Beam Extraction Research Articles

Magnetic field configuration effects on a miniature DC ion thruster with an inverted potential structure

To improve the mass utilization and total efficiency of miniature direct current (DC) ion thrusters, this article proposes a graphene–oxide–semiconductor (GOS) electron source as a cathode in the discharge chamber and neutralizer. To apply the GOS device to the discharge chamber, multiple electron sources were positioned downstream to provide sufficient electrons for plasma generation. Accordingly, the anode was placed upstream of the discharge chamber; thus, the potential structure was inverted from that of a conventional device. Based on this concept, two discharge chamber types were developed with different magnetic field configurations (1- and 2-Cusp types) using a filament cathode and their performance was experimentally evaluated. Probe diagnostics results showed that plasma was generated mainly in the center of the discharge chamber in 1-Cusp, whereas it was generated in the entire discharge chamber in 2-Cusp. Regarding ion beam extraction with 211-hole grids, 1-Cusp exhibited a higher screen current than 2-Cusp. However, using 391-hole grids with an expanded ion-extraction region, 2-Cusp outperformed 1-Cusp with 211-hole grids, attaining a minimum discharge loss of 273 W/A (excluding filament power) and a maximum mass utilization efficiency of 0.48.

Impact of beam-coupling impedance on the Schottky spectrum of bunched beam

The Schottky monitors of the Large Hadron Collider (LHC) can be used for non-invasive beam diagnostics to estimate various bunch characteristics, such as tune, chromaticity, bunch profile or synchrotron frequency distribution. However, collective effects, in particular beam-coupling impedance, can significantly affect Schottky spectra when large bunch charges are involved. In such conditions, the available interpretation methods are difficult to apply directly to the measured spectra, thus preventing the extraction of beam and machine parameters, which is possible for lower bunch charges. To study the impact of impedance on such spectra, we introduce a method for building Schottky spectra from macro-particle simulations performed with the PyHEADTAIL code, applied to LHC beam conditions. In this case, the use of a standard Fast Fourier Transform (FFT) algorithm to recover the spectral content of the beam becomes computationally intractable memory-wise, because of the relatively short bunch length compared to the large revolution period. To circumvent this difficulty, a semi-analytical method was developed to efficiently compute the Fourier transform. The simulated Schottky spectrum is then compared against theoretical formulas and measurements of Schottky signals previously obtained with lead ion beams in the LHC where impedance effects are expected to be limited. Furthermore, this study provides preliminary interpretations of the impact of beam-coupling impedance on proton Schottky spectra by incorporating longitudinal and transverse resonator-like impedance models into the simulations. A theoretical framework is also introduced for the case of the longitudinal impedance, allowing the extension of the existing theoretical formalism.

Prototype of repetitive pulsed power supply for the RCS beam extraction kicker in the CSNS upgrade

A repetitive pulsed power supply prototype has been designed and implemented for the RCS beam extraction kicker in the CSNS upgrade. The pulsed power supply features an ultra-low impedance Blumlein PFN to generate a fast front and high-current pulse, and a saturable magnetic switch to sharpen the pulse’s rising edge at the nanosecond time scale. A detailed report is provided on the practical engineering issues arising from the ultra-low impedance Blumlein PFN in the paper. Additionally, we have utilized simulation and experimental methods to evaluate the effectiveness of the saturation magnetic switch in sharpening the excitation current at the nanosecond time scale. The developed pulsed power supply is capable of generating a pulse current of 6660 A with a rise time of 248 ns, a pulse flat-top duration of 628 ns, a time jitter of ±4.5 ns, and a flat-top ripple of ≤±1 % at a repetition rate of 25 Hz. The design and experimental results of this pulsed power supply are presented in detail in this paper.

