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Radio Frequency cavities for the MAX IV storage rings has arrived

Monday, November 25, 2013

On October 31st, three out of totally eight Radio Frequency (RF) cavities for the MAX IV storage rings were unloaded at the MAX-lab goods delivery. The remaining four (the MAX IV prototype cavity is already delivered and tested) will arrive from the German company Research Instruments GmbH (RI) this year.

The RF cavities are being unloaded at MAX-lab.

Three different laboratories has gone together for a common cavity type

RI will deliver in total twelve cavities of this type, since also the Polish light source SOLARIS will use the same type of 100 MHz cavities for their storage ring, and the Danish light source ASTRID2 as well (with a small modification to reach their RF of 105 MHz). It has been an exciting time during which three different laboratories has gone together for a common cavity type. Even the Synchrotron Light Research Institute in Thailand is planning to upgrade to use this type of cavity.

Originally designed and manufactured in-house at MAX-lab

The cavity and its tuning mechanism was originally designed and manufactured in-house at MAX-lab already in the first years of 2000, when MAX-lab performed a major RF change on the existing MAX-II storage ring. One advantage of these low frequency cavities is that they can provide large enough energy acceptance to the storage rings, without much RF overvoltage. In other words, the power consumption is diminished. In this respect one could think of super conducting cavities, but one must then remember that also liquid Helium need power to be produced, and as well all cryogenic equipment/infrastructure. In fact, in the 3 GeV ring, these 100 MHz cavities will have an efficiency of about 75%, from the cavity input power to the electron beam and light production, which is quite extraordinary. This is important, remembering that our 3 GeV ring will eventually produce almost 500 kW of synchrotron radiation.

The RF cavities will soon be installed at MAX IV.

Higher Harmonic Cavities

Another advantage of the 100 MHz system is that electron bunches in the storage rings can be made relatively long, lowering the electron density so that many unwanted effects on the beam can be avoided. The trick to elongate the electron bunches is to introduce so-called Higher Harmonic Cavities (HHC) in the storage rings. Also here, the three laboratories MAX IV, Solaris and ASTRID2 have together ordered a number of 300 MHz (3rd harmonic) cavities. Except for the prototype (which is already installed and tested in the MAX-III ring) these cavities will arrive in the beginning of next year. In total nine such cavities will be produced. The design has been made by MAX-lab, while RI performs the manufacturing.

Design and construction of the RF transmitter made by the RF group

In connection to these cavities it is worthwhile to mention the power source, i.e. the RF transmitter that delivers the RF power to the cavity. For the 100 MHz cavities we use transmitters based on tetrode tubes. These kinds of transmitters are used in radio broadcasting since the 1920s, and are not yet beaten by the solid-state amplifiers (as is the case for higher frequencies). For the 300 MHz cavities the electron beam itself will be the power source. The electron beam passively drives those cavities, since it contains multiples of the 100 MHz. In fact, there are no 300 MHz transmitters commercially available. However, we would be helped a lot by such a device, since the cavities need to be conditioned with RF power before they reach their full performance. Therefore, the design and the construction of the 300 MHz RF transmitter with 6 kW output power is made in-house at MAX-lab by the RF-team.. In this way we can perform the RF-conditioning immediately at the arrival of the 300 MHz cavities. This will save many months of our storage ring commissioning period.

The team behind the RF transmitter. From left: Sven-Olof Heed, David Olsson and Aleksandar Mitrovic.

Text: Åke Andersson
Photo: Madeleine Schoug (cavities & RF-team)