Cesium Injection Research Articles

Analysis and optimization of cesium dynamics for the HUST negative ion source

The high-density beam in the NBI negative ion source relies on surface production on the converter with low work function, which is facilitated by cesium injection. Maintaining the appropriate cesium conditions during the source operation sequence is essential for achieving optimal cesium coverage on the converter. To better understand cesium cycle in the source and to enhance beam production performance during operation sequences, analysis of cesium dynamics and optimization of cesium operation have been carried out. The analysis model is established based on the Monte-Carlo method with cesium sources, cesium transport and surface process during the operation sequence considered. Cesium distribution and converter surface coverage can be obtained from the model using input parameters from the HUST negative ion source. For the operation in the HUST negative ion source, the influence of temperature settings and time sequence is discussed, with suggested parameters for cesium injection proposed to achieve appropriate cesium coverage on converter surface through operation sequences.

Experimental results of a magnetic field modification to the radio frequency driver of a negative ion source

A magnetic field produced by a current flowing through the plasma grid (PG) is one of the solutions to reduce the collisional loss of negative ions in a negative ion source, which reduces the electron temperature in front of the PG. However, the magnetic field diffused into the driver has some influence on the plasma outflowing. In order to investigate the effect of changing this magnetic field on the outflowing of plasma from the driver, a circular ring (absorber) of high permeability iron has been introduced at the driver exit, which can reduce the magnetic field around it and improve plasma outflowing. With the application of the absorber, the electron density is increased by about 35%, and the extraction current measured from the extraction grid is increased from 1.02 A to 1.29 A. The results of the extraction experiment with cesium injection show that both the extraction grid (EG) current and H− current are increased when the absorber is introduced.

Beam extraction under Cs-free conditions in HUST negative ion source

A radio-frequency driven negative hydrogen ion source is under development at Huazhong University of Science and Technology, which aims to investigate the physics of production and extraction of negative ions. The source operational parameters are investigated to investigate the source plasma characteristics and to optimize the extraction properties before cesium injection, which means, in this study, that negative ions were mostly produced by reactions in the plasma volume. The magnetic filter field generated by a plasma grid (PG) current flowing through the PG can cool down the electron temperature below 1 eV, but simultaneously increased the plasma density in the extraction region. The plasma asymmetry in the direction perpendicular to the PG filter field, is discussed and compared with that in other ion sources. As the PG current increased, the asymmetry factors tended to increase first and then decrease. Biasing the PG positively with respect to the source body can reduce the co-extracted electrons effectively, resulting in reducing the electron to H− ion current ratio. An extracted negative beam with a current density of about 19 A m−2 and an energy of 12 keV was achieved under 20 kW RF power and 0.3 Pa filling pressure, with a ratio of electron current to ion current of about 54.

SPIDER, the Negative Ion Source Prototype for ITER: Overview of Operations and Cesium Injection

An overview of the recent operations and the main results of cesium injection in the Source for the Production of Ions of Deuterium Extracted from Rf plasma (SPIDER) negative ion source are described in this contribution. In experiments without cesium injection, all SPIDER plants were tested to verify the basic expectations on the operational parameters (e.g., electron cooling effectiveness of magnetic filter field) and to determine its operational region. For beam properties, it was shown that the current density varies across the beam in the vertical direction. In preliminary cesium experiments, the expected increase of negative ion current and simultaneous decrease of co-extracted electrons were found, along with the influence of the control parameters (polarization of the plasma electrodes, magnetic filter field) on the SPIDER beam uniformity in the horizontal and vertical directions. It was shown that non-Gaussian tails can be identified in the angular distribution on the plane perpendicular to the beam propagation direction. Stray particles, nonhomogeneous beam and large divergence might result in unexpected heat and particle loads over ITER neutral beam injector (NBI) accelerator grids; it is the goal of SPIDER to assess and possibly to identify suitable methods for controlling these beam features. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, is outlined. Such improvements are expected to allow SPIDER to pursue the ITER requirements in terms of negative ion current, electron-to-ion ratio, and beam duration.

