Deyang Yu

A two-dimensional beam-profile monitor has been constructed based on a scanning Faraday cup array, a 128-channel picoammeter system, and a LabVIEW-based data acquisition system. The Faraday cup array consists of 128 small Faraday cups, while each is connected to an independent picoammeter of the picoammeter system. It is driven across the ion beam by a servo motor and images the two-dimensional beam profile by directly measuring the absolute electric current distribution. The benefit from the avoidance of readout switching between different Faraday cups is that the crosstalk is negligible during a beam scanning. Typically, this monitor scans a beam cross section of 76.8 × 76.8 mm2 within 18–420 seconds. Tests with 50-keV O5+ and 1.8-MeV Xe25+ beams show that an ion beam with beam flux greater than ∼1 pA/mm2 can be clearly distinguished and a spatial resolution of ∼0.6 mm can be achieved. The total beam current measured by this device was calibrated using a Faraday cup, showing a systematic negative deviation of ∼6%–8% relative to the reference current, which is mainly attributed to inlet diameter tolerance and alignment errors.

We demonstrate that secondary ions sputtered from a macrocapillary's inner surface by the primary beam induce premature saturation of the guiding field, hindering fast ion guiding. By suppressing secondary ion sputtering with grooved surfaces, we achieve a guiding-field potential difference exceeding 1 kV, 2 orders of magnitude larger than previously achieved in stable ion guiding. This enables stable guiding of a 20-keV/q O^{5+} beam at offset angles up to 15°, with transmitted ions retaining their initial energy and charge state. A self-consistent field model accurately reproduces these results, revealing that guiding performance relies on managing secondary ions and the channel's voltage tolerance, while remaining robust against beam fluctuations. This Letter paves the way for ionic fibers that, much like optical fibers for light, passively and adaptively control ion transport.

We explored a distinct mechanism for matter creation via electron-positron pair production during bound-bound transitions in the deexcitation of muonic atoms. For ions with nuclear charges Z ≥ 24, transitions from low-lying excited states to the 1s-muon state can lead to the production of electronpositron pairs. We show that the Breit interaction determines the transition probabilities for states with nonzero orbital momentum. We show that the pair production arises mainly from the decay of the 2p states. Thus, the Breit interaction governs electron-positron pair production in bound-bound muon transitions. This process offers a unique opportunity to explore quantum electrodynamics in strong fields, as well as a class of nonradiative transitions involving electron-positron pair production.

We present a Faraday Cup Array (FCA) beam profile monitor designed for rapid monitoring of high-intensity direct current ion beams. The monitor consists of 2112 small Faraday cups (FCs) arranged uniformly in a grid pattern over an area of 89.2 × 89.2 mm2. These FCs are arranged in rows and columns, forming 64 independent channels, which are evenly split into 32 x-channels and 32 y-channels, with each channel comprising 33 FCs. Each channel is connected to an independent channel of a 64-channel picoammeter, allowing all channels to conduct current measurements simultaneously and independently. The monitor is capable of directly detecting the projections of beam current distribution in both horizontal and vertical directions with a spatial resolution of 2.75 mm within 1 s. We tested it using He2+ beams with energies of 120 and 200 keV, corresponding to beam powers of 15 and 27.5 W, respectively. The temperature increases on its front and back sides were monitored during the test. We also studied the thermal conduction and temperature rise through simulations when it was exposed to ion beams.

The line ratios in X-ray emission resulting from charge exchange between highly charged ions (HCIs) and neutral atoms are not only crucial for accurately modeling astrophysical X-ray emissions but also offer a unique perspective on the charge exchange processes happening during collisions. The K X-ray spectra following charge exchange between Mg¹¹⁺ and He are presented for a collision velocity of 1489 km/s (11.5 keV/amu). The spectra were measured by two Silicon Drift Detectors capable of resolving the Mg¹⁰⁺ Kα, Kβ, Kγ, and Kδ+ lines. The line intensity ratios of Kβ, Kγ, and Kδ+ relative to the Kα line, as well as the hardness ratio, were obtained. The experimental results were compared with the theoretical results from a cascade model that utilizes the state cross-sections produced by multichannel Landau–Zener (MCLZ) calculation. It was discovered that the K X-ray spectrum features can be reproduced well by MCLZ theory when the contributions of both single electron capture (SEC) and autoionizing double capture (ADC) processes are included. This finding implies that the ADC feeding mechanism is significant and should be taken into account for the X-ray emission during charge exchange between highly charged ions and multielectron atoms.