Papers

In the storage ring proposed to measure the proton electric dipole moment (EDM), quadrupole misalignments of a few microns mimic a genuine EDM through a geometric (Berry) phase; at a 10 micron rms misalignment the false signal is up to a thousand times the target sensitivity of 1e-29 e-cm. The misalignment splits into an antisymmetric component, which distorts the closed orbit and is visible to the beam-position monitors, and a symmetric component, whose orbit signature is strongly suppressed. Standard single-beam orbit correction therefore removes only the antisymmetric part. The proposed design reaches the symmetric one by beam-based alignment (BBA), which is effective but relatively slow and invasive. Using symplectic beam and spin tracking, we show that correcting the two counter-rotating beams jointly recovers the symmetric component from the orbit itself; it is carried by the sum of the two orbits, whereas their difference is blind and a single beam is far too weak to recover it. The static BPM offset is degenerate with the symmetric misalignment in this sum, so the scheme does not replace the initial absolute alignment. However, the drift is differential and the offset cancels, so continuous two-beam correction maintains the alignment after a single initial BBA calibration, reducing the repeated realignment required by the original proton EDM proposal.

It is shown that the convective gain of Stimulated Raman Backscatter (SRBS) within a single laser wavelength in the strongly coupled regime is equivalent to the Pierce parameter commonly used to describe high gain x-ray Free Electron Lasers (XFELs). The comparison utilizes the fact that the gain is a Lorentz invariant so the Pierce parameter is written in terms of the FEL parameters in the electron beam's rest frame. It is thus inferred that there is a direct correspondence of the physics between SRBS in the strong-coupled regime and high gain XFELs in the Compton regime.

In multi-petawatt laser plasma accelerators, achieving high-density relativistic electron beams is typically accompanied by large transverse divergence, limiting the attainable effective electron density needed for high-flux interaction regimes relevant to laboratory astrophysics. Here we report experimental demonstration of self-generated magnetic pinching (SMP), a collective mechanism that actively regulates transverse beam dynamics using a Laguerre-Gaussian laser at strong relativistic intensity (~8 x 10^19 W/cm^2) interacting with an underdense plasma. The electron beam evolves from a two-lobe high-charge injection structure into a compressed, high-density profile, yielding a threefold reduction in divergence and nearly an order-of-magnitude enhancement in effective beam density compared with a Gaussian driver. Particle-in-cell simulations agree with the experimental observations and reveal that a self-generated azimuthal magnetic field governs the electron dynamics within the SMP regime, which is defined by the forming condition S = 0.717 l a0 [ne(10^18 cm^-3)]^-3/4 = 1, where l, a0, and ne are topological charge, laser amplitude, and plasma density, respectively. A transient kick from a dense inner sheath electron population drives collective magnetic pinching, transforming an initially separated electron distribution into a compressed and well-collimated beam. For higher-power laser systems, the forming condition can be extended to higher plasma densities and larger orbital angular momentum modes, potentially enabling electron beams with charges exceeding several nC and effective densities above 10^19 cm^-3. This mechanism provides a route to overcoming transverse expansion and enhancing rare interaction processes relevant to high-flux particle sources.

Realization of a low-emittance muon beam through the acceleration of keV-scale muons requires the injection of a suitably matched beam into an accelerator, since beam mismatch can lead to emittance growth and reduced acceleration efficiency. In one such scheme, muons are first thermalized to room temperature and then injected into a linear accelerator. Non-destructive diagnostics are challenging because of the low energy and low intensity. We developed a compact low-energy muon diagnostic system compatible with the accelerator under construction at J-PARC. The system is designed to evaluate beam conditions required for precise tuning prior to acceleration. Commissioning with low-energy muon sources shows the system's capability to identify low-energy muon signals and measure beam profiles.

This contribution presents the method used for beam energy monitoring at the European XFEL. It relies on high-performance trajectory fitting of beam position monitor data against an online model of the accelerator lattice. Unlike most other methods, it can be applied to virtually any section of the lattice with sufficient dispersion without the need for manual adaptation. Our implementation is fast enough to measure the energy of many thousands of bunches per second in realtime on conventional x86 hardware shared with other processes. A short outline of the technique is presented and illustrated with data from the accelerator.

Plasma is produced in a Large Volume Plasma Source (LVPS; dia. = 1 m, height = 1m) using CW microwaves (= 400 - 600 W, 2.45 GHz), in a compact ECR plasma source (CEPS) attached to LVPS, at hydrogen gas pressures = 1 - 3 mTorr. Plasma expands along the CEPS magnetic field into LVPS. A Hiden Analytical HPR 60 molecular beam mass spectrometer (MBMS) is used to measure the H^- ion energy distribution functions (IEDFs) in the downstream plasma. Previous plasma characterization studies in LVPS indicated favourable downstream plasma conditions for volume production of H^- ions. Measurements conducted with the MBMS probe aligned facing the plasma flow = 80 cm downstream, gave typical H^- count rates = 3 x 10^5 counts /s, at = 400 W, = 1 mTorr, along with a distinct high energy tail (<= 20 eV). These and other results are analyzed in detail. The positive ion spectrum showed the H_3^+ count to be consistently high in all cases (= 60-70 %); the counts for H_2^+ and H^+ were =30-35 % and a =few %. Combining the Langmuir probe (LP) and MBMS data it is possible to determine the approximate densities in front of the MBMS probe aperture. At = 500 W and = 2 mTorr, one finds: n_(H^+) = 9.6 x 10^9 cm^(-3), n_(H_2^+) = 1.7 x 10^10 cm^(-3) and n_(H_3^+) = 4.3 x 10^10 cm^(-3). The corresponding H^- density, = 80 cm downstream is n_(H^-) = 3.9 x 10^8 cm^(-3). Accounting for all H^- losses due to scattering and destruction, one finds the effective mean free path for H^- loss to be = 12.4 cm. Noting that H^- formation takes place about = 10 - 30 cm downstream of the source exit, the approximate average H^- density in the formation zone is determined as = 5.5 x 10^10 cm^(-3). This value is remarkably encouraging for H^- production in volume mode, considering the large chamber volume and area, as well as the very moderate power used for the experiments.

Betatron coupling near the difference resonance has been explored and adopted in several fourth-generation storage rings, yet its influence on high-order-mode-driven transverse coupled-bunch instabilities has not been systematically established. Using the Hefei Advanced Light Facility (HALF) as an example, we investigate this effect through theoretical analysis and macroparticle tracking simulations. The theoretical predictions agree well with macroparticle tracking simulations in both the weak- and strong-instability regimes, validating the description over the parameter ranges considered. For the vertical HOMs studied with HALF parameters, suitable betatron coupling effectively suppresses the instability, offering potential benefits for transverse stability control.

We present an optimization procedure using machine learning (ML) libraries for optimization of electron beam transport for maximal bunch compression and optimal operation of a bunched-beam Superradiant FEL. This is exemplified for the parameters of the 6MeV ORGAD Accelerator at Ariel University that is driving a THz Superradiant waveguide FEL. For superradiant emission (proportionally to the number of electrons squared), the bunch duration ${\sigma}_t$ at the undulator should be shorter than the optical period (2${\pi}/{\omega}$) of the radiation. Also, the beam transport optimization must confine the transverse dimensions of the beam to enter the undulator waveguide. The variables of the ML Bayesian optimization are the RF parameters and the currents of the coils and quads along the beamline. The beam dimensions and duration are provided from full 3D GPT simulations that are automatically driven by the ML exploration and exploitation algorithms. Twenty-five simulation iterations sufficed to arrive to an optimal beam transport design.