Stable Beam Current Research Articles

Investigation of dosimetric effect of beam current fluctuations in synchrotron-based proton PBS continuous scanning

Objective. In proton pencil beam scanning (PBS) continuous delivery, the beam is continuously delivered without interruptions between spots. For synchrotron-based systems, the extracted beam current exhibits a spill structure, and recent publications on beam current measurements have demonstrated significant fluctuations around the nominal values. These fluctuations potentially lead to dose deviations from those calculated assuming a stable beam current. This study investigated the dosimetric implications of such beam current fluctuations during proton PBS continuous scanning. Approach. Using representative clinical proton PBS plans, we performed simulations to mimic a worst-case clinical delivery environment with beam current varies from 50% to 250% of the nominal values. The simulations used the beam delivery parameters optimized for the best beam delivery efficiency of the upcoming particle therapy system at Mayo Clinic Florida. We reconstructed the simulated delivered dose distributions and evaluated the dosimetric impact of beam current fluctuations. Main results. Despite significant beam current fluctuations resulting in deviations at each spot level, the overall dose distributions were nearly identical to those assuming a stable beam current. The 1 mm/1% Gamma passing rate was 100% for all plans. Less than 0.2% root mean square error was observed in the planning target volume dose-volume histogram. Minimal differences were observed in all dosimetric evaluation metrics. Significance. Our findings demonstrate that with our beam delivery system and clinical planning practice, while significant beam current fluctuations may result in large local move monitor unit deviations at each spot level, the overall impact on the dose distribution is minimal.

Improved 26Al and 10Be cathode performance and data analysis at KIST

Addition of a metal powder interface between solid metal cathode and sample powder was investigated for26Al and10Be analysis by accelerator mass spectrometry (AMS). Cathode quality and consistency improved, leading to more intense, stable and longer-lived ion beam currents, while cathode matrix sensitivity to powder fineness, homogeneity and precipitate to metal powder mixing ratios was reduced. Now, ratios of 1:0.75–2 for Al2O3 + Ag in Cu cathodes and 1:0.9–4 for BeO + Nb in Al or Cu cathodes are optimal. Furthermore, a survey of sputter energies showed that 5 keV–6 keV greatly increases cathode life, current stability, and cathode consumption efficiency for both BeO and Al2O3as opposed to the widely used 7 keV for High Voltage Engineering SO110 ion sources. Finally, some data flagging criteria that have recently been incorporated into the26Al and10Be internal data analysis software are explained.

Efficiency improvement of an E × B Penning discharge source by enhanced cross-field transport of electrons

Precise control of the particle motion in externally applied electric and magnetic fields is of great significance in the development of the E × B source to generate high-density plasma and deliver a stable ion beam current. Especially, in the E × B Penning discharge source, the heating and energy relaxation of the beam electrons is concentrated in the plasma column along the magnetic field line. Plasma researchers have thus far focused on the relevant physical phenomena of the partially magnetized plasma that arises from the gradient of the plasma properties in the E × B Penning source. Here, we point out that current methods of radially centered electron confinement do not guarantee efficient ion beam extraction, and newly introduce the improvement of the efficiency of a cylindrical E × B Penning source targeting radial extraction of ion beam. We concentrate on the method to enhance the cross-field transport of electrons toward the extraction region. The generation of a spatially asymmetric sheath structure allows the beam and energetic electrons to be transported to the extraction region via the E × B drift of the electrons. The transported electrons contribute to expansion of the electron heating and ionization regions to the extraction region by breaking of axial symmetry of the sheath, thereby increasing the temperature and density of the electrons in the extraction region as the magnetic field strength increases. The enhanced discharge efficiency defined as the ratio of the electron density to the discharge current is noticeable, recording approximately twice the improved efficiency compared to the conventional mode with symmetric sheath structure.

