually extinguishes the instability.

Effect of ion beam on the characteristics of ion acoustic Gardner solitons and double layers in a multicomponent superthermal plasma

The nonlinear propagation of ion acoustic solitary waves (IASWs) is investigated in an unmagnetized plasma composed of a positive warm ion fluid, two temperature electrons obeying kappa type distribution and penetrated by a positive ion beam. The reductive perturbation method is used to derive the nonlinear equations, namely, Korteweg-de Vries (KdV), modified KdV (mKdV), and Gardner equations. The characteristic features of both compressive and rarefactive nonlinear excitations from the solution of these equations are studied and compared in the context with the observation of the He+ beam in the polar cap region near solar maximum by the Dynamics Explorer 1 satellite. It is observed that the superthermality and density of cold electrons, number density, and temperature of the positive ion beam crucially modify the basic properties of compressive and rarefactive IASWs in the KdV and mKdV regimes. It is further analyzed that the amplitude and width of Gardner solitons are appreciably affected by different plasma parameters. The characteristics of double layers are also studied in detail below the critical density of cold electrons. The theoretical results may be useful for the observation of nonlinear excitations in laboratory and ion beam driven plasmas in the polar cap region near solar maximum and polar ionosphere as well in Saturn's magnetosphere, solar wind, pulsar magnetosphere, etc., where the population of two temperature superthermal electrons is present.

The ion streaming effect on Weibel instability in the relativistic dense plasma

The Weibel instability plays an important role in stopping hot electrons and energy deposition mechanism in fast ignition of inertial fusion process. In this paper, the ion Weibel instability in counter propagating electron‐ion plasmas is investigate. The obtained results show that the growth rate of Weibel instability will be decreased about 40% with the anisotropy velocity as vxe = 2vze = 20; the ion density ratio, b = n0i1/n0i2, and density gradient, are increasing 50 and 90% respectively. The ion streaming in density gradient of dense plasma leads to increasing the Weibel instability growth rate and its amplification through ion streaming in the large wavenumber. The maximum unstable wavenumber has been decreased with decreasing the ion beam density ratio. For fixed ion density ratio, increasing 90% of the density gradient in the near of fuel plasma corona leads to reducing growth rate and unstable wavenumber about 43 and 42% respectively.

Crystallization mechanism of Fe78Si13B9 amorphous alloy induced by ion bombardment

The crystallization behavior of Fe78Si13B9 amorphous alloys induced by Ar ion bombardment was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. The crystallization process of amorphous alloys can be controlled by adjustment of ion incident angles (θ), beam density and substrate temperature. The compressive stress caused by ion bombardment will result in the decrease of critical nucleation energy and induce the crystallization of amorphous alloy even at very low temperature, and the thermal effect converted from energetic ions will improve the crystallization of amorphous alloys. The crystallization process of amorphous alloy induced by ion bombardment was a stress induced phase transformation process assisted by thermal effect.

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

Experiments in the DIII-D tokamak show that many overlapping small-amplitude Alfvén eigenmodes (AEs) cause fast-ion transport to sharply increase above a critical threshold in beam power, leading to fast-ion density profile resilience and reduced fusion performance. The threshold is above the AE linear stability limit and varies between diagnostics that are sensitive to different parts of fast-ion phase-space. Comparison with theoretical analysis using the nova and orbit codes shows that, for the neutral particle diagnostic, the threshold corresponds to the onset of stochastic particle orbits due to wave-particle resonances with AEs in the measured region of phase space. The bulk fast-ion distribution and instability behavior was manipulated through variations in beam deposition geometry, and no significant differences in the onset threshold outside of measurement uncertainties were found, in agreement with the theoretical stochastic threshold analysis. Simulations using the ‘kick model’ produce beam ion density gradients consistent with the empirically measured radial critical gradient and highlight the importance of including the energy and pitch dependence of the fast-ion distribution function in critical gradient models. The addition of electron cyclotron heating changes the types of AEs present in the experiment, comparatively increasing the measured fast-ion density and radial gradient. These studies provide the basis for understanding how to avoid AE transport that can undesirably redistribute current and cause fast-ion losses, and the measurements are being used to validate AE-induced transport models that use the critical gradient paradigm, giving greater confidence when applied to ITER.

