The RFQ accelerator


The radio frequency quadrupole is a new type of charged particle accelerator and the one at MSL is the first one of the 4-rod type designed for heavy ions, i.e, ions with a lower charge than atomic number. The RFQ is made of copper and it is enclosed in a copper-plated steel tank which also constitutes the vacuum vessel. A high frequency (108.5 MHz) AC signal is connected to the four rods (a). Together with the supports and their foundations the rods constitute a resonance circuit tuned to the high frequency. This gives rise to high electric fields between the rods. The connection to the four rods are made in such a way that near-by rods always have opposite polarity, which is depicted by the different colours in the illustration. The forces from the radial electric fields between the rods keep the ions together in a narrow beam (b) that can pass through the RFQ. Due to the wave-shaped rods, an electric field directed along the rods is also generated which alternates with the radio frequency. Only those particles which arrive at the correct time and are moving with the correct velocity will be accelerated through. In order to minimise particle loss, the first part of the RFQ is designed to bunch the beam in short pulses. These bunches are accepted by the electric field and are then accelerated in the second part of the RFQ. The energy of the ions are thus increased from 10 keV/u to 300 keV/u which corresponds to an increase in velocity from 1.4 to 7.8 x 103 km/s.

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For more information about the RFQ, contact Anders Källberg email: