A novel optimization platform and its applications to the TRIUMF energy recovery linac

A software platform for global optimization was developed to study the TRIUMF Energy Recovery Linac. The platform is parallel capable and allows for combinations of accelerator modeling tools such as MADX and Genesis. Many parameters are coupled, including RF parameters which are shared for all linac passes. The platform can study dynamic relationships between different processes, a practice not easily performed with standalone optimization. Tradeoffs are presented to give insights on how objectives are related and the repercussions of design decisions. INTRODUCTION The TRIUMF E-linac is a 50 MeV, 10 mA average current CW driver for rare isotope beams (RIB). The E-linac can be upgraded to an Energy Recovery Linac (ERL) for simultaneous ERL and RIB operations. The design of the upgrade is complicated. Many parameters are coupled, including RF parameters which are shared between three beams (RIB pass and two ERL passes). Our goals are: • Create a computational platform for multiobjective optimization (MOO), capable of massively parallel computation. • Capable of using different modeling tools, or engines, in combination to produce global models of machines. • Use the platform to set up a global ERL model. • Use the platform to study the physics of the TRIUMF ERL. • Create a baseline from optimization results, complete with layout coordinates and optics requirements. More details on the capabilities of the platform can be found in [1, 2]. The complete platform design and implementation can be found in [3], with a description of MOO. OPTIMIZATION METHOD Objectives and constraints used in the optimization are • Maximize gain, therefore FEL power. • Energy recovery: Edmp = Ein =7.5 MeV, or equivalently, pdmp = pin =7.5 MeV/c dump energy same as injection energy. • σx ≤ 3 mm, σy ≤ 3 mm minimize beam loss via beam scraping by restricting transverse beam size everywhere. • σx,EDBT ≤ 3 mm, σy,EDBT ≤ 3 mm constrain beam size in the dump section EDBT. This is complicated ∗ Work supported by Natural Sciences and Engineering Research Council of Canada and National Research Council of Canada by the large energy spread δ obtained from lasing and deceleration, which is converted to beam size by the EDBT dipole. This constraint overlaps with the previous, but is listed again for emphasis. • beam loss ≤ 10−5 lasing creates a momentum tail which can cause beam loss in the return arc. • σx,dmp = 7 mm, σy,dmp = 7 mm blow up beam at the dump to reduce radiation heating. • αx = αy = ηx′ = 0 at both arc centers look for designs with arc symmetries in βx , βy , and ηx . Symmetries make tuning easier. The layout of the arc optics is symmetric to accommodate these conditions. ηy is zero everywhere and does not need to be considered. • Maximize ERIB high energy is desired for RIB transport. Free parameters in the optimization are: • RF phases and amplitudes (four independent cavities) • Drift lengths (25 cm minimum separation between elements to support diagnostics) • Quad gradients (upper limit set by existing TRIUMF magnet designs) • Chicane dipoles bend angle The main linac geometry is designed and not subject to optimization. Care is also taken to make sure the ERL fits in the TRIUMF E-hall. Figure 1: Dual ERL-RIB layout. Layout of the ERL is shown in Fig. 1. Modeling of the machine starts at the linac. After acceleration an RF separator kicks the ERL beam into the recirculation loop. The beam lases and is then decelerated by a second pass. The energy recovered beam is kicked to EDBT for disposal. The RIB pass is modeled as well, with the RF separator kicking the beam into RIB transport EHAT. Components of the ERL are modeled by a combination of engines: MADX, DIMAD, Genesis, and the Empirical Model (EM) [4]. EM is a TRIUMF code for modeling cavities. The optimization platform handles all transitions between engines and distribution of work on parallel clusters. THCWC4 Proceedings of ICAP2015, Shanghai, China ISBN 978-3-95450-136-6 164 C op yr ig ht © 20 15 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s B-1 Beam Dynamics Simulation Engines can be easily switched because the platform treats them as independent modules.