Observation of Beam Ion Instability in Spear3*

Weak vertical coupled bunch instability with oscillation amplitude at μm level has been observed in SPEAR3. The instability becomes stronger when there is a vacuum pressure rise by partially turning off vacuum pumps and it becomes weaker when the vertical beam emittance is increased by turning off the skew quadrupole magnets. These confirmed that the instability was driven by ions in the vacuum. The threshold of the beam ion instability when running with a single bunch train is just under 200 mA. This paper presents the comprehensive observations of the beam ion instability in SPEAR3. The effects of vacuum pressure, beam current, beam filling pattern, chromaticity, beam emittance and bunch-by-bunch feedback are investigated in great detail. INTRODUCTION In an electron accelerator, ions generated from the residual gas molecules can be trapped by the beam. Then these trapped ions interact resonantly with the beam and cause beam instability and emittance blow-up. Most existing light sources use a long single bunch train filling pattern, followed by a long gap to avoid multi-turn ion trapping. However, such a gap does not preclude ions from accumulating during one passage of the single bunch train beam, and those ions can still cause a Fast Ion Instability (FII) as predicted by Raubenheimer and Zimmermann [1]. FII has been observed in ALS [2], and PLS [3,4] by artificially increasing the vacuum pressure by injecting helium gas into the vacuum chamber [2,3,4] or by turning off the ion pumps [3] in order to observe the beam ion instability. In some existing rings, for instance B factory, the beam ion instability was observed at the beginning of the machine operation after a long period of shutdown and then it automatically disappeared when the vacuum was better. However, when the beam emittance becomes smaller, the FII can occur at nominal conditions as observed in PLS [5], SOLEIL [6] and SSRF [7]. This paper reports the observations of beam ion instabilities in SPEAR3 under different condition during a period of one year, which includes single bunch train instability (FII) and multi-bunch train instability. Note that the instability may be not the same even with the same beam due to the change of the vacuum with time. SPEAR3 has a circumference of 234 m with a harmonic number of 372. SPEAR3 runs with six bunch train filling pattern in order to suppress the possible beam ion instability. Table 1 lists the main parameters of SPEAR3. The vacuum of SPEAR3 ranges from 0.1 to 0.5 nTorr, which varies from section to section. Table 1 Typical Parameters of SPEAR3 Physics Symbol/Unit Horizontal Emittance nm 10 Vertical Emittance pm 14 Bunch Number 280 Harmonic Number 372 Beam Energy GeV 3 Circumference m 234 Bunch spacing ns 2.1 RF frequency MHz 476.315 Radiation Damping time τx /τy /τz [ms] 4.0/5.3/3.2 Vacuum nTorr 0.1~0.5 DEPENDENCE OF VERTICAL INSTABILITY ON EMITTANCE AND VACUUM PRESSURE Beam-ion instability was first observed in 200mA operation with a single bunch train filling pattern. Figure 1 shows the observed typical beam spectrum. The observed unstable modes of the lower vertical sidebands agree with the theoretical prediction of the beam ion instability and the frequencies of the unstable modes agree with theory. There is no horizontal instability. The observed bunch oscillation amplitude increases along the bunch train and saturates at the order of beam size as shown in Fig. 2. Coupled-bunch instabilities driven by the traditional impedances either do not saturate or saturate at much larger amplitudes. This indicates the observed instability is driven by the ions in the vacuum. Further tests have been carried out to confirm it. All skew quadrupole magnets were turned off to increase the vertical emittance. Figure 3 shows the measured vertical lower sidebands observed on the beam spectrum analyzer when the skew quadrupole magnets were on and off. When the skew quadrupole magnets were off, the maximum frequency of the observed sidebands reduced from 26.0 MHz to 13.0 MHz, and the maximum amplitude also dropped as predicted by the theory of beam ion instability. This indirectly confirms that the instability is driven by ions in the vacuum instead of the impedance inside the vacuum chamber because the traditional impedance doesn’t change with the beam emittance. In order to further test that the vertical instability is driven by ions, the vacuum pressure was raised by partially turning off the vacuum pumps. Figure 4 shows the vertical sidebands at different average vacuum pressures along the ring with 300 mA total beam current. ____________________________________________ *Work supported by DOE contract No. DE-AC02-76SF00515 # Email address: [email protected] SLAC National Accelerator Laboratory, Menlo Park, CA 94025 SLAC-PUB-14595 The beam fill pattern was a six bunch train, and there was no vertical beam instability at pressure 0.37 nTorr when all vacuum pumps were on. The instability appeared as the pressure increased, and it became stronger with a higher pressure. This directly confirms that the vertical instability observed in SPEAR3 is driven by ions in the vacuum chamber. Figure 1: Beam spectrum at 200 mA with a single bunch train filling pattern. There are 280 bunches along the bunch train. The large peaks are the revolution harmonics and the low peaks are the vertical lower sidebands. 0 100 200 300 400 0 2 4 6 8 10 12 14 Bunch number R M S a m p li tu d e ( μ μ μ μ m ) Figure 2: Measured beam’s vertical oscillation amplitude with single bunch train filling pattern. Bunches 1-280 and 326 are filled with a total beam current of 200 mA. 5 10 15 20 25 30 k=0.12% 0 5 10 15 20 25 30 5 10 15 20 25 30 A m p (a .u .)