Beam Current Measurements Research Articles

Tracing the atomic mass unit to the kilogram by ion accumulation

An experimental approach for linking the atomic mass unit to the kilogram with an uncertainty sufficiently small for a future re-definition of the kilogram is described. The concept consists of accumulation of ions from an ion beam up to a weighable mass and measurable total charge. The main problems and influencing factors connected with ion beam technology, weighing and current measurement together with the corresponding experimental solutions are discussed in detail. The first experiments with consistent results, but still large uncertainty, are described.

First operation of a superconducting RF-gun

For the first time, a superconducting RF gun where a photocathode is inside a superconducting cavity has been working stably over a period of seven weeks. The gun with a half-cell cavity has been operated in the cryostat at 4.2K. A maximal field strength of 22MV/m in the cavity and maximal beam energy of 900keV have been obtained. Measurements of beam current, transmission, energy spread and transverse emittance are presented and discussed.

ＳＱＵＩＤと超伝導磁気シールドを用いた高感度ビーム電流モニターの開発

Beam diagnostics are an essential constituent of any accelerator. There are a large variety of beam parameters, and total current is one of the most important parameters for accelerators. A current monitor is used to operate an accelerator efficiently and to improve the performance of the machine. A Faraday cup is the most fundamental current detection process, in which charged particles are stopped in the cup. However, this destructive method cannot be applied for high-current or high-energy beams because the total energy carried by the beam can destroy and activate the intercepting material. Therefore, non-destructive beam current measurement requires the use of current transformers that detect the magnetic field produced by the pulsed or DC beam. On the other hand, a new type of beam current monitor using a low-temperature superconducting (LTS) magnetic shield and an LTS SQUID was developed to measure the faint ion beams that are below the lowest measurable limit of the DC current transformer (DCCT) for atomic-physics studies. Recently, a prototype of a highly sensitive SQUID current monitor for measuring the intensity of faint beams, such as radioisotope beams, was completed for the RIKEN RI beam factory. This monitor is composed of a high-temperature superconducting (HTS) magnetic shield and an HTS SQUID. The first measurements using ion beams were carried out in the CNS experimental hall and RIKEN Ring Cyclotron (RRC). This paper first describes the principle of the conventional current monitor such as the Faraday cup and current transformers. Second, the progress of the LTS SQUID current monitor is discussed, and finally, the present status of the prototype of the HTS SQUID current monitor at RIKEN and the results of the first beam measurement are given.

Intrabeam scattering analysis of measurements at KEK’s Accelerator Test Facility damping ring

We derive a simple relation for estimating the relative emittance growth in x and y due to intrabeam scattering (IBS) in electron storage rings. We show that IBS calculations for the ATF damping ring, when using the formalism of Bjorken-Mtingwa, a modified formalism of Piwinski (where eta squared divided by beta has been replaced by the dispersion invariant), or a simple high-energy approximate formula all give results that agree well. Comparing theory, including the effect of potential well bunch lengthening, with a complete set of ATF steady-state beam size vs. current measurements we find reasonably good agreement for energy spread and horizontal emittance. The measured vertical emittance, however, is larger than theory in both offset (zero current emittance) and slope (emittance change with current). The slope error indicates measurement error and/or additional current-dependent physics at the ATF; the offset error, that the assumed Coulomb log is correct to within a factor of 1.75.

Active pixel detector for ion beam profiling

The use of ion implanters has been vital to the downward scaling of device dimensions in silicon technology. To achieve good process control, the ion beam characteristics within the implanter must be well known. Hybrid pixel technology is advanced as a possible solution to the problem of real-time ion beam profiling and dosimetry. The proof-of-principle prototype detector is described. The read-out electronics comprises a charge integrating chip designed in Europractice. The detector has been tested in an argon ion beam. The results show real-time beam profile acquisition and current measurement. The effects of secondary electron emission are qualitatively observed for different beam energies and must be addressed for accurate beam profiling.

