Fast ion beam-plasma interaction system

A device has been constructed for the study of the interaction between a fast ion beam and a target plasma of separately controllable parameters. The beam of either hydrogen or helium ions has an energy of 1-4 keV and a total current of 0.5-2 A. The beam energy and beam current can be varied separately. The ion source plasma is created by a pulsed (0.2-10-ms pulse length) discharge in neutral gas at up to 3 x 10(-3) Torr. The neutrals are pulsed into the source chamber, allowing the neutral pressure in the target region to remain less than 5 x 10(-5) Torr at a 2-Hz repetition rate. The creation of the source plasma can be described by a simple set of equations which predict optimum source design parameters. The target plasma is also produced by a pulsed discharge. Between the target and source chambers the beam is neutralized by electrons drawn from a set of hot filaments. Currently under study is an unstable wave in a field-free plasma excited when the beam velocity is nearly equal to the target electron thermal velocity (v(beam) approximately 3.5 x 10(7) cm/s, Te = 0.5 eV).     

An ultra-low energy (30-200 eV) ion-atomic beam source for ion-beam-assisted deposition in ultrahigh vacuum

The paper describes the design and construction of an ion-atomic beam source with an optimized generation of ions for ion-beam-assisted deposition under ultrahigh vacuum (UHV) conditions. The source combines an effusion cell and an electron impact ion source and produces ion beams with ultra-low energies in the range from 30 eV to 200 eV. Decreasing ion beam energy to hyperthermal values (≈10(1) eV) without loosing optimum ionization conditions has been mainly achieved by the incorporation of an ionization chamber with a grid transparent enough for electron and ion beams. In this way the energy and current density of nitrogen ion beams in the order of 10(1) eV and 10(1) nA/cm(2), respectively, have been achieved. The source is capable of growing ultrathin layers or nanostructures at ultra-low energies with a growth rate of several MLs/h. The ion-atomic beam source will be preferentially applied for the synthesis of GaN under UHV conditions.     

Ultraviolet stimulated electron source for use with low energy plasma instrument calibration

We have developed and demonstrated a versatile, compact electron source that can produce a mono-energetic electron beam up to 50 mm in diameter from 0.1 to 30 keV with an energy spread of <10 eV. By illuminating a metal cathode plate with a single near ultraviolet light emitting diode, a spatially uniform electron beam with 15% variation over 1 cm(2) can be generated. A uniform electric field in front of the cathode surface accelerates the electrons into a beam with an angular divergence of <1° at 1 keV. The beam intensity can be controlled from 10 to 10(9) electrons cm(-2) s(-1).     

Resolving power of lens-less low energy electron point source microscopy

Simulations show the resolving power of lens-less low energy electron point source microscopy to be approximately 15 Å  for electron energies of between 14 and 105 eV. This resolution can be improved to 10 Å  by employing a composite hologram method. However, these values fall short of predictions of at least 3 Å  resolution for electron energies in this range because the limited divergence angle of the electron beam had not previously been taken into account. Also shown is that electron coherence from field emitting tips that terminate in a single atom will not limit the resolving power as much as the beam divergence angle. The penetration depth of electrons with energies of between 50 and 100 eV, into biological molecules, was estimated from the inelastic mean free path length to be 25 Å . This places an upper limit on the size of biological molecules for which internal structural information can be obtained using low energy electrons.     

Characteristics of low-energy ion beams extracted from a wire electrode geometry

Beams of argon ions with energies less than 50 eV were extracted from an ion source through a wire electrode extractor geometry. A retarding potential energy analyzer (RPEA) was constructed in order to characterize the extracted ion beams. The single aperture RPEA was used to determine the ion energy distribution function, the mean ion energy and the ion beam energy spread. The multi-cusp hot cathode ion source was capable of producing a low electron temperature gas discharge to form quiescent plasmas from which ion beam energy as low as 5 eV was realized. At 50 V extraction potential and 0.1 A discharge current, the ion beam current density was around 0.37 mA/cm(2) with an energy spread of 3.6 V or 6.5% of the mean ion energy. The maximum ion beam current density extracted from the source was 0.57 mA/cm(2) for a 50 eV ion beam and 1.78 mA/cm(2) for a 100 eV ion beam.     

Characterization of plasma parameters, first beam results, and status of electron cyclotron resonance source

Electron cyclotron resonance (ECR) plasma source at 50 keV, 30 mA proton current has been designed, fabricated, and assembled. Its plasma study has been done. Plasma chamber was excited with 350 W of microwave power at 2450 MHz, along with nitrogen and hydrogen gases. Microwave power was fed to the plasma chamber through waveguide. Plasma density and electron temperature were studied under various operating conditions, such as magnetic field, gas pressure, and transversal distance. Langmuir probe was used for plasma characterization using current-voltage variation. The nitrogen plasma density calculated was approximately 4.5 x 10(11) cm(-3), and electron temperatures of 3-10 eV (cold) and 45-85 eV (hot) were obtained. The total ion beam current of 2.5 mA was extracted, with two-electrode extraction geometry, at 15 keV beam energy. The optimization of the source is under progress to extract 30 mA proton beam current at 50 keV beam energy, using three-electrode extraction geometry. This source will be used as an injector to continuous wave radio frequency quadrupole, a part of 100 MeV proton linac. The required root-mean-square normalized beam emittance is less than 0.2pi mm mrad. This article presents the study of plasma parameters, first beam results, and status of ECR proton source.     

