Energy Efficient Klystron Operation at Saturation: Possibility due to Novel Modulator

Abstract

The European Spallation Source (ESS) will be the world s most powerful pulsed neutron source by the end of the decade. The beam of protons will be accelerated by 155 amplifiers, out of which 126 are klystron amplifiers. Following the present state of art, the Low Level RF (LLRF) system will generate an input signal for the amplifier that drives the cavity to the field with an amplitude and phase that are within 0,1% and 0,1 degrees of the set value. To achieve this, ESS needs the LLRF overhead of 25%. As the differential gain of klystron at the saturation is nearly zero, the LLRF system needs to operate the klystron back-off from the saturation in the linear region by about 15–25 %. This leads to the two disadvantages: need for infrastructure with higher power capability and operation of the klystron in energy inefficient zone. The present paper discusses the novel concept of operation of klystron at saturation and modulating the cathode high voltage delivered by the modulator to achieve modulation of the klystron RF output power. This avoids klystron operation in the linear region and will reduce the requirement on the amplifier power by 25% and at the same time increases the efficiency at the operating point by 15-20%. In the context of ESS operation, it will lead to energy savings of 7.5GW-hr per year. The energy saving leads to an operational cost saving of about 10 MEUR for 25 years. It will also minimize the wasted energy conversion into heat, thus minimizing the cooling and the ventilations costs.