Ion transport and gas collision effects in a radio frequency quadrupole cooler: installation in the Eltrap solenoid and beam calculations

Radio frequency quadrupole coolers (RFQCs) are very suitable to cool ion beams with moderate energy spread, typically ions of exotic nuclear species (like $^{132}$ Sn $^{1+}$ ) as in the Selective Production of Exotic Species project at the Laboratori Nazionali di Legnaro, whose ion source supplies 40 keV ions. Beam dynamics includes ion–gas collisions (with a balance of cooling and diffusion effects), acceleration and deceleration and radiofrequency confinement, which can be supplemented by static magnetic field effects. Insertion of a prototype RFQC in the solenoid of the Eltrap machine is also discussed here, with innovations in beam extraction and in modelling, now also based on stochastic equations. Practical consideration on gas pumping and voltage distribution are also included. Typical limits of RFQC are discussed, with special attention to the extracted beam root mean square emittance, which is shown to strongly depend not only on cooler parameters, but also on extraction optics.

Effect due to Cs injection upon current oscillation of the beam extracted from the J-PARC negative hydrogen ion source

A high-speed emittance measurement system installed at the J-PARC RF negative hydrogen (H-) ion source test stand can construct emittance diagrams of the beam of a specific frequency component. The driving 2 MHz RF power produces small amplitude oscillating components at 2 MHz and at higher harmonic frequencies in the total H- ion beam detected by a Faraday installed downstream of the emittance measurement system. Magnitudes of oscillating component at 2 MHz and 4 MHz frequencies become appreciable as the slits of the emittance measurement system separate the H- ion beam into the position/angle resolved phase space elements. The reconstructed emittance diagrams of H- ion beams for the operation condition with continuous supply of Cs realizing large H- ion current to total extraction current ratios indicate that 2 MHz and 4 MHz frequency oscillating beam components propagate surrounding the center DC beam. Both 2 MHz and 4 MHz oscillating beam components comprise two to three beam particle groups in the phase space. The DC component of the H- ion beam exhibit a single peak in the angular distribution. Meanwhile, oscillating components often take the minima where the DC H- ion beam takes the maximum in the angular distribution diagram. Thus, the regions that produce large amplitude oscillations of the H- ion beam are considered located at the circumference of the beam extraction aperture. One possible explanation for observing larger amplitude beam oscillation can be the enhanced H- ion surface production at the plasma facing the wall of the beam extraction aperture. The operation without the Cs delivery to the source increased electron current coextracted with H- ions and changed the structure of the beam emittance diagram. In the direction perpendicular to the magnetic fields created by a magnetic filter and electron suppression magnets, the peak of the 4 MHz oscillating beam components appeared at the center of the DC beam. This observation suggests the extraction of H- ions formed in the central region by the volume production process near the internal RF antenna. Mechanisms that can excite and diminish the beam fluctuation amplitudes of the extracted H- ion beam are discussed.

A very low energy ion beam extraction system design of the GTS ECR ion source at GANIL

The GTS ion source, operated at 14.5GHz, provides multiply charged heavy ion beams for the ARIBE facility at GANIL. The facility variety is limited by the efficiency of the extraction especially in the few keV/q domain. In order to improve the ion source, the extraction system was upgraded numerically using IBSimu. The shape of the plasma and the puller electrodes was changed. It causes the increase of the electric field near the plasma meniscus and allows the use of lower extraction voltages at constant total ion beam current. The required beam parameters were taken from the experimental data. The opportunity of effective low energy beam formation (at a few keV/q or several hundreds eV/q beam energy) was studied. The high current and low energy ion beam production will provide new possibilities for ARIBE facility users.

Potential impact of surface microstructure change on reduction of emission current in tungsten filament

When a helium beam is extracted, a significant decrease in arc current is observed in the filament arc ion source of the Neutral Beam Injector. Microstructural changes in the tungsten filament surface of the ion source are observed by electron microscopy to investigate the cause of the reduction of arc current. Helium bubbles with 100–800nm diameters appear immediately beneath the surface within 1μm, and the bubbles develop into surface hole structures with surface erosion by sputtering. The decrease in arc current with helium beam extraction can be explained by a reduced effective surface area, due to the increase in hole porosity. It has been shown that the reduction of the arc current can be recovered by removing the helium-irradiation-affected layer by sputtering with argon discharge.

