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Hans Frischholz (1938 – 2022)

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Hans working on a 1 MW cw klystron for LEP in the test stand in hall 112, in 1983. (Image: CERN)

After a long illness borne with courage and dignity, our colleague and friend Hans Frischholz passed away on 19 September 2022. Hans was a renowned specialist on radiofrequency (RF) power systems and made many important contributions to the ISR, LEP and the LHC.

Hans joined CERN in 1968, working in the ISR division’s RF group led by Wolfgang Schnell. He worked on the development of the RF cavities and amplifiers with Fritz Ferger and later took full responsibility for their operation, leading a small team. Hans and his team were also responsible for the construction, commissioning and operation of the beam loading compensation systems. These high-power systems were designed to allow the RF system to cope with high-intensity injected PS beams but they were also crucial for the acceleration of ISR beams to 31.4 GeV. Hans made a major contribution to the success of the ISR.

In preparation for the LEP project, Hans studied the design of the 1 MW high-power klystrons needed to power the copper cavity RF system. He collaborated with industry on the construction of prototypes. An important aim, successfully achieved, was reaching the high specification for DC to RF conversion efficiency at full power. Hans coordinated the external manufacture of 16 klystrons and the testing of the production series at CERN. Hans and his relatively small team then carried out the installation and commissioning of the whole RF power system for LEP1, comprising 8 RF units each made up of a complex arrangement of waveguides, circulators and loads. They oversaw its successful operation throughout the LEP1 period. During the LEP2 upgrade, running through the 1990s, Hans and his team accomplished the huge task of installing a further 36 klystrons needed to drive a total of 288 superconducting RF cavities. This was a vital contribution to the success of the LEP2 project and the achievement of ultimately reaching 3600 MV circumferential voltage, enabling LEP energies of up to 104.5 GeV.

Following his LEP achievements, Hans took responsibility for the design of the 400 MHz LHC klystrons and their RF power systems. The klystron design was based on work he had done during an earlier sabbatical at Stanford. In 2003, Hans took his well-deserved retirement, after many years of successful dedication, smoothly handing his activities and responsibilities over to his colleagues.

He will be remembered fondly by all who had the pleasure of working with him, not only for his accomplishments and his professionalism but also for his informal and friendly style.

We offer our heartfelt condolences to his family.

His friends and former colleagues