8.5 Configuration of the RAON BLM system
8.5.1 Final layout of RAON BLM system
Taking all these results and our discussions together, we arrive the final layout of the RAON BLM system as shown in Fig. 8.18. From the low energy sections (LEBT to SCL3) through the bending sections (P2DT, CSS) to the high energy sections (SCL2), we plan to install at least one BLM per warm section in the superconducting linac. In SCL3, the BLC will be installed at each beam diagnostic box, whereas it will be installed every 3 diagnostic boxes in SCL2 to produce a strong current signal for 1 W/m loss (>100 nA). The ACCTs (DBCM network) are shown in red, which are placed at the start and end of the important acceleration structures.
Figure 8.16: Proportional counter (PC) current signal for gamma detection. Expected current signal in the beam loss detectors calculated from the radiation levels in Fig. 8.10 for 1 W/m slow losses, and the sensitivities of the detectors introduced in Fig. 8.15. The 1(2) label in the beam name refers to whether detector1(2) was used.
From this design study of the RAON BLM system, we aim to establish a strategy for operating the high-power heavy ion (from Proton to Uranium beams) accelerator facility safely and efficiently. Beam loss situations and their locations can be detected reasonably quickly and accurately. At present, PC and PD prototyping are underway, and test results with radiation sources will be reported elsewhere.
Figure 8.17: Plastic detector (PD) current signal for neutron detection. Expected current signal in the beam loss detectors are calculated from the radiation levels in Fig. 8.9 for 1 W/m slow losses, and the sensitivities of the detectors introduced in Eq. (8.2). The 1(2) label in the beam name refers to whether detector1(2) was used.
Figure 8.18: RAON BLM system layout. DBCM networks (pairs of ACCTs) are shown in red. Beam diagnostic chambers containing BLMs (BLC, PD, or PC) are shown in green.
Chapter 9
Summary and Discussions
In this dissertation, we summarized the mathematical derivations of the equations of motion (EOM) of a single particle, and the envelope equations of 2D coasting beam and 3D bunched beam. We obtained the space-charge fields of uniform density K-V beam and Gaussian density beams in transverse and longitudinal spaces.
We have studied the space-charge driven resonances in high-intensity linear accelerators under the periodic focusing channels. We clarified the difference between the parametric instability (also called coherent resonance) and the incoherent particle resonance which are the main concepts for halo forma- tion mechanisms in high-intensity accelerators.
Parametric instability is related to the exponential growth of the beam amplitude, which comes from the perturbation of oscillation frequencies in envelope equations. The parametric resonances are originated from beam mismatches of the uniformly distributed K-V beams. We verified the envelope instability stop bands in the tune depression space by using analytical calculations of linearized perturbed envelope equations. The envelope instability of B and Q modes is manifested when the zero-current phase advance is larger than 90◦, and the parametric nth-order resonances are manifested when the perturbed envelope oscillation has the wave number of 2πl/n(l=1,2,· · ·). We investigated the particle trajectories on the phase space with matched, mismatched, and parametric resonance conditions by using particle-core model. Furthermore, we have studied the self-consistent evolution of the space-charge dominant beams by solving the linearized Vlasov-Poisson equation of K-V beam.
Incoherent particle resonance is related to the extra forces in EOM of harmonic oscillation, which is originated from the non-linear space-charge fields. For realistic Gaussian density beams, the 4σ=360◦ fourth-order particle resonance is prominently observed and only the envelope instability (second-order parametric resonance) occurs while the higher-order parametric resonances are suppressed by Landau damping. In recent studies, it has been found that the space-charge driven fourth-order particle resonance is observed in linear accelerators and predominantly manifested over the envelope instability even for the initially well-matched conditions of non-linearly distributed beams.
From our studies in this dissertation, we reported the interplay of the 4σ=360◦fourth-order particle resonance and the envelope instability along the periodic focusing lattices in linacs. We investigated the
general stop band of the 4σ=360◦fourth-order particle resonance, which isσ0>90◦andσ<90◦. The fourth-order particle resonance stop band is wider than the envelope instability stop band, and inevitably manifested over short periods and the envelope instability is induced after long periods only within the envelope instability stop bands following the fourth-order particle resonance. It has been proven by using both analytical calculations and multiparticle simulations of initially well-matched Gaussian beams in terms of the relative emittance growth and halo parameter growth along the linacs.