Uniformity optimization of ion beam sputtering coating equipment based on strong current ion source

The widespread application of ion beam sputtering coating, especially in optical devices, requires the improvement of beam current intensity and uniformity of large-area uniform coatings. The advent of high current Penning sources offers a potential solution. This study introduces an automated optimization simulation method based on a three-electrode extraction system to investigate its influence on ion beam quality and uniformity. Focusing on high current intensity and uniformity, our simulation explores the effects of plasma electrode, inhibition electrode, and extraction electrode angles and distances on ion beam performance. Evaluation metrics include average beam intensity density, average energy of a single particle, and reciprocal variance of each macro particle position, which are achieved through normalization functions, allowing comprehensive comparison of simulation results. To assess coating efficiency, we estimate sputtering yield and depth. The study identifies patterns among electrodes and emphasizes the influence of different ion ratios on beam extraction. The results indicate that optimizing the angle of the plasma electrode and the distance of the suppressed electrode yields a highly uniform ion beam for low charge ions. However, for highly charged ions, similar optimization will reduce the current strength, so compensation needs to be achieved through electrode shape optimization. This research provides a model for systematically optimizing the three-electrode extraction system, guiding researchers in achieving optimal solutions based on ion source characteristics and application requirements. Additionally, we introduce a method of estimating the sputtering depth of mixed ion beams. This study provides valuable insights for advancing ion beam sputtering coating technology and reference for making the decision on design and application of ion source.

New event based timing system for the Taiwan Light Source

The Taiwan Light Source (TLS) is a third generation synchrotron light source that has been in operation since 1993. However, the legacy timing system is obsolete and needs to be upgraded to an event based timing system. This upgrade is intended to replace obsolete components and improve functionality. The new timing system must coordinate the operation of the linac, the White circuit based booster synchrotron, and the storage ring for beam generation, injection, extraction, and accumulation, while also supporting top up operation of the storage ring. To achieve this, a cPCI (CompactPCI) based EPICS IOC (Input Output Controller) and expandable fanless embedded computers have been adopted for the new TLS timing system, replacing the existing VME-based ILC (Intelligent Local Controller) to provide an easy to maintain control environment. However, the resonant excitation of the booster magnets with the event system requires special attention in the design and implementation phases. This paper summarizes the design ideas and implementation process for addressing this issue.

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.

Simulation study on the slow extraction for the improvement of the beam shill structure at J-PARC Main Ring

J-PARC Main Ring (MR) delivers a slow extracted 30 GeV proton beam to the Hadron Experimental Facility using third-order resonance. One of the critical properties required for the proton beam is the flatness of the time structure of the extracted beam (spill structure). We performed a simple beam simulation of the MR slow beam extraction to investigate the effect of the current ripples of the main magnet power supplies on the beam spill structure. In addition, we investigated in the simulation the effects of the feedback control system on the betatron tune using fast Q magnets and the transverse RF system to improve the spill structure. The simulation qualitatively reproduced the measured spill structures. Now the attempts to optimize their parameters for better spill structure are ongoing.

Calculation of negative ion beam loss for two region arc plasma negative ion source

Korea Atomic Energy Research Institute (KAERI) is currently developing a Two Region Arc Plasma (TRAP) negative ion source for neutral beam injection (NBI) systems of a fusion tokamaks which is characterized by two distinct regions with different plasma temperatures without the use of external filter field magnets. The source has recently been designed, manufactured, and installed for testing. Plasma discharge characterization experiments of the TRAP ion source are presently in progress, with planned beam extraction experiments. The beam extractor of the TRAP ion source is capable of accelerating hydrogen ions up to 30 keV and comprises four grids with slit-type aperture structure. Understanding the negative ion beam loss in the extractor is crucial for expecting potential issue within the beam extractor and comprehending the plasma discharge characteristics through negative ion beam. Using a 3-D Monte Carlo program (molflow+), we calculated the vacuum pressure distribution within the TRAP ion source experimental device, and based on these results, we calculated the loss of negative ion beams within the beam extractor and the neutralization efficiency from the beam exit to the target, an effect arising from residual gases. The computational results reveal that approximately 20 ∼ 50 % of the negative ion beam is lost within the beam extractor, and a reduction of more than 60 % in negative current occurs from the beam exit grid to the target within the test chamber due to neutralization of negative ions.