Reflection-time-of-flight spectrometer for two-electron (e,2e) coincidence spectroscopy on surfaces

In this article, a novel time-of-flight spectrometer for two-electron-emission (e,2egamma,2e) correlation spectroscopy from surfaces at low electron energies is presented. The spectrometer consists of electron optics that collect emitted electrons over a solid angle of approximately 1 sr and focus them onto a multichannel plate using a reflection technique. The flight time of an electron with kinetic energy of Ekin approximately 25 eV is around 100 ns. The corresponding time- and energy resolution are typically approximately 1 ns and approximately 0.65 eV, respectively. The first (e,2e) data obtained with the present setup from a LiF film are presented.     

Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions

The growth of ultrathin films, deposited by laser ablation, crucially depends on the energy of the ablated species. Therefore, a time-of-flight (TOF) spectrometer has been constructed and measurements have been carried out in order to determine the energy distribution of laser-ablated Fe and Pt atoms and ions in the plasma created by nanosecond pulses of a frequency-doubled neodymium doped yttrium aluminum garnet laser. The experiments have been performed in ultrahigh vacuum at relatively low laser power. For measuring the spectra of the neutrals, a cross-beam electron source for postionization and electric as well as magnetic fields for repelling the ions are employed. Nevertheless, measurements of neutral particles are restricted to low plasma densities due to electrostatic shielding within the plasma, leading to an inefficient deflection of charged particles by electrostatic and magnetic fields. Test measurements have been performed by utilizing the TOF spectrometer as a pressure gauge and also by chopping the electron beam, running the TOF spectrometer as a residual gas mass spectrometer. The spectra of the laser-ablated plasmas have shown plasma conditions with a Debye length of approximately 10(-4) m, densities of 10(15)-10(16) m(-3) and ion energies up to 150 eV. Neutral spectra have shown an unexpectedly low fraction of neutrals (10(-3)-10(-4)) and hyperthermal energies up to several 10 eV, possibly contributed by recombination of ions and electrons in the plasma. Even though gas spectra had demonstrated the expected sensitivity of the TOF spectrometer for low-energy neutrals, no thermally evaporated neutral atoms could be found.     

16厘米低能强流宽束离子源

介绍了一个新型16厘米束径多会切磁场低能强流宽束离子源(MCLB-16)。由于采用新型多会切磁场和优化的低能引出系统,所以该源在薄膜辅助沉积的能量(200eV～800eV)范围内,具有较好的低能特性。源的最大引出束流可达650mA。可用反应气体或惰性气体工作。源在使用氧气时,可连续工作数十小时。该源可用于各种高性能薄膜制备的辅助沉积,也可用于制备大面积类金钢石膜(DLC)。叙述了该源的结构及性能。     

A photoelectron-photoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of sigma=18 ps and energy resolution Delta E/E=3.5%

We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5 microm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to sigma=18 ps. We observed that our electron coincidence detector has a timing resolution better than sigma=16 ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of DeltaE(FWHM)/E=3.5% for electrons with kinetic energy near 2 eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about Deltam(FWHM)/m=14150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to sigma=115 microm.     

Description of a polarized source of He i radiation for surface studies

A polarized radiation source has been designed and built for use in angular-resolved photoelectron spectroscopy on surfaces of single crystals under ultrahigh vacuum. The light from a discharge in helium is polarized by a triple reflection from a series of gold mirrors, producing a plane-polarized beam of He i radiation (21.22 eV) with a polarization of 80%. The plane of polarization can be rotated through more than 180 degrees without breaking vacuum. Details of the construction of the lamps are given, together with how it is integrated into the overall operation of the electron spectrometer. As an illustration of the lamp's potential, data are shown on the photoemission studies of the (110) and (111) faces of copper as a function of both the orientation angle of the crystal and the polarization vector. A brief discussion of the future development and possible uses of the lamp is included.     

Time-of-flight electron velocity spectrometer using transverse electric field deflectors

A time-of-flight electron velocity spectrometer using two transverse electric field deflectors as fast electronic shutters has been constructed. Square-wave voltage pulses applied to the two shutters limit the transmitted current to electrons in selected narrow velocity ranges. Analysis of the magnitude of this transmitted electron current as a function of the time delay between the two shutter openings leads to the electron velocity or energy spectrum in the incident charged particle beam. Time resolutions of 5 ns (FWHM) can be obtained with the present apparatus for electrons in the 5-100 eV range.     