Experimental Study of the Cathode Modulation for the TH2103A Klystrons on HL-2A Tokamak

The 3.7 GHz high power klystrons TH2103A are widely used in the lower hybrid current drive system for the tokamak experiment. In order to avoid the tetrode and simplify the power supply system, the cathode modulation for high power TH2103A klystron is experimentally studied. Both the independent Anode Power-Supply (APS) and a Passive Potential Divider (PPD) configuration are investigated. The beam current goes to zero in 10 ms in the APS mode directly. As a result, a stable beam current is achieved in the PPD mode when the resistor from the anode to the ground is less than 30 k Ω, the cathode voltage is below − 45 kV and its rise-time is between 100 and 150 μs. The threshold of the resistor and the rise-time may be caused by the inductance of the wire-wound resistors. Although the anode current exceeds 10 mA when the cathode voltage rises and falls, 5 years operation validates the safety of the klystron. A practical and simple method is developed to operate the anode modulation klystron by cathode modulation on HL-2A, without using a dedicated tetrode.

36Cl preparation method for Chinese Karst samples (Tiankeng)

Tiankeng, a typical Karst relief of the Late Quaternary Period, in the south of China, plays an important role when researching the surface shape and evolution of the Karst relief. The accumulation of Chlorine-36 (T1/2 = 301 ka) through the interaction of cosmic ray particles with Ca, K and Cl in rocks allow it to be used as the ideal nuclide for determination of the exposure age and erosion rate of limestone. In this work, an improved 36Cl sample preparation method for Tiankeng limestone was presented. The Isotope Dilution-AMS technique allows for the simultaneous measurement of 36Cl and Cl with a single AMS sample preparation. The barium introduced during the sample preparation was removed to avoid ionizer poisoning and attain a higher and more stable beam current. The method is particularly suitable for limestone samples, as well as for soil, sediment and water samples.

Studying Nuclear Structure at the extremes with S3

The in-depth study of the regions of Superheavy elements and the proton drip line around 100Sn are two major challenges of today’s Nuclear Physics. Performing detailed spectroscopic studies on these nuclei requires a significant improvement of our detection capabilities. The Super-Separator-Spectrometer S3 is part of the SPIRAL2 facility at GANIL. Its aim is to use the high stable beam currents provided by the new LINAC to reach rare isotopes by fusion-evaporation.

Investigations on the time evolution of the plasma density in argon electron-beam plasma at intermediate pressure**Supported by National Natural Science Foundations of China (No. 11375187) and the Foundation of State key Laboratory of China (No. SKLIPR1510).

The time evolution of the argon electron-beam plasma at intermediate pressure and low electron beam intensity was presented. By applying the amplitude modulation with the frequency of 20 Hz on the stable beam current, the plasma evolution was studied. A Faraday cup was used for the measurement of the electron beam current and a single electrostatic probe was used for the measurement of the ion current. Experimental results indicated that the ion current was in phase with the electron beam current in the pressure range from 200 Pa to 3000 Pa and in the beam current range lower than 20 mA, the residual density increased approximately linearly with the maximum density in the log-log plot and the fitting coefficient was irrelative to the pressure. And then three kinds of kinetic models were developed and the simulated results given by the kinetic model, without the consideration of the excited atoms, mostly approached to the experimental results. This indicated that the effect of the excited atoms on the plasma density can be ignored at intermediate pressure and low electron beam current intensity, which can greatly simplify the kinetic model. In the end, the decrease of the plasma density when the beam current was suddenly off was studied based on the simplified model and it was found that the decease characteristic at intermediate pressure was approximate to the one at high pressure at low electron beam intensity, which was in good accordance with the experimental results.

真空エレクトロスプレーを用いるイオン銃の開発

Atmospheric pressure electrospray had been used previously in our laboratory to generate massive cluster ion beam for charged droplet ionization; a method we called electrospray droplet impact (EDI). Previous ion gun that employed atmospheric pressure electrospray lacks adequate beam current and density for surface and interface analysis. To improve the EDI beam performance, we have proposed a technique for producing a stable electrospray of aqueous solution under vacuum. It is well known that vacuum electrospray of volatile liquids is difficult because of the freezing of the liquids by evaporative cooling under vacuum. Vacuum electrospray of aqueous solution was achieved by irradiating the tip of the electrospray emitter with infrared or near-infrared laser to prevent the freezing under high vacuum (<0.05 Pa). A stable and large beam current was also obtained by optimizing the vacuum electrospray conditions. On the basis of these results, the vacuum electrospray can be expected to be a high-performance massive cluster ion gun.

