The RFQ accelerator
RFQ
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length 1.54 m
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injection energy 10 keV/u
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extraction energy 300 keV/u
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frequency 108.5 MHz
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max. voltage 70 kV
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charge/mass 0.25
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.
Click on images to get larger versions.
For more information about the RFQ, contact Anders Källberg email:
kallberg@msl.se