Diffraction of 0.5keV electrons from free-standing transmission gratings

A nanostructured grating was used to diffract a low-energy (500 eV) electron beam, and the current transmitted into the zeroth diffraction order was greater than 5% of the incident beam current. This diffraction efficiency indicates that the 55-nm-wide grating bars absorb electrons but the 45-nm-wide slots between bars transmit electron de Broglie waves coherently. The diffraction patterns can be asymmetric, and can be explained by a model that incorporates an electrostatic potential energy for electrons within 20 nm of the grating structure calculated by the method of images.     

A cryogenic source of slow monochromatic positrons

A cryogenic source of slow monochromatic positrons based on ²²Na radioactive isotope has been developed and produced at the Joint Institute for Nuclear Research. A monochromatic beam is formed from a continuous β⁺ spectrum with energies of 0–0.5 MeV using a solid neon moderator frozen onto a copper substrate that is cooled to temperatures of 5–7 K. The efficiency of condensation of neon onto the substrate is >99.8%. A slow positron beam with an intensity of 5.8 × 10³ particles/s and a mean energy of 1.2 eV at a spectrum width of 1 eV has been extracted from a ²²Na-based test source. The fraction of decelerated positrons is 1% of the total flux.     

Development of a low-energy and high-current pulsed neutral beam injector with a washer-gun plasma source for high-beta plasma experiments

We have developed a novel and economical neutral-beam injection system by employing a washer-gun plasma source. It provides a low-cost and maintenance-free ion beam, thus eliminating the need for the filaments and water-cooling systems employed conventionally. In our primary experiments, the washer gun produced a source plasma with an electron temperature of approximately 5 eV and an electron density of 5 × 10(17) m(-3), i.e., conditions suitable for ion-beam extraction. The dependence of the extracted beam current on the acceleration voltage is consistent with space-charge current limitation, because the observed current density is almost proportional to the 3∕2 power of the acceleration voltage below approximately 8 kV. By optimizing plasma formation, we successfully achieved beam extraction of up to 40 A at 15 kV and a pulse length in excess of 0.25 ms. Its low-voltage and high-current pulsed-beam properties enable us to apply this high-power neutral beam injection into a high-beta compact torus plasma characterized by a low magnetic field.     

Plasma enclosed in a magnetic field produced by flexible surface magnets

A homogeneous steady state plasma with a usable volume of approximately 200 l and with an electron temperature of 1-2 eV and a plasma density of approximately 10(9)-10(10) cm(-3) is produced in a discharge chamber the outside of whose walls is covered with flexible magnetic strips. This magnet arrangement can be built at a fraction of the cost of a conventional system using rigid surface magnets. The magnetic multipole field leads to an increase of the plasma density by one to two orders of magnitude and it is also found to cause trapping of high energy electrons originating from the discharge region.     

A differentially pumped low-energy ion beam system for an ultrahigh-vacuum atom-probe field-ion microscope

An ultrahigh-vacuum (UHV) differentially pumped low-energy (50-3000 eV) ion beam system for the in situ irradiation of specimens in a UHV atom-probe field-ion microscope (FIM) was designed and constructed. The ion beam system consisted of a Finkelstein-type ion source, an Einzel lens, and a magnetic mass analyzer. The ion source was connected to the analyzer chamber by small apertures which resulted in differential pumping between the ion source and the analyzer chamber; during a typical in situ irradiation of a specimen in the atom-probe FIM the total pressure was maintained at approximately 10(-7) Torr. In the case of helium ion irradiation the optimum ion-current density was approximately 0.5 microA cm(-2) for 300-eV He+ ions at the atom-probe FIM specimen. After the completion of a helium ion irradiation the pumpdown time from 5 x 10(-7) to approximately 3 x 10(-10) Torr in the atom-probe FIM chamber was 0.5 h.     

Reactions of polypeptide ions with electrons in the gas phase

Reactions of electrons in the energy range below 70 eV with polypeptide cations and anions are reviewed, as well as their applications for the structural analysis of polypeptides. At very low energies (相似文献   

Complex diagnostics of a high-current accelerator during generation of high-power bremsstrahlung

A diagnostic system has been developed for the YHH-10 high-current electron accelerator with a beam current of ≤ 60 kA, an electron energy of ≤4 MeV, and a pulse duration of ≤80 ns. The X-ray diagnostic method is used in the diagnostic system to determine the energy of accelerated electrons at the entrance into the converter target. A measuring transducer of the electron energy is based on determining the static transfer characteristic of the converter target, which relates (under the known conditions) the energy of accelerated electrons to the beam current and the dose rate at a selected point of the bremsstrahlung field on the electron beam transport axis. Physically, the transducer is located in the target unit of the accelerator.