The Research of ZnSe Aspheric Vacuum Plasma Sputtering Polishing Surface Roughness

In this paper, ZnSe aspheric optical element in the vacuum plasma polishing technology was studied, the paper mainly discusses the principle of plasma sputtering polishing and the effects of process parameters on ZnSe aspheric surface roughness. Firstly, the relationship between sputtering yield and ion energy and the relationship between density of the beam and the incident angle were gained by simulation research with the use of the SRIM software. And the relationship between polishing efficiency and process parameters was studied. Secondly, the relationship between polishing result and polishing rate under different process parameters was studied by the vacuum plasma polishing removal experiment with the use of the vacuum plasma equipment. The effects of rf power, ion beam energy and beam density on the polishing result were studied. The relationship between plasma characteristics and substrate surface roughness after polishing and polishing efficiency were established. Finally, with the mechanism of plasma polishing gained by simulation and experimental, the vacuum plasma polishing process parameters were obtained.

Ion beam induced luminescence spectra of lithium fluoride at high-and low-temperature

A new ion beam induced luminescence (IBIL) measuring setup, equipped with a custom-made heating/cooling sample stage (the attainable temperature ranges from 80 K to 900 K), has been established on the GIC4117 tandem accelerator in Beijing Normal University. As the yield of back scattering ions is proportional to the beam flux, an Au-Si surface barrier detector is employed to count the back scattering ions synchronously with collecting the IBIL spectra under the multi-channel scaler (MCS) mode of the multichannel analyzer, making it possible to online monitor the beam current. Then, the yield of back scattering ions is used to correct the intensity of the IBIL spectrum and calculate the ion fluence, for eliminating the influence of the beam current fluctuation. IBIL spectra of pure lithium fluoride (LiF) at different temperatures (100, 200, 290, 450, 550 K) under the 2 MeV H+ irradiation are acquired and the significant influence of temperature on luminescence centers is observed. The emission bands relating to exciton recombination (296 and 340 nm) and impurities (400 nm) are more prominent at low temperatures and present quite lower intensities at high temperatures. Moreover, these luminescent intensities decay with ion fluence increase obviously at high temperatures after initially increasing in the early period of irradiation. The initial increase of the disturbed exciton peak at 296 nm can be attributed to the strained bonds produced by nuclear elastic scattering at a low fluence, which was not observed in previous IBIL measurements under high ionization energy density or high ion beam flux. This observed increase indicates that the emission feature may also originate from the emitting centers relating to point defects, not just from exciton transition near lattice or impurities. The luminescent intensities of F2 color centers (peaked at 670 nm) are dominant at all temperatures, while the luminescent intensities of F3+ color centers (peaked at 540 nm) are not obvious at low temperatures and the luminescent intensities of F3-/F2+ color centers (peaked at 880 nm) are weak at high temperatures. The luminescent intensities of these F-type centers reach saturated values at lower fluences at high temperatures. The different evolution behaviors under different temperatures can be due to the influence of temperature on the vacancy migration rate and the non-radiative recombination. In addition, the surface charge accumulation may lead to the luminescent intensities of color centers reaching saturated values at higher fluences, compared with the previous IBIL measurements of LiF. The self-absorption effect would reduce the intensities of F3+ color centers because of the absorption of F-type centers at low temperatures, while the effect is weak at high temperatures due to the degradation of F-type centers.

Investigation of the homogeneity of a high-power ion beam formed by a diode with a closed electron drift

The results of an investigation of the energy-density distribution over the cross section of a pulsed ion beam formed with a passive-anode diode in the mode of magnetic insulation and a closed electron drift in the anode–cathode gap are presented. Diodes of two types are studied: with external magnetic insulation (Br diode) on the BIPPAB-450 accelerator (400 kV, 80 ns) and self-magnetic insulation of electrons (spiral diode) on the TEMP-4M accelerator (250 kV, 120 ns). In the investigated diodes, various processes are used to form anode plasma: a breakdown over the surface of a dielectric coating on the anode and ionization of the anode surface with accelerated electrons (Br diode), as well as explosive emission of electrons (spiral diode). To analyze the ion-beam energy density, thermal-imaging diagnostics is used with a spatial resolution of 1–2 mm. The energy-density is calculated from the one-dimensional Child–Langmuir relationship. It is shown that a continuous plasma layer is efficiently formed on the working anode surface for all the investigated diodes. The anode-plasma concentration is rather high, and the beam-energy density is limited by the space charge of ions, but not by the plasma concentration. It is found that, when the magnetic field in the Br-diode anode–cathode gap decreases or the electron current in the spiral diode increases, the energy density of the high-power ion beam rises significantly, but the beam homogeneity decreases.

