Transverse Collimation Research Articles

Commissioning and operation of the transverse collimation system at the RCS of CSNS

In high-intensity proton synchrotrons, controlling beam loss during machine operation is crucial to prevent radiation damage. Uncontrolled beam loss poses a significant challenge in achieving higher beam intensity and power in these synchrotrons. The beam collimation system plays a vital role in removing halo particles and localizing beam loss, serving as a critical tool for controlling uncontrolled beam loss in high-intensity proton accelerators. To address the issue of uncontrolled beam loss, a two-stage transverse collimation system was designed at the rapid cycling synchrotron (RCS) of China Spallation Neutron Source (CSNS). However, during the beam commissioning, it was found that the collimation efficiency was compromised due to the small ratio of the physical acceptance to the collimator acceptance and insufficient phase advances between the primary and secondary collimators. To address this issue, the designed two-stage collimator was modified to a one-stage collimator. Through optimization of the collimation system, the beam loss was effectively localized within the collimator area. As a result, CSNS achieved the design power of 100 kW with minimal uncontrolled beam loss.

Features of Dynamics and Instability of Plasma Jets Expanding into an External Magnetic Field in Laboratory Experiments with Compact Coaxial Plasma Generators on a Large-Scale “Krot” Stand

Structural instabilities that develop during pulsed injection of dense plasma jets into vacuum in the presence of an external quasi-homogeneous magnetic field are studied by high-speed photography using ICCD cameras. The experiments are carried out in the chamber of the “Krot” stand, which has record-breaking dimensions in its class of installations (diameter—3 m, length of the working section—10 m), and makes it possible to study plasma dynamics by various diagnostic methods at scales of more than 1 m both along the magnetic field and in the direction transverse to the magnetic field. During injection along the magnetic field, a transverse collimation of the flow of ionized matter and the development of a flute instability of the plasma boundary are observed, which, at the late stages of expansion, leads to the plasma leaving the injection region in the form of several jets across the field. During transverse injection, the formation of a collimated flow, a “plasma sheet,” is observed, in which, as the plasma moves across the field, inhomogeneous structures develop in the direction of injection.

Collimation method studies for next-generation hadron colliders

In order to handle extremely-high stored energy in future proton-proton colliders, an extremely high-efficiency collimation system is required for safe operation. At LHC, the major limiting locations in terms of particle losses on superconducting (SC) magnets are the dispersion suppressors (DS) downstream of the transverse collimation insertion. These losses are due to the protons experiencing single diffractive interactions in the primary collimators. How to solve this problem is very important for future proton-proton colliders, such as the FCC-hh and SPPC. In this article, a novel method is proposed, which arranges both the transverse and momentum collimation in the same long straight section. In this way, the momentum collimation system can clean those particles related to the single diffractive effect. The effectiveness of the method has been confirmed by multi-particle simulations. In addition, SC quadrupoles with special designs such as enlarged aperture and good shielding are adopted to enhance the phase advance in the transverse collimation section, so that tertiary collimators can be arranged to clean off the tertiary halo which emerges from the secondary collimators and improve the collimation efficiency. With one more collimation stage in the transverse collimation, the beam losses in both the momentum collimation section and the experimental regions can be largely reduced. Multi-particle simulation results with the MERLIN code confirm the effectiveness of the collimation method. At last, we provide a protection scheme of the SC magnets in the collimation section. The FLUKA simulations show that by adding some special protective collimators in front of the magnets, the maximum power deposition in the SC coils is reduced dramatically, which is proven to be valid for protecting the SC magnets from quenching.

Thermal analysis and tests of W/Cu brazing for primary collimator scraper in CSNS/RCS

To the transverse beam collimation system in a rapid cycling synchrotron, an important component is the primary collimator, which improves emittance of the beam halo particles such that the particles outside the predefined trajectory can be absorbed by the secondary collimators. Given the material properties and power deposition distribution, the beam scraper of the primary collimator is a 0.17 mm tungsten foil on a double face-wedged copper block of 121.5 mm × 20 mm. The heat is transferred to the outside by a ϕ34 mm copper rod. In this paper, for minimizing brazing thermal stress, we report our theoretical analysis and tests on brazing the tungsten and copper materials which differ greatly in size. We show that the thermal stress effect can be controlled effectively by creating stress relief grooves on the copper block and inserting a tungsten transition layer into the copper block. This innovation contributes to the successful R&D of the primary collimator. And this study may be of help for working out a brazing plan of similar structures.

Collective transverse cavity cooling of a dense molecular beam

Based on a classical point particle description, we study light induced transverse collimation and phase space compression of a fast molecular beam traversing the crossover region of a high-Q optical cavity and a standing-wave laser beam. The molecules scatter pump laser light, far detuned from any molecular transition, into the resonant cavity mode. We show that despite a very small probability for this process for a single particle, collective enhancement can lead to significant transverse cooling and collimation above the self-organization threshold. The analytical formulae for this threshold derived from homogeneous one-dimensional (1D) models give surprisingly good estimates for the pulsed 3D case. We compare ring and standing wave cavity geometries and show that phase space compression can be achieved even for extremely small particle field coupling if compensated by a large density and pump power. In the vicinity of the critical point the compression time can be reduced by tailored seeding of the cavity mode or using a second buildup cavity for the pump field.

