The debuncher for the FAIR Collector Ring – The trick is in the rotation
A first so-called debuncher for FAIR has reached GSI in autumn 2015 and is currently in commission in a testing facility. Over the course of the year the scientists want to thoroughly evaluate it, as it will be used to improve the properties of the ion beams. It weighs almost two tons and has a height of two meters and a length of more than one meter. In total five. debunchers will be installed in the FAIR Collector Ring (CR).
The GSI and in future the FAIR accelerators get billions of charged atomic nuclei, so called ions, up to high speed. In the collision of the beam with a material sample, called a target, a beam of new nuclei not existent on Earth is produced. They are the goal of the researchers, as they could e.g. give information about the element synthesis in stars. But the desired objects are unstable and decay after a short time – so you better hurry up with your studies.
And also another problem arises: The desired particles move with different speeds in all directions in reference to an ideal particle. They have, in the language of the scientists, a large momentum spread. "A cooling of the beam i.e. a reduction of the momentum spread is needed to accumulate or to analyze those nuclei. But the established cooling methods like stochastic cooling are only effective if the momentum spread is small to begin with. Otherwise cooling can not be performed for all particles. And for particles, which can be cooled it will take too long and the nuclei have decayed before they are prepared for their experiments", explains Dr. Oleksiy Dolinskyy, head of the FAIR project department "Collector Ring".
To reduce momentum spread the debunchers are necessary. They are positioned in the FAIR Collector Ring (CR) where the new nuclei are cooled. The word debuncher derives from the word bunches. That's what the tighly-packed heaps of ions in the acceleration process are called. Electric fields form the bunches and take care that the positively charged ions don't repel each other, but cuddle closely together. Already during the acceleration very short bunches of 50 nanoseconds duration can be generated, which lead to equally short bunches of new atomic nuclei via the collision with the target. Not a lucky coincidence, but a prerequisite for the now following process of debunching.
So we have two parameters to look at: the momentum spread and a time component. In combination those two span a so called phase space, in which the ions move. To reduce momentum spread, the scientist use a trick: The debunchers rotate the bunch in the phase space transforming a large momentum spread into a bunch with a long duration, and vice versa changing a long (short) bunch to a small momentum spread. This method reduces the momentum spread by a factor of three in the CR,. After that they stretch the beam around the whole ring and generate a continuous or "coasting" beam, and thus prepare the beam for stochastic cooling.
The debuncher operates at a frequency from 1.1 to 1.5 megahertz, and each debuncher is able to deliver a voltage of 40 kilovolts for the bunch rotation, in sum 200 kilovolts. They are a German in-kind contribution for FAIR with GSI being responsible for the delivery. "We worked out the specifications and a conceptual design, and then contracted out the detailed design as well as the production", says Dr. Ulrich Laier from the FAIR project department "Ring RF". "Three companies have been entrusted with the three main components of the first model and will also build the other four debunchers after our acceptance." RI Research Instruments from Bergisch-Gladbach built the cavity, Ampegon PPT GmbH from Dortmund built the amplifier and OCEM Power Electronics from Bologna, Italy, built the power supply.