Study on beam size correction free from Coulomb interaction

The effect of Coulomb interaction in a variable shaped e-beam (EB) lithography system was studied. The effect of Coulomb interaction was estimated by a simulation. The simulation results predicted that beam blur due to Coulomb interaction in a variable shaped EB lithography system operated at the beam current density as high as 10 A/cm2 was not negligible for measurement and correction of beam size based on beam profile measurement, which deteriorates beam size accuracy by several hundredth micron. A new beam size correction method free from the effect of Coulomb interaction, which was based on beam current measurement, was implemented. The beam size accuracy of the new correction method was compared to that of conventional one, experimentally. While beam size errors exceeding 35 nm were found with the conventional correction method based on beam profile measurement, beam size errors were less than 10 nm with the new correction method based on beam current measurement. A beam size correction method based on beam current measurement is indispensable for a variable shaped EB lithography system operated at high current density in order to avoid the effect of Coulomb interactions and to attain high CD accuracy.

Measurement of low currents in an external beam set-up

It is well known that an accurate measurement of beam currents in an external set-up is not a straightforward task. Charge collection in a Faraday cup can be altered by ionisation along the beam path in atmosphere. These problems are more and more relevant when weaker currents are used, as in the ion beam analysis of precious objects where beam-induced damage should be strictly avoided. We performed a calibration of the integrated charge values obtained with a Faraday cup in an external set-up, at different beam current intensities, versus the “true” values. Effects of wrong charge integration have been detected, with increasing relevance at currents below 3 nA. The measurement of ultra-weak currents in atmosphere adds further problems for their integration. By measuring the X-ray yield obtained from a rotating chopper made of thick nickel in front of the target we obtain a sufficiently accurate beam current monitoring and charge-equivalent normalisation down to below 1 pA.

Measurement of the beam current in the compact storage ring based on the conservation of RF power

AURORA is an electron storage ring of the weak focusing type composed of a single body superconducting magnet and does not allow the use of a conventional method for the measurement of stored beam current. A new method has been developed, in which the power conservation in the RF accelerating system and the normalized load impedance are taken into account and combined in to a single equation. By measuring the beam loading through the radiation power measurement at various magnitudes of stored beam current and referring to the equation, the absolute value of the beam current can be obtained.

Results from the scaled final focus experiment

Vacuum ballistic focusing is the straightforward method to obtain a heavy-ion beam spot size necessary to drive an inertial confinement fusion target. The beam is first expanded then focused to obtain the desired convergence angles at the exit of the last element. This is done in an attempt to achieve a focal spot size in which emittance is the limiting factor; however, aberrations and space charge will influence the spot radius. Proper scaling of particle energy, mass, beam current, beam emittance, and magnetic field replicates the dynamics of a full driver beam at the focus in a small laboratory experiment. By scaling the beam current to ∼100 A, 160 keV Cs + has been used to study experimentally a proposed driver design at one-tenth scale. Once a nominal focal spot is achieved, the magnet strengths are deliberately de-tuned to simulate the effect of an off-momentum slice of the beam. Additionally, several methods will be used to inject electrons into beam following the last focusing element in order to study the neutralization of space charge and its effect on the focus. Transverse phase space and beam current density measurements at various stages of the focus will be presented as well spot size measurements from the various trials. This data will be compared to the results of a PIC model of the experiment.

Design and performance of an HTS current comparator for charged-particle-beam measurements

Superconducting direct current comparators are well established for operation at liquid helium temperatures. We have begun investigations into HTS Cryogenic Current Comparator designs, which incorporate a flux concentrator and readout SQUID also made from HTS. This work aims to produce a system capable of high accuracy, non-invasive, measurements of an ion or electron beam current in the range /spl sim/10 nA to 1 mA. We outline a proposal for a precise determination of the faraday fundamental constant, using purely physical systems, and employing an HTS CCC for charged particle beam current measurement. In addition estimates of the potential current sensitivity are reported, based on measurement results of a prototype system, and proof-of-principle measurements of ion beam currents are described. The design and construction of a robust, compact cryogenic unit suitable for further, more realistic, field trials are discussed.

