Yong Ho Chin, a leading theoretical accelerator physicist at the High Energy Accelerator Research Organization (KEK) in Japan and chair of the beam dynamics panel of the International Committee for Future Accelerators (ICFA) since November 2016, unexpectedly passed away on 8 January.
In 1984, Yong Ho received his PhD in accelerator physics from the University of Tokyo for studies performed at KEK under the supervision of Masatoshi Koshiba, who won the Nobel Prize in Physics jointly with Raymond Davis Jr and Riccardo Giacconi in 2002. Yong Ho participated in the design and commissioning of the TRISTAN accelerator, and later in the designs of the KEKB and J-PARC accelerators, along with major contributions to JLC (the Japan Linear Collider) and ILC (the International Linear Collider). In the 1980s and 1990s he spent several years abroad, at DESY and CERN in Europe, and at LBL (now LBNL) in the US.
In his long and distinguished career, Yong Ho made numerous essential contributions in the fields of beam-coupling impedances, coherent beam instabilities, radio-frequency klystron development, space–charge and beam–beam collective effects. He considered his “renormalisation theory for the beam–beam interaction”, developed during his last six months at DESY in the 1980s, as his greatest achievement. However, in the accelerator community, Yong Ho Chin’s name is linked, in particular, to two computer codes he wrote and maintained, and which have been widely used over the past decades.
The first of these codes, developed by Yong Ho in the 1980s, is MOSES (MOde-coupling Single bunch instabilities in an Electron Storage ring), which computes the complex transverse coherent betatron tune shifts as a function of the beam current for a bunch interacting with a resonator impedance. The second well-known code, written by Yong Ho in the 1990s, is the ABCI (Azimuthal Beam Cavity Interaction) code for impedance and wakefield calculations. This served as a time-domain solver of electromagnetic fields when a bunched beam with arbitrary charge distribution goes through an axisymmetric structure, on or off axis.
In the mid-1990s, Yong Ho’s work expanded to two-stream beam instabilities. He rightly foresaw that such instabilities could potentially limit the performance of KEKB and organised and co-organised several international workshops to address this issue early on. Subsequently, he was put in charge of the development and modelling of the X-band klystron for the JLC. He also greatly contributed to the development of the multi-beam klystron now in use for large superconducting linacs, and to the optimisation of the J-PARC accelerators.
Yong Ho returned to the field of collective effects more than 10 years ago and he remained extremely active there. Over the past few years, together with two other renowned accelerator physicists, Alexander W Chao and Michael Blaskiewicz, he developed a two-particle model to study the effects of space–charge force on transverse coherent beam instabilities. The purpose of this model was to obtain a simple picture of some of the essence of the physics of this intricate subject and at the same time provide a good starting point for newcomers joining the effort to solve this long-lasting issue.
As illustrated by his role as chair of an ICFA panel, and by his co-organisation of a large number of international workshops and conferences (including PAC and LINAC), Yong Ho was devoted to serving the international physics community. He was a productive author, diligent referee and esteemed editor for several journals. In 2015 he was recognised with an Outstanding Referee Award by the American Physical Society, and just a few months ago, in the summer of 2018, Yong Ho was appointed associate editor of Physical Review Accelerators and Beams.
Yong Ho was a very good lecturer, teaching at different accelerator schools, including the CERN Accelerator School. He was also in charge of a collaboration programme in which young accelerator scientists were invited to spend a few weeks at KEK.
Yong Ho was a wonderful person and an outstanding scientist. We are very proud to have had the chance to work and collaborate with him. His passing away is a great loss to the community and he will be sorely missed.