Low current performance of the Bern medical cyclotron down to the pA range

A medical cyclotron accelerating H− ions to 18 MeV is in operation at the Bern University Hospital (Inselspital). It is the commercial IBA 18/18 cyclotron equipped with a specifically conceived 6 m long external beam line ending in a separate bunker. This feature is unique for a hospital-based facility and makes it possible to conduct routine radioisotope production for PET diagnostics in parallel with multidisciplinary research activities, among which are novel particle detectors, radiation biophysics, radioprotection, radiochemistry and radiopharmacy developments. Several of these activities, such as radiobiology experiments for example, require low current beams down to the pA range, while medical cyclotrons are designed for high current operation above 10 μA. In this paper, we present the first results on the low current performance of a PET medical cyclotron obtained by ion source, radio-frequency and main coil tuning. With this method, stable beam currents down to () pA were obtained and measured with a high-sensitivity Faraday cup located at the end of the beam transport line.

Study on the steady operating state of a micro-pulse electron gun

Micro-pulse electron gun (MPG) employs the basic concept of multipacting to produce high-current and short-pulse electron beams from a radio-frequency (RF) cavity. The concept of MPG has been proposed for more than two decades. However, the unstable operating state of MPG vastly obstructs its practical applications. This paper presents a study on the steady operating state of a micro-pulse electron gun with theory and experiments. The requirements for the steady operating state are proposed through the analysis of the interaction between the RF cavity and the beam load. Accordingly, a MPG cavity with the frequency of 2856 MHz has been designed, constructed, and tested. Some primary experiments have been finished. Both the unstable and stable operating states of the MPG have been observed. The stable output beam current has been detected at about 3.8 mA. Further experimental study is under way now.

Quantitative Microanalysis at Low Voltage with a WDS Electron Microprobe Equipped with a FE Column

Thanks to its precision, its reproducibility and its stability, Electron Microprobe is a well suited technique for accurately analyzing nearly all chemical elements at concentration levels down to few 10’s ppm with a spatial resolution of about 1 μm, which is relevant to microstructures in a wide variety of materials and mineral specimens. With the development of the Schottky emitter and its implementation as electron source in Electron Microprobe, small features are commonly analyzed down to sub-micrometer scale. Thanks to the high brightness of the Schottky emitter, a fine focussed electron beam can be achieved with both high and stable beam currents even at low accelerating voltages (≤10 keV). Since X-rays are generated from a much larger diameter than the diameter of the incident electron beam, it is necessary to optimize the two interdependent parameters, accelerating voltage and beam diameter, in order to take full advantage of the FEG electron source for X-ray analysis. The electron beam diameter increases when decreasing the electron beam energy. The interaction volume within which scattered electrons generate X-rays – decreases with the electron energy. Thus a small beam diameter is not always synonym of a small interaction volume and optimized conditions are obtained when the analytical spatial resolution is primarily limited to the diameter of the X-ray emission volume in a specific material. The ability to accurately quantify precipitate phases on the micrometer and sub-micrometer scale when working at low beam energy with high spatial resolution will be illustrated in examples such as Dunite, igneous rock locally enriched with Platinum. The analytical resolution determined from X-ray maps will be presented. All samples were analyzed with the CAMECA SXFiveFE.