Investigation of Surface Ionization Detector for Cesium Oven Evaporation Rate

To assure stable cesium injection for negative ion source, the surface ionization detector (SID) is applied for diagnosis. Consisted of emitter and collector, the SID is operating based on bias voltage setting to drive the ionized cesium ions from emitter to the collector for detection. An analysis model of SID for cesium oven evaporation rate is established, investigating the process of cesium transportation in vacuum, thermionic ionization and cesium ion collection. Cesium transportation from nozzle to SID in vacuum is studied on the basis of free molecule flow model, and the cesium density distribution is obtained after having estimated cesium flux density ratio intercepted by SID from nozzle. The collection period is analyzed by particle tracking model, providing the relationship of SID current and ionized cesium current under adaptive bias voltage along with characteristics between SID current and bias voltage reproduced corresponding to experimental results. The model is adjustable and flexible to deal with different 3D structures, geometry and electric field setting. SID analysis for cesium oven of HUST negative ion source is carried out to monitor oven operation and to provide evaporation rate features under various oven temperatures along with error analysis.

Investigation of RF driver equivalent impedance in the inductively coupled SPIDER ion source

SPIDER experiment includes an RF inductively coupled plasma source working at 0.3 Pa of gas pressure (H/D) where plasma is generated and heated by eight RF drivers, fed by four RF circuits. A single RF circuit is composed of two drivers connected in series, attached to a capacitive matching network, fed by 200 kW 1 MHz RF oscillator through a coaxial transmission line. The knowledge of driver impedance in different experimental conditions is a valuable window for understanding the characteristics of generated plasma. Direct measurement of driver impedance in SPIDER is not possible, it has been estimated via the development of a suitable electrical model of the RF circuit using as input, measurements at the oscillator’s output. This paper reports on the progress in the modeling of the SPIDER RF circuit and discusses a procedure to estimate the driver impedance exploiting also the recently available experimental measurements from the dual directional coupler. The impedance obtained with this approach, derived from measurements obtained in a recent SPIDER campaign, is reported for various operating conditions, such as RF power, gas pressure, plasma grid current, cesium injection rate, type of gas (hydrogen and deuterium), etc.

Spatially resolved diagnostics for optimization of large ion beam sources

Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density and beamlet currents; at the edges of the central segments, both the beam and electron density appear to decrease (probably maintaining fixed electron-to-ion ratio); at the bottom of the source, an increase of cesium injection can compensate for the vertical drifts that cause a much higher presence of electrons and a lower amount of negative ions.

First Prototype of a Cesium Oven Design for Negative-Ion-Source-Based Neutral Beam Injector at ASIPP

The RF-driven negative ion source has a steady state characteristic and will be a promising ion source for neutral beam injection (NBI) in the future. Cesium (Cs) injection is an efficient method to enhance the yield of negative ions during plasma discharge. In order to support the engineering and physical research in this field, the Cs oven prototype has been developed for negative ion source based neutral beam injector at ASIPP. This article presents the design details of a Cs oven system, including the constant temperature control system, the mechanical structure of Cs oven, and the surface ionization detector (SID). SID is a measurement method for the cesium flux in the nozzle. The experiment results of constant temperature control system show that the control accuracy and function meet the requirements of device operation. Meanwhile, the simulation analysis of Cs vapor concentration has been carried out in this paper. According to the simulation results, the graph of total Cs flux is given in the article, which presents the reference for the subsequent device testing.

In Situ Formation of Zwitterionic Ligands: Changing the Passivation Paradigms of CsPbBr3 Nanocrystals.