A new method of experimental investigation of high-power electron beam

An equipment for measuring radial current distribution in electron beam produced by technology electron optical system with power up to 10 kW is described. The principle of operation of apparatus and examples of beam characteristics are presented. Two movable plates: one water-cooled protection plate and other- analysing plate behind it are used. Every plate is moving in the direction of one of two perpendicular axis in planes parallel to the focal beam plane. A slit in every plate, which is perpendicular to the axis of its movement, determines the point of beam current density measurement. The electron beam is analysed in the plane where its cross-section is big enough (3–5 mm) relative to a very narrow slit (0.05 mm). The beam changes focus and deflects regularly to avoid melting or damaging of the protection plate. During the measurements of the electron beam current density in any point of beam spot, the electron beam is in a static position and the beam position respectively, measuring the current passing through two slits is one and the same. As a result, high resolution and many details of the current distribution in the beam focus spot can be obtained. This gives a possibility to adjust the electron gun equally in different runs and to obtain the same welding results at other equal technology conditions. A comparison between electron beams of the different electron guns is possible.

Pockels cell voltage probe for noninvasive electron-beam measurements

Accurate measurements of beam position and current are critical for the operation of the high-energy electron accelerators used for radiographic applications. Traditional short-pulse (e.g., 70 ns) machines utilize B-dot loops to monitor these parameters with great success. For long-pulse (e.g., 2 micros) accelerators, beam position and current measurements become more challenging and may require new technology. A novel electro-optic voltage probe has been developed for this application and provides the advantages of complete galvanic isolation, excellent low-frequency performance, and no time integration requirement. The design of a prototype sensor is presented along with preliminary accelerator test data.

The effect of the first dynode's curvature on the gain of a type 226EM electron multiplier used as a detector of highly charged ions

The gain of a type 226EM electron multiplier with BeCu dynodes having a cylindrical shape was measured in terms of its dependence on the position where ions bombarded the surface of the first dynode. This position-impact dependence of the gain exhibits a great decrease if the bombarded position on the first dynode's surface departs from the input window into the second dynode region. The presented results show that the gain of any 226EM as well as those of other electron multipliers equipped with curved first dynodes depends not only on the charge state and the kinetic energy of detected ion species but also on the ion beam's density profile and on the impact position on the first dynode's surface. An application of the 226EM for measurement of an ion beam current is discussed. A beam of ions with a kinetic energy of 690 keV was used as a probing beam.

Beam dynamics studies with the heavy-ion linear induction accelerator MBE-4

Current amplification of heavy-ion beams is an integral feature of the induction linac approach to heavy-ion fusion (HIF). In this paper we report on amplification experiments conducted on a single beam of the Multiple Beam Experiment (MBE-4), a heavy-ion (Cs+) induction linac. Earlier MBE-4 experiments [H. Meuth et al., Nucl. Instrum. Methods Phys. Res. A 278, 153 (1989)] had demonstrated up-to-9× current amplification but had been accompanied by an up-to-2× increase of normalized transverse emittance. Experiments to pinpoint the causes of this emittance growth indicated various factors were responsible, including focusing aberrations and mismatch difficulties between the injector diode and the accelerator transport lattice, a localized quadrupole misalignment problem, and the interaction of transversely large beams with the nonlinear elements of the focusing lattice. Following ameliorative measures, new current amplification experiments, both with and without acceleration, showed that current amplifications of up to 3× and line charge density increases of up to ≈2× could be achieved without increasing the beam’s normalized transverse emittance. Finally, both the transverse beam dynamics, and beam current and energy measurements were accurately modeled by numerical particle-in-cell simulations and longitudinal dynamics codes, respectively.

