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CORNELL LABORATORY FOR ACCELERATOR-BASED SCIENCES AND EDUCATION

CLASSE NEWS | 8 May 2014

CESR-TA measures Fast Ion Instability

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A slice of the data taken during the April CesrTA run. Click for enlarged view.

CESR Test Accelerator (CesrTA) has made the first detailed bunch-by-bunch measurement of the Fast Ion Instability (FII), an effect that has vexed accelerators for decades. In FII, the residual gas in the accelerator vacuum system can be positively ionized by the negatively-charged circulating electron beam (which contains a series of electron bunches, referred to as a train) during a single passage. The negatively-charged beam then traps the positively charged ions. The beam-ion coupling leads to an instability that limits the total charge in each bunch and the number of bunches in a train. This effect is a serious concern for the next generation of colliders and light sources, limiting luminosity and brightness, and must be well-understood in order to devise a method of mitigation.

The plot above shows a slice of the data taken during the April CesrTA run. It shows the vertical motion of each bunch (the band depicting the RMS amplitude) as a function of bunch number, with bunch 1 denoting the head of the train. As the pressure of the injected argon gas is increased, the motion becomes larger for the tail of the train, making the train less stable. However, it was also shown that by using a fast feedback system, the degradation of beam quality due to FII was negligible. This demonstrates that a fast feedback system may be sufficient for future light sources to compensate for the Fast Ion Instability.