CsPbBr3 nanocrystals (NCs) passivated by conventional lipophilic capping ligands suffer from colloidal and optical instability under ambient conditions, commonly due to the surface rearrangements induced by the polar solvents used for the NC purification steps. To avoid onerous postsynthetic approaches, ascertained as the only viable stability-improvement strategy, the surface passivation paradigms of as-prepared CsPbBr3 NCs should be revisited. In this work, the addition of an extra halide source (8-bromooctanoic acid) to the typical CsPbBr3 synthesis precursors and surfactants leads to the in situ formation of a zwitterionic ligand already before cesium injection. As a result, CsPbBr3 NCs become insoluble in nonpolar hexane, with which they can be washed and purified, and form stable colloidal solutions in a relatively polar medium (dichloromethane), even when longly exposed to ambient conditions. The improved NC stability stems from the effective bidentate adsorption of the zwitterionic ligand on the perovskite surfaces, as supported by theoretical investigations. Furthermore, the bidentate functionalization of the zwitterionic ligand enables the obtainment of blue-emitting perovskite NCs with high PLQYs by UV-irradiation in dichloromethane, functioning as the photoinduced chlorine source.

Implementation of Nanoscale Secondary‐Ion Mass Spectrometry Analyses: Application to Ni‐Based Superalloys

Secondary‐ion mass spectrometry (SIMS) is probably the most widely used chemical analysis technique in semiconductor science and in metallurgy because of its ultimate sensitivity to all elements and in particular to light elements providing semiquantitative information on the depth distribution of elements, for instance, doping elements (i.e., B, H), contaminants (i.e., C and O), chemical gradients, and segregation in thin films and at interfaces, etc. With the size shrinking of systems, high‐resolution 3D chemical imaging is becoming a prerequisite for the development of new materials at the nanoscale and for the deep understanding of the correlation between their properties and functionalities. Herein, the development of an innovative analytical SIMS implemented in a focused ion beam (FIB) (using Ga source)/scanning electron microscope (SEM) is reported. The equipment enables to give elemental chemical mapping at very high resolution (<30 nm) by precise optimization of the secondary‐ion optics and detection, while preserving excellent sensitivity, thanks to the integration of a gas injection system (GIS), which improves a positive (negative) ionization rate with oxygen (cesium) injection. The capability of the technique is demonstrated with nanoscale characterization of Ni‐based superalloys with broad applications in the manufacturing of engine parts and accessories for aircraft and aerospace equipment.

Shape Tailoring of Iodine‐Based Cesium Lead Halide Perovskite Nanocrystals in Hot‐Injection Methods

AbstractAll‐inorganic lead‐halide perovskite nanocrystals continue to attract the attention of the scientific community due to their optical potential. In this work, we demonstrate that synthetic approaches based on the modulation of the ligand‐to‐metal ratio in hot‐injection methods can provide direct access to a variety of colloidal nanocrystal morphologies without resorting to post‐synthetic transformations. The shape tailoring observed for iodine‐based perovskite nanocrystals suggests that nanowires can evolve through the oriented attachment of nanocubes in the presence of high ligand‐to‐metal ratios. Conversely, mixed‐halide perovskite nanoplatelets are generated in low‐surfactant regimes, due to the peculiar reactivity of halo‐plumbate species generated before cesium injection, which favors an anisotropic growth of the nanoparticles. These results contribute to deepen the knowledge on the shape evolution of perovskite nanoparticles, thus opening the way to the development of new synthetic approaches.

Experimental study for 15–20 mA dc H− multicusp source

Recently, a new H- source and test stand was developed at CIAE. The design of this new source is based on the experience on our previous 10-15 mA H(-) ion source and the source at TRIUMF. Major efforts include the study of the virtual filter magnetic field, confining magnetic field, filament shape and location, the vacuum improvement on the extracting area, the extraction optics, new control and interlock system of the power supplies. More than 15 mA of H-beam was obtained for 36 h with stability of +/-0.5%. The normalized emittance of 0.48pi mm mrad (4 rms normalized emittance) were measured with approximately 8 mA dc beam. Further experimental studies are proceeding in an effort to reach 20 mA with reasonable emittance at this moment. More study plans are conducted, e.g., building a longer source body and using cesium injection to get better emittance, which will be presented as a separate paper at this conference.