Boron carbide coating deposition on tungsten substrates from atomic fluxes of boron and carbon

A device used for both coating deposition and material testing is presented in the paper. By using lock chambers, sputtering targets are easily exchanged with sample holder thus allowing testing of deposited samples with high power density electron or ion beams. Boron carbide coatings were deposited on tungsten samples. Methods of increasing coating adhesion are described in the paper. 2 μm boron carbide coatings sustained 450 heating cycles from 100 to 900 C. Ion beam tests have shown satisfactory results.

MHD Simulation for Neutron Yield, Radiations and Beam-Ion Properties in the Spherical Plasma Focus

The developed spherical plasma focus model is used to investigate the neutron production and radiations emitted at the pinch. Theory of the neutron yield and radiations emitted in the radiative phase of the spherical plasma focus device is included in this paper. Since the neutron yield in the spherical plasma focus is from the beam-target neutron production mechanism, the beam-target neutron production is used for calculating the neutron yield. The achieved neutron yield with 14.5 Torr of DT mixture (1:1) filling gas, 25 kV of charging voltage and 432 muF of capacitor bank is 1.3 × 103. This result agrees to a large extend with other experimental measured value. The effect of the bremsstrahlung, radiative recombination and line radiations as the energy loss terms and of the Joule heating with the plasma resistance as the energy gain term on the plasma column is investigated. Plasma-ion density, beam-ion density and beam-ion speed are calculated under the diode voltage in the radiative phase of the spherical plasma focus.

High current density ion beam obtained by a transition to a highly focused state in extremely low-energy region

A high current density (≈3 mA/cm(2)) hydrogen ion beam source operating in an extremely low-energy region (E(ib) ≈ 150-200 eV) has been realized by using a transition to a highly focused state, where the beam is extracted from the ion source chamber through three concave electrodes with nominal focal lengths of ≈350 mm. The transition occurs when the beam energy exceeds a threshold value between 145 and 170 eV. Low-level hysteresis is observed in the transition when E(ib) is being reduced. The radial profiles of the ion beam current density and the low temperature ion current density can be obtained separately using a Faraday cup with a grid in front. The measured profiles confirm that more than a half of the extracted beam ions reaches the target plate with a good focusing profile with a full width at half maximum of ≈3 cm. Estimation of the particle balances in beam ions, the slow ions, and the electrons indicates the possibility that the secondary electron emission from the target plate and electron impact ionization of hydrogen may play roles as particle sources in this extremely low-energy beam after the compensation of beam ion space charge.

Simulation study of high-frequency energetic particle driven geodesic acoustic mode

High-frequency energetic particle driven geodesic acoustic modes (EGAM) observed in the large helical device plasmas are investigated using a hybrid simulation code for energetic particles and magnetohydrodynamics (MHD). Energetic particle inertia is incorporated in the MHD momentum equation for the simulation where the beam ion density is comparable to the bulk plasma density. Bump-on-tail type beam ion velocity distribution created by slowing down and charge exchange is considered. It is demonstrated that EGAMs have frequencies higher than the geodesic acoustic modes and the dependence on bulk plasma temperature is weak if (1) energetic particle density is comparable to the bulk plasma density and (2) charge exchange time (τcx) is sufficiently shorter than the slowing down time (τs) to create a bump-on-tail type distribution. The frequency of high-frequency EGAM rises as the energetic particle pressure increases under the condition of high energetic particle pressure. The frequency also increases as the energetic particle pitch angle distribution shifts to higher transit frequency. It is found that there are two kinds of particles resonant with EGAM: (1) trapped particles and (2) passing particles with transit frequency close to the mode frequency. The EGAMs investigated in this work are destabilized primarily by the passing particles whose transit frequencies are close to the EGAM frequency.