Improvement of four anode rods ion source

In this work, an improved form of a saddle field ion source has been designed and constructed. It consists of four anode rods made from copper and two copper cathode discs. The two cathode discs are placed symmetrically on both sides of the four anode rods. The electrical discharge and output ion beam characteristics were measured at different pressures using argon gas. The optimum distance between each two anode rods was determined. Also the optimum distance between the four anode rods and any cathode disc was obtained. It was found that the optimum distance between each two anode rods equal to 6 mm, while the optimum distance between the four anode rods and any cathode disc equal to 16 mm, where a stable discharge current and maximum output ion beam current can be obtained. The effect of negative extraction voltage applied to both the extractor electrode and Faraday cup on the output ion beam current was studied. The sputter yield of copper and aluminum targets using argon ions of different energies was determined.

Porous iron pellets for AMS 14C analysis of small samples down to ultra-microscale size (10–25μgC)

Abstract We developed the use of a porous iron pellet as a catalyst for AMS 14C analysis of small samples down to ultra-microscale size (10–25 μgC). It resulted in increased and more stable beam currents through our HVEE 4130 14C AMS system, which depend smoothly on the sample size. We find that both the expected decrease of oxalic acid standards and increase of backgrounds with decreasing sample size, due to increasing influence of contamination, are reproducible. Using a mass-dependent background correction for dead (1.0 ± 0.4 μgC) and modern (0.25 ± 0.10 μgC) contamination, we obtain reliable results for small samples down to 10 μgC and possibly smaller. Due to our low graphitization yield for ultra-small samples (increases from 40% to 80% on average with sample size), we measured graphite standards as small as 3 μgC. The standard deviation of the corrected activity is about 5% for a 10-μgC HOxII standard. Here we report the iron pellet technique, which is new to the best of our knowledge. It is generally applicable for AMS 14C laboratories that want to measure small samples down to ultra-microscale size. As an illustrative test-case, we analyze 14C data for IAEA-C5, C7 and C8 samples with masses ranging from 15 to 300 μgC.

Operating characteristics of the hollow anode glow discharge ion source

In this work, a new shape of a glow discharge ion source with axial extraction has been designed and constructed. High output ion beam current can be extracted axially in a direction normal to the discharge region without using extraction system. Optimization of the distance between the anode and the cathode has been determined using argon gas. It is found that the optimum gap distance between the anode and the cathode is equal to 3.5 mm, where stable discharge current and maximum output ion beam current can be obtained. The discharge characteristics of the ion source at different operating gas pressures have been measured at this optimum distance between the anode and the cathode. A disk of Teflon insulator has been put between the anode and the cathode. This disk was covering the cathode area and reducing the discharge area on the cathode surface for discharge confinement, therefore, a higher output ion beam current could be obtained.

Investigation of Factors That Affect the Sensitivity of Accelerator Mass Spectrometry for Cosmogenic 10Be and 26Al Isotope Analysis

We report experiments designed to improve accelerator mass spectrometry (AMS) of (10)Be and (26)Al for a wide range of geological applications. In many cases, the precision of the AMS isotope ratio measurement is restricted by counting statistics for the cosmogenic isotope, which are in turn limited by the intensity of AMS stable ion beam currents. We present data obtained at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratories (LLNL) indicating that AMS ion beam currents are impacted by certain elemental impurities. For (10)Be analysis, the AMS ion beam current is most adversely affected by the presence of titanium (which can be challenging to separate chemically during sample preparation because of its tendency toward stable refractory forms) and aluminum (which can coelute with beryllium during cation exchange chromatography). In order to minimize impurities that suppress AMS ion beam currents, we evaluate, using inductively coupled plasma atomic emission spectroscopy (ICP-AES), a widely used chemical separation protocol involving a multiacid digestion scheme, preseparation elemental analysis, anion exchange chromatography, ad hoc selective precipitation, cation exchange chromatography, and postseparation elemental analysis.