Laser mode effects on the ion acceleration during circularly polarized laser pulse interaction with foil targets

Ion acceleration by circularly polarized laser pulses interacting with foil targets is studied using two dimensional particle-in-cell simulations. It is shown that laser pulses with transverse super-Gaussian profile help in avoiding target deformation as compared with the usual Gaussian pulse. This improves monochromaticity of the accelerated ion spectrum. Two kinds of surface instabilities have been found during the interaction. These instabilities can potentially break the target and destroy the quasimonoenergetic character of the final ion spectrum. Combined laser pulses with super-Hermite and Gaussian modes are used to improve the ion acceleration and transverse collimation.

Derivation and Validation of a Sensitivity Formula for Slit-Slat Collimation

An analytic formula is derived for the sensitivity of collimators achieving transverse collimation with a slit and axial collimation with a slat assembly whose septa may be parallel or focus on a line. The formula predicts sin(3) phi dependence on the incidence angle and, in the particular case of parallel slats, 1/h dependence on the distance from the slit. More complex expressions for sensitivity that do not diverge at points near the slit or the focal line of the slat assembly are also derived. The predictions of the formulas are checked against simple cases for which solutions are available from direct calculation as well as against Monte Carlo simulation and published experimental data. Agreement is good in all cases analyzed. An approximate penetration model is also introduced: it involves the use of a sensitivity-effective slit width and septal length. Its predictions are compared to simulation results. Agreement was found to be compatible with statistical fluctuation (+/- 0.3%) for geometric sensitivity and better than 3% of total sensitivity in the worst case of septa designed for high-energy (364.5 keV) photons.

Macro-particle simulation study on transverse halo collimator for J-PARC linac

To realize the transverse emittance required for the ring injection, a transverse halo collimation system of the charge-exchange type has been developed for J-PARC (Japan Proton Accelerator Research Complex) linac. In this type of collimation system, negative hydrogen ions constituting a tail portion are charge-exchanged to protons with thin foil collimators. Then, the generated protons are separated by a bending magnet to be delivered to a dedicated beam dump. Because a two-species beam is transported along the collimation system, a special care is to be taken to avoid the uncontrolled beam loss along the beam transport. To this end, it is of practical importance to develop an accurate numerical model that has the capability of both particle tracking with space-charge effects and modeling of scattering at the charge-exchange foil. In this paper, a numerical model is developed for the collimation system using a three-dimensional particle-in-cell code, IMPACT, and a Monte-Carlo simulation tool kit, GEANT. Uncontrolled beam loss along the collimation system has been evaluated with this model, and the optimum foil thickness has been determined for the collimation system.

Macro-particle simulation of multi-species beams in CSNS/LRBT

At the Chinese Spallation Neutron Source (CSNS), a transverse collimation system consisting of foil scrapers and 60° triplet cells in the Linac to Ring Beam Transport line (LRBT) is designed, and the scraped particles are transported to the linac development dump. For hands-on maintenance, it is important to study the beam loss in transporting the stripped protons. Due to the lack of appropriate simulation codes to track the mixed beams, a new simulation code Stripping Collimation and Mixed Beam Transport (SCOMT) was developed to tackle the problem. The macro-particles simulation code tracks simultaneously the main H−, the fully stripped proton and the partially-stripped H0 beams. It also takes into account the stripping efficiency of H− in foils, nuclear absorption, nuclear elastic scattering and multiple scattering effect, etc. The SCOMT simulations were carried out at the LRBT for the design optimization. It is found that the second-time foil traverse of both the protons and the H0 particles has an important contribution to the beam loss. The foil thickness has been optimized to reduce the total beam loss.

Design of the two-stage collimation system for CSNS/RCS

The transverse collimation system of the Chinese Spallation Neutron Source (CSNS) Rapid Cycling Synchrotron (RCS) is expected to have a collimation efficiency of over 95%. The transverse collimation system is a two-stage collimation system, including a primary collimator and some secondary collimators. Here, the collimation efficiency of CSNS/RCS is deduced in the 2D case. The relationship between the collimation efficiency with phase advances and scattering angle is clearly shown. Finally, collimators are optimized with ORBIT code.

2D laser collimation of a cold Cs beam induced by a transverse B field

We describe transverse collimation of a continuous cold cesium beam (longitudinal temperature 75 μK) induced by a two-dimensional, blue-detuned near-resonant optical lattice. The mechanism described for a lin-‖-lin configuration is made possible by the application of a transverse magnetic field B ⊥. The phenomenon described differs from gray molasses, for which any small magnetic field degrades cooling, as well as from magnetically induced laser cooling in red-detuned optical molasses, where there are no dark states. The lowest transverse temperature is experimentally found to vary as B ⊥ 2 . The collimated flux density shows a dip as a function of B ⊥, the width of which is proportional to the cube root of the laser intensity, general features predicted by our semiclassical model. This technique provides a sensitive tool for canceling transverse magnetic fields in situ at the milligauss level.

Studies of the collimation system for the TTF FEL at DESY

A collimation scheme for the protection of the undulators at the TESLA Test Facility phase 2 (TTF2) is discussed. The transverse and energy collimation, and the protection against off-energy and mis-steered bunches are proposed in a beam line which, besides the collimation elements, integrates the fast feedback system and matches the beam to the undulator entrance.

Laser Manipulation of Atoms and Atom Optics

In this paper experiments on laser cooling, collimation and manipulation of a sodium atomic beam, such as the transverse collimation and decollimation of an atomic beam by a standing wave or a misaligned standing wave, longitudinal cooling of an atomic beam by a diffuse light field, sub-Doppler cooling in a blue detuned standing wave, are reported. The basic concept on atom optics is developed. An experiment on a method for the injection of atoms into an atomic cavity is also discussed.