High yields from the Stockholm electron beam ion source CRYSIS

CRYSIS is an electron beam ion source (EBIS) with a superconducting solenoid. Highly charged ions are delivered to the acceleration and storage ring CRYRING [ K. Abrahamsson et al., Nucl. Instr. Methods B 79 (1993) 269.], SMILETRAP [C. Carlberg et al., Phys. Scr. T 59 (1995) 196.] and to low energy atomic and surface physics experiments. Stable electron beam currents up to 700 mA are obtained, in order to enhance the ion yield out of the EBIS. Measurements of the total charge per pulse at different working conditions and electron beam current density measurements were done. At electron beam currents of 600 mA yields up to 2.5 × 1010 charges per pulse could be measured.

Quantitative EDS analysis in the environmental ESEM using a bremstrahlung intensity-based correction for primary electron beam variation and scatter

Quantitative EDS analysis of bulk samples in the scanning electron microscope (SEM) or electron microprobe requires, as a fundamental parameter, a stable and reproducible primary electron beam current Beam current is usually measured with a Faraday cage positioned in the electron column below the objective aperture or in the specimen holder. Reproducibility and stability within 1%/hour is a minimum condition.Primary beam current measurement in the ESEM or any high pressure SEM is difficult to measure. Electron-gas interaction in the biased chamber generates a positive ion flow highly amplified relative to the primary beam (Danilatos, 1990) and generates an x-ray signal from the gas. The latter signal amplitude is dependent on primary beam current, chamber pressure and backscatter electron signal from the specimen (Griffin et al. 1993). These interactions prevent quantification of EDS data standardised to Faraday cage primary beam current measurements or x-ray counts from a reference standard.

Non-intercepting continuous beam current measurement for a high-current ion implanter

For high accuracy dosage determination, a parametric current transformer (PCT) has been built into the target end-station of a Danfysik Model 1090 high-current implanter. The PCT is an instrument designed for continuous measurement of the intensity of a charged particle beam (a.c. or d.c.) without intercepting it. The PCT can be positioned outside the vacuum chamber. A special PCT has been designed for this application with an inner diameter of 340 mm. It is mounted around the beam scanning chamber, between the scanning magnets and the target position. Since the PCT is magnetically sensitive, it has been shielded. With the current transformer installed and with a well-known scanning area, it is possible to monitor the implanted dose with an accuracy better than 5%. The present article describes the current measurement system and its applications.

K130 beam current measurement system

A measurement system for very low currents, developed to be used in the K130 cyclotron at University of Jyväskylä, is described. The beam intensity measurement is implemented with a current preamplifier and signal multiplexor. The measurement is controlled and visualised with a commercial data acquisition card integrated in a PC.

Development of a beam current monitor by using an amorphous magnetic core

The high performance amorphous magnetic core monitor (ACM) for the measurement of electron beam currents has been developed. This monitor is composed of an amorphous magnetic core, radiation shields, a winding, magnetic absorbers, a ceramic vacuum duct and a SMA connector. The ACM showed the very fast rise and fall times ( < 1 ns), the high sensitivity (5 V/A at 50 Ω load), the good linearity, and good S/N ratio due to the high permeability of the amorphous magnetic core. The monitor works as a primary transformer. The time-response was simulated by an electric circuit analysis code.

Pressure compensated dose control in high current ion implantation systems

High power ion implantation into photoresist coated wafers poses special problems for the process engineer. As the incoming ion enters the photoresist layer, molecules and atoms from the photoresist are liberated and become part of the background gas through which the ion beam must travel. Subsequent atomic collisions between the background gas atoms and the beam ions result in some degree of neutral beam flux which cannot be measured by conventional faraday systems. The Baton high current ion implantation systems utilize faradays to measure beam current and ion gauges to measure pressure simultaneously. The beam current and pressure measurements are combined to provide a means of estimating the degree of neutral particle flux and controlling dose rate based on these measurements. This paper describes the pressure compensated dose control concept, presents procedures for optimizing the control, and presents typical performance results.