It has been considered that the fourth-order particle resonance together with the envelope instability set the main operational limit in high-intensity linear accelerators that the zero current phase advance should be less than 90◦. However, it is highly desirable to overcome this operational limit in order to expand the range of choices of accelerator parameters including beam current and external focusing forces for the future development of higher intensity accelerators. Previous studies have been discussed mainly in the suppression of the parametric mode instabilities along the circular accelerators. Also, it has been recently reported that the active mitigation of the envelope instability can be achieved under a fast acceleration along the linacs. However, the fourth-order particle resonance still remains an important source for inducing emittance growth and halo formations, whereas the envelope instability is handled mainly by fast acceleration.
In that reason, we focus more on the mitigation of the 4σ =360◦ fourth-order particle resonance which is inevitably manifested independently of the lattice structures and more prominently observed along the high-intensity accelerators. We proposed the novel approach of using spinning beams to mitigate the space-charge driven 4σ=360◦ fourth-order particle resonance in high intensity linear ac- celerators. It has been found that the spinning beams have an intrinsic characteristics that can mitigate the impact of the fourth-order particle resonance on the emittance growth and halo formations over short periods and the subsequent envelope instability after all. We investigated the spinning effects on the fourth-order particle resonance by using analytical models and multiparticle simulation studies of 2D coasting beams and 3D bunched beams with and without acceleration under solenoid and quadrupole focusing channels. Also, we explore the longitudinal dynamics on the space-charge driven fourth-order particle resonance for the first time along the linacs, and report the spinning in longitudinal space of 3D bunched beams with and without acceleration.
3D coupling effects on the emittance exchange between transverse and longitudinal directions are also studied by TraceWin PIC simulation code. We expand the simulation studies of the parametric mode instability stop bands which is simply represented by Hofmann chart to the fourth-order particle resonance stop bands as a function of initial emittance ratio and tune ratios in transverse and longitudinal spaces.
Finally, we reported the design study of the beam loss monitoring systems for the real accelerator facility which has been constructed in Daejeon, Korea. It is important to detect the beam loss situations as fast as possible to protect the damage of beam line components. Monte Carlo simulations using MCNPX code were performed to study the details of beam loss-induced neutron and gamma radiations, and to determine the appropriate types of beam loss detectors that will be installed in the accelerator facility.
In the future work, we will discuss about the technical issues of generating the spinning beams for real experimental setups in linear accelerators. The positive effects and the negative effects of the spinning beams should be considered simultaneously, and it is important to define the precise goal of applying the spinning beams in high-intensity linacs which is to mitigate the halo evolution caused by non-linear space-charge driven particle resonances. Even though we proposed the spinning beam appli- cation in the linear accelerators, the application on the circular accelerators can also be the interesting topics to be addressed in the future.
Even if σ0<90◦ (i.e., without fourth-order resonance nor envelope instability), spinning beams could be applied to reduce beam losses from initial mismatches and machine imperfections, motivated by the mechanical stability principle of the spinning flying objects. These topics will be addressed in detail by using multiparticle simulations under more realistic linac structures.
In this dissertation, we chose the emittance growth rate and halo parameters as the figures of merit in the high-intensity linacs. However, another figures of merit can be chosen such as different definitions for the halo particles and the beam qualities depending on the requirements of user facilities. Because there is no exact definition about the ‘halo’ of the charged particle beams, the requirements of the accelerator users set the most important standards about which characteristics of the beams should be focused on designing and operating the accelerator facilities.
Overcoming the 90◦ limit in high-intensity accelerators has been the important issue for achieving the higher intensity and higher quality beam in the future. We strongly believe that the spinning beam dynamics on mitigating the space-charge driven particle resonances reported in our dissertation would be the important starting point to excite the new paradigms in the beam dynamics communities and lead to the interesting research direction on surpassing the 90◦limit in high-intensity accelerators. Also, it is essential to understand the detail and difference about the characteristics of halo formation mechanisms under the strong space-charge effects. Indeed, our studies that clarify the two concepts of halo formation mechanisms (coherent and incoherent resonances) in high-intensity linacs would be the good guideline for beam dynamics physicists. We will continue to find the novel ways to overcome the operation limits and make possible to upgrade the charged particle beam accelerators that can be applied for a wide range of scientific studies all over the world.
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