Simulation of cesium injection and distribution in rf-driven ion sources for negative hydrogen ion generation

Cesium seeded sources for surface generated negative hydrogen ions are major components of neutral beam injection systems in future large-scale fusion experiments such as ITER. Stability and delivered current density depend highly on the cesium conditions during plasma-on and plasma-off phases of the ion source. The Monte Carlo code CSFLOW3D was used to study the transport of neutral and ionic cesium in both phases. Homogeneous and intense flows were obtained from two cesium sources in the expansion region of the ion source and from a dispenser array, which is located 10 cm in front of the converter surface.

Microfabricated alkali atom vapor cells

We describe the fabrication of chip-sized alkali atom vapor cells using silicon micromachining and anodic bonding technology. Such cells may find use in highly miniaturized atomic frequency references or magnetometers. The cells consist of cavities etched in silicon, with internal volumes as small as 1 mm3. Two techniques for introducing cesium and a buffer gas into the cells are described: one based on chemical reaction between cesium chloride and barium azide, and the other based on direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption and coherent population trapping resonances were measured in the cells.

Development of a high current H− source for European spallation source

For the European spallation source, a volume source based on the high efficiency source is being developed. The source will be optimized to produce high current densities in pulsed operation. A pulse generator delivering 1–1.5 ms pulses was installed. Furthermore, cesium was supplied to the plasma generator from an external oven. The cesium injection was optimized for a low e/H− ratio and a high current. We obtained a current density of 70 mA/cm2. This way, with an aperture radius of 4.25 mm, an H− current of 40 mA was extracted at an extraction voltage of 22 kV. After a description of the source and the experimental setup, measurements of the beam current density and the e/H− ratio will be presented in this article.

Large-area radio frequency plasma sources for fusion applications

Four powerful inductively coupled radio frequency (rf) plasma generators producing 90 A, 55 kV hydrogen beams were manufactured for the second injector of ASDEX Upgrade. The design of the source and the power supply was optimized based on the experience with a prototype source. During conditioning of the first two sources, it was demonstrated that power efficiency, transmission of the beam power and reliability meet all requirements. The start of the neutral beam injection with the second injector is scheduled for October 1997. Recently, also negative ions in rf sources are being investigated within a collaboration with CEA Cadarache. In this development project a positive ion prototype rf source is being utilized with a small extraction area (48 cm2) and a 700 G cm magnetic filter. Without cesium a current density of 4.5 mA/cm2 was achieved so far at elevated pressure (1.6 Pa), the H−-current density being proportional to the rf power. At medium pressure (0.6 Pa) the current density is lower approximately by a factor of 5, but preliminary results with cesium injection show a relative increase by almost the same factor in this pressure range. Probe measurements indicate electron temperatures of 2.5–4.5 eV close to the plasma grid. Attempts to improve the performance by better plasma confinement, different wall materials, and filter strengths are under way.

The origins of two-state spontaneous membrane potential fluctuations of neostriatal spiny neurons