Methods for High-Precision 14C AMS Measurement of Atmospheric CO2 at LLNL

Development of radiocarbon analysis with precision better than 2% has the potential to expand the utility of 14CO2 measurements for carbon cycle investigations as atmospheric gradients currently approach the typical measurement precision of 2–5%. The accelerator mass spectrometer at Lawrence Livermore National Laboratory (LLNL) produces high and stable beam currents that enable efficient acquisition times for large numbers of 14C counts. One million 14C atoms can be detected in approximately 25 min, suggesting that near 1% counting precision is economically feasible at LLNL. The overall uncertainty in measured values is ultimately determined by the variation between measured ratios in several sputtering periods of the same sample and by the reproducibility of replicate samples. Experiments on the collection of 1 million counts on replicate samples of CO2 extracted from a whole air cylinder show a standard deviation of 1.7% in 36 samples measured over several wheels. This precision may be limited by the reproducibility of oxalic acid I standard samples, which is considerably poorer. We outline the procedures for high-precision sample handling and analysis that have enabled reproducibility in the cylinder extraction samples at the &lt;2% level and describe future directions to continue increasing measurement precision at LLNL.

Longitudinal quadrupole instability and control in the Frascati DAΦNE electron ring

A longitudinal quadrupole ($q$-pole) instability was limiting the maximum stable current in the DA$\ensuremath{\Phi}$NE ${e}^{\ensuremath{-}}$ ring at a level of $\ensuremath{\sim}700--800\text{ }\text{ }\mathrm{mA}$. In order to investigate the phenomenon, the instability threshold has been measured as a function of various machine parameters as radio frequency voltage (${V}_{\mathrm{rf}}$), momentum compaction (${\ensuremath{\alpha}}_{c}$), number of bunches, fill pattern, etc. An unexpected interaction with the longitudinal feedback system, built to control the dipole motion, has been found and a proper feedback tuning has allowed increasing the threshold. The maximum stable beam current has now exceeded 1.80 A and it is no longer limited by the quadrupole instability.

On Landau Damping by Bunch Filling Pattern in the Pohang Light Source Storage Ring

The stable beam current of the Pohang Light Source (PLS) is normally limited to about 150 mA despite the design value of 400 mA at 2.0 GeV due to the coupled bunch mode instabilities (CBMIs) which are generated by the interaction between higher order modes of RF cavities and circulating beams. In order to improve this situation, a longitudinal feedback system (LFS) using parallel digital signal processors was installed during the summer maintenance period in 1999. Besides the cure of the CBMIs, this programmable LFS is useful for various beam diagnostics such as the synchrotron oscillation phases and their spreads, and the beam pseudo-spectra that are the beam spectra without beam revolution harmonics. With this bunch-to-bunch diagnostic information, it is found that properly arranged bunch filling patterns can increase the stored beam current significantly without an active longitudinal feedback due to Landau damping by bunch-to-bunch synchrotron frequency spreads.

Effect of magnetic field structure near cathode on the arc spot stability of filtered vacuum arc source of graphite

Amorphic diamond films deposited by the filtered vacuum arc (FVA) method have attracted much attention due to their superior mechanical and optical properties. However, the instability of the arc limits the continuous operation of the FVA source, resulting in a poor productivity. In the present work, we investigated the effects of the cathode shape and the structure of the magnetic field near the cathode on the erosion behavior by both computer simulations and experimental studies. Arc instability in the configuration of parallel magnetic polarities of the source magnet and the extraction could be suppressed by placing a permanent magnet of opposite polarity behind the cathode. We show further that oscillation of the current of the source magnet was effective in extending the area of the arc spot movement. A tapered cathode exhibited a more stable arc than a cylindrical cathode, as confirmed by the time variation of the beam current. By using the oscillating current of the source magnet and a tapered cathode of diameter 80 mm, a continuous operation for 2000 min with an arc current of 60 A was obtained, at which more than 90% of the cathode volume could be used. A stable beam current of about 350 mA was obtained under the present operating conditions.

New design of an ion source for accelerator mass spectrometry at the Lund Pelletron

The requirements for negative ion sources designed for accelerator mass spectrometry (AMS) are, e.g., a high and stable beam current, a high efficiency, a low memory effect, and a quick system for switching between different samples. For the AMS program at the Lund 3 MV Pelletron tandem accelerator a new caesium sputtering source was constructed and installed a few years ago. Some new mechanical features of the source, as well as tests of how well the requirements are fulfilled are reported.