(Invited) Photo-Assisted Electron Emission from Silicon-Based Electron Emission Devices

Ultrafast pulsed electron sources with spatial coherence have attracted significant interest in emerging applications such as scientific instruments of ultrafast electron microscopy [1-3] and electron beam lithography [4], and also as sources of X-ray [5] and micro- and THz-waves [6-9]. In particular, high-frequency electron bunch trains generated directly from the cathode surface can not only dramatically reduce dimensions and weight through the elimination of the pre-modulation circuit, but can also provide extremely high efficiency and improve the power in the electron beam devices due to coherence effects.One solution is employing photoemission from electron sources driven by ultrashort laser pulses for such emerging applications. Metallic flat photocathodes and recently metallic nanotips have been employed. However, a low quantum efficiency (QE), typically of approximately 0.01 %, and a large work function require a high-power UV laser to generate an intense electron beam. Negative electron affinity (NEA) semiconductor photocathodes such as NEA GaAs can provide QE as high as ~ 10 % [10], operating using visible laser light irradiation. However, the NEA surface is fragile and easily damaged by residual gas adsorption or beam extraction.In this presentation, we introduce the research and development of gated Si-FEAs with a volcano structure and planar-type electron emission devices based on a MOS diode structure and discuss the potential of the devices as a photocathode of the devices operating without NEA surface treatment. We investigate the optical properties of emitted electrons from the devices under visible laser light irradiation. The results indicate that Si-based electron emission devices with a fully optimized structure hold promise as a photocathode with a high QE and a high-speed response for generating a train of ultrashort electron bunches, operating without NEA surface treatment.

Surface-barrier detector with smoothly tunable thickness of depleted layer for study of ionization loss and dechanneling length of negatively charged particles channeling in a crystal

A new method for the experimental study of ionization loss of relativistic negatively charged particles moving in a crystal in the channeling regime using a semiconductor surface-barrier detector with smoothly tunable thickness of the depleted layer is proposed. The thickness of the depleted layer in a flat semiconductor detector can be smoothly regulated by the value of the bias voltage applied to the detector. Therefore, the energy distribution of the ionization loss of relativistic particles which cross the detector and move in the channeling regime in the detector crystal can be measured along the path of the particles by varying the bias voltage of the detector and the dechanneling length can be found. Available literature data on experimental and theoretical researches of the dechanneling length are reviewed. The significant disagreement between the experimental and theoretical data is noted. Comparison of experimental data obtained by the detector-target with smoothly tunable thickness of the depleted layer with calculations can help to develop theoretical description of the dynamics of motion of negatively charged particles channeling in a crystal. A better understanding of the dechanneling length properties can be useful in the production of positrons and other particles such as neutrons by an electron beam in crystals, as well as in the development of crystalline undulators, and in the crystal-based extraction of electron beams from a synchrotron.

Experimental Validation of Low-Intensity Beam Extraction Mode for Proton Imaging at the Prometheus Synchrotron

Experimental Validation of Low-Intensity Beam Extraction Mode for Proton Imaging at the Prometheus Synchrotron

Modeling the influence of an electric thruster’s ion beam on its global EMC

This research deals with electromagnetic compatibility (EMC) in the field of electric propulsion (EP). To complete previous investigations, the emissions of a fully operating radio-frequency thruster (RIT)—including its extracted ion beam—were numerically analyzed. The ion beam was simulated and investigated with various characteristics. The simulations were performed by means of transient co-simulation. It is clear that the ion beam had a significant impact on the thruster emissions. Properties such as divergence angle and the conductivity of the beam, which can be directly attributed to the operating point of the plasma discharge inside the thruster, play a major role. The next steps will be to bring together all the knowledge gained about the emissions of the individual thruster components as well as the peripheral electronics.