In vivo intracellular recordings of spontaneous activity of neostriatal spiny cells revealed two-state behavior, i.e., characteristic shifts of membrane potential between two preferred levels. The more polarized level, called the Down state, varied among neurons from -61 to -94 mV. The more depolarized level, called the Up state, varied among neurons form -71 to -40 mV. For any one neuron, the membrane potential in the Up and Down states was constant over the period of observation (from 15 min to 4 hr), and the cells spent little time in transition between states. The level of membrane potential noise was higher in the Up state than in the Down state. Spontaneous membrane potential fluctuations were not abolished by experimental alteration of the membrane potential, but the time spent in each state was altered when intracellular current was used to vary the baseline membrane potential. Neither the sodium nor the calcium action potential that could be evoked by depolarization of spiny neurons was required for the occurrence of spontaneous shifts of membrane potential. Blockade of these action potentials using intracellular injection of QX314 and D890, respectively, altered neither the incidence of the membrane potential shifts nor the preferred membrane potential in either state. In contrast, antagonism of voltage-dependent potassium channels with intracellular cesium altered membrane potential shifts. In the presence of QX314 and D890, intracellular injection of cesium caused little or no change in the Down state and a large depolarizing shift in the Up state (to about -20 mV). Under these circumstances, the neuron responded to current in a nearly linear manner, and membrane conductance was found to be increased in the Up state, attributable to a membrane conductance with the same reversal potential as that of the synaptic potential evoked by cortical stimulation. These results indicate that the event underlying the Up state is a maintained barrage of synaptic excitation, but that the membrane potential achieved during the Up state in neostriatal spiny neurons is determined by dendritic potassium channels that clamp the membrane potential at a level determined by their voltage sensitivity. Neostriatal spiny neurons ordinarily receive enormously powerful excitation, which would drive the cells to saturation, and probably destroy them, if it were not for these potassium currents.

Cesium injection into a large rf-driven hydrogen negative-ion source

A large rf-driven hydrogen negative-ion source has been developed for a neutral beam injection (NBI) system in the fusion application. The rf plasma generator is a metal-walled multicusp bucket source, the dimensions of which are 30 cm×30 cm in cross section and 20 cm in depth. An induction coil antenna is fully immersed in the rf plasma generator, and is used for coupling the rf power into plasma. A strong external magnetic filter is generated in front of the plasma electrode for negative-ion production, and a filter field strength of more than 1000 G cm is required to reduce the electron temperature in the extraction region as low as 1 eV. Precisely 5.5 mA of a negative-ion current was obtained from a single aperture, corresponding to 4.1 mA/cm2 of current density. A small amount of cesium was supplied to the rf plasma generator from a cesium oven. As a result, a lowering of the operational gas pressure and improvement of the negative-ion production efficiency were observed. The optimum bias voltage was reduced by the cesium injection, which may be related to the reduction of the plasma potential.

Negative ion production in large volume source with small deposition of cesium

Experimental data on the enhancement of D− (H−) negative ion production due to cesium injection into a large volume multiampere negative ion source (MANTIS) are described. The directed deposition of small cesium amounts (5–100 mg) from a compact, movable oven, placed into the central part of a MANTIS gas-discharge box was used. A calorimetrically measured D− beam with an intensity up to 1.6 A and an extracted current density up to 4.2 mA/cm2 (beam energy 25 kV) was obtained. Exactly 30 mg of cesium provides at least one month of source operation (1000 pulses with a discharge pulse duration of 4 s). The effect of cesium on NI enhancement was immediately displayed after the distributed Cs deposition, but it needed some ‘‘conditioning’’ of cesium by tens of discharge pulses (or by several hours ‘‘pause’’) in the case of a localized Cs deposition. No degradation of extraction-acceleration voltage holding on within the tested range of cesium injection was observed.

Recent progress on high power negative ion sources at JAERI

Abstract R & D's on high power negative ion source at JAERI are reviewed. A method of cesium injection into volume source has drastically progressed in development of the high current source. The H− ion current was enhanced up to 10 A only by seeding cesium of a few hundreds mg into a volume production type magnetically filtered multicusp source. Reductions of the optimum gas pressure and the electron current were also observed in the operation of cesium seeded sources. These effects are helpful in stable ac-celeration of the ion beams. High energy negative ion beam acceleration was demonstrated in the cesium seeded sources. So far, we have succeeded to accelerate 0.13 A H− ion beam up to 350 keV for 1 s with good beam optics of the divergence ωl/e ∼ 5 mrad. Two methods of the beam focussing were attempted; i.e. aperture displacement and curved grids extraction. The beam focussing by aperture displacement was found to be useful for the high power beam steering, while the beam merge experiment were being p...