Slit extraction and emittance results of a permanent magnet minimum-B quadrupole electron cyclotron resonance ion source

We present an experimental and simulation study of high charge state ion beams produced with a permanent magnet electron cyclotron resonance ion source (ECRIS) with minimum-B quadrupole magnetic field topology and slit extraction system. The unconventional topology generates fan-shaped plasma flux, favouring a rectangular aperture for beam extraction. We demonstrate successful low-energy transport of the slit beam, achieving up to 25 times higher beam currents compared to a round extraction aperture. Maximum beam intensities are 1.5 and 15 μA for argon 11＋ and 9＋, respectively. The results indicate microwave power-limited beam production. Emittance values are comparable to traditional ECRIS. Unlike in conventional ECRIS, the emittance increases monotonically with increasing charge state due to slit extraction, magnetic topology, and beam transport characteristics. The experimental transport efficiency for helium ranges from 32% to 50%, depending on the total beam current. The losses primarily occur due to the dipole magnet’s insufficient vertical aperture.

Influences of coupling magnetic field of 2 cm electron cyclotron resonance ion thruster on plume neutralization

An electron cyclotron resonance ion thruster (ECRIT) has advantages of long service life and simple structure. The external coupling magnetic field of the ECRIT is one of factors that influences the plume neutralization process and coupling voltage of the neutralizer. The coupling magnetic field varies with the installation direction of the ion source and neutralizer as well as the direction of the internal magnetic pole. Calculating the distribution law of the coupling magnetic field and experimentally studying the influence of the coupling magnetic field on plume neutralization are very important. This paper focuses on two power rates and two flow rates of the ion source and conducts experiments on neutralization within the acceleration voltage range of 350 to 1 450 V to study the influence of changes in the direction and position of the ion source and the influence of the neutralizer′s magnetic pole on the ion beam extraction and maximum coupling voltage. The results indicate that the ion beam extraction is not influenced by the direction of the magnetic pole and the installation direction of the ion source and neutralizer. When the ion source is installed relatively perpendicular to the neutralizer, its coupling voltage is reduced. At the same time, changing the direction of the magnetic pole of the neutralizer into the opposite of the direction of the magnetic pole of the ion source also reduces the coupling voltage of the neutralizer. When the ion source is installed perpendicular to the magnetic pole of the neutralizer in the opposite direction, the coupling voltage is the lowest.

Paving the road towards ITER relevant long deuterium pulses at ELISE by investigating improved operational scenarios

Hydrogen plasma pulses of 1000 s with an extracted current density of over 90 % of the ITER target value (while also fulfilling the requirements for the electron-ion ratio and the global top/bottom beam homogeneity) were demonstrated at the ELISE test facility with its half ITER-size negative hydrogen ion source in pulsed extraction mode, i.e. short beam extraction phases of ≈10 s each ≈150 s, and using the so-called potential rods. In deuterium roughly 67 % of the ITER target for the extracted current density has been achieved for pulses longer than 45 minutes, limited by a pronounced vertical asymmetry and increase with timeinthe co-extracted electron current.In order to stabilize the co-extracted electron current during long pulses, particularly in deuterium, improved operational scenarios have to be developed. At ELISE, it was investigated the possibility to remove the potential rods and instead to set the bias plate to a positive voltage with respect to the source vessel. In this operational scenario short pulses in deuterium can be achieved with an extracted negative ion current density of up to 90 % of the ITER target value.In order to improve the physics insight, a 2D fluid code is applied. The first general result is that the modified potentials of surfaces close to the extraction system strongly affect the fluxes of charged particles towards different surfaces and consequently can result in a reduction of the co-extracted electron current.The temporal behaviour during long pulses with pulsed extraction, in particular of the co-extracted electrons, demonstrates a significant impact of the beam extraction on the caesium dynamics. Thus, CW extraction is mandatory for developing fully ITER relevant operational scenarios. After an extensive upgrade, ELISE is now capable of performing CW pulses, i.e. long-pulse beam extraction.

Hot Gap Measurement of a Three-Grid System for a 30 cm Diameter Ion Thruster With Beam Extraction

Hot Gap Measurement of a Three-Grid System for a 30 cm Diameter Ion Thruster With Beam Extraction