Direct Observation of the Fast Beam-Ion Instability

Abstract

Direct evidence of the fast beam-ion instability (FBII) was obtained in the Pohang Light Source by measuring the bunch-by-bunch parameters from the snapshots of the beam image. With the direct observation, we confirmed the FBII signals and clarified uncertainties of the blowup factors quantitatively: bunch size blowup of ϳ2s y and the oscillation amplitude of ϳ0.75s y . Suppression of the FBII was also demonstrated in the presence of the multiple gases or an extra clearing gap in the bunch train. [S0031-9007(98)07688-1] PACS numbers: 29.27.Bd, 41.75.Ht As the bunch current and the number of bunches become very large in future low emittance accelerators, a new kind of beam instability, so-called the fast beamion instability (FBII) [1,2], has become an important subject of the beam physics. The FBII is distinguished as a transient beam instability excited by the beamgenerated ions accumulated in a single passage of the bunch train, while the conventional ion trapping [3,4] is excited by the trapped ions accumulated in the periodic beam potential over multiple passages of the beam. One of the characteristic signals of the FBII is a coherent beam-ion oscillation with increasing amplitude along the bunch train. According to the linear theory [1,2], the initial amplitude of oscillation y͑t, z͒ grows quasiexponentially as y͑t, z͒ ϳ exp͓͑z͞l͒ p t͞t ͔ with the phase factor v i z͞c 2 v b t, where z is the position within a bunch train, l is the length of the bunch train, t is the characteristic growth time of the FBII, and v i , v b are the ion and betatron frequencies, respectively. As the amplitude increases, however, the nonlinearity becomes important. Computer simulation studies [1,2,[5][6][7] have shown that the amplitude of oscillation saturates at about s y of the bunch size due to the nonlinearity of the beam-ion interaction. This fully developed FBII will cause harmful effects on the performance of the planned colliding beam facilities like the KEKB [7], PEP-II [8], and the future linear colliders. A more serious problem would be that the growth time of the FBII is too fast to be damped effectively by the existing active feedback systems, nor can it be cured by including a clearing gap long enough to overfocus the ions out of the beam path.There have been experimental studies to confirm the FBII in ALS [9], TRISTAN AR [10], PEP-II [11], and PLS [12]. The first observation of the FBII was made in the ALS with the injection of helium gas into the storage ring to raise the growth rate. Both the transverse and longitudinal beam feedback systems were used to suppress the coupled bunch instabilities, and a chargecoupled-device (CCD) camera was used to observe the FBII by measuring the projected beam size of the whole bunch train. When the 80 nTorr He was injected, the projected vertical beam size blew up by a factor of 2 to 3. The growth time of the FBII was also estimated by measuring the onset of the FBII as a function of the length of the bunch train. On the other hand, a single pass beam position monitor (SBPM) w...

Fast beam-ion instability driven longitudinal damping

“…1 and 2(a). These facts are well agreed with our past FBII experimental results in the PLS storage ring [2], [3]. Generally, after the installation of a new vacuum component, the population of CO is downed to 5% within one month operation of the PLS storage ring.…”

“…2(b). This saturation can be generated when the vertical oscillation amplitude of the beam centroid is higher than about one RMS vertical beam size, σ y 90.0 µm, due to the nonlinearity of the beam-ion force and the detuning of the ion oscillation frequency during the strong vertical beam shaking [1], [3]. Since it is difficult to measure the vertical beam size clearly because of the vertical beam shaking, we can not calculate the ion oscillation frequency exactly with the linear theory [1].…”

Phase space tracking of coupled-bunch instabilities

“…Although such measurements have been used at the Advanced Light Source [8] and the Pohang Light Source [9], they are not always conclusive during commissioning, since conditions such as beam orbit, vacuum pressure, coupling, beam size, feedback, etc. are sometimes not well controlled.…”

Fast ion instability at the Cornell Electron Storage Ring Test Accelerator

“…Such studies provide insight into phenomena that are likely to limit the performance of next-generation colliders and storage rings (e.g., intra-beam scattering, electron cloud growth, and FII). FII has been qualitatively observed at many accelerator facilities: by injecting gas or turning off vacuum pumps [3][4][5], or by reducing the beam emittance to increase the trapping potential under nominal vacuum [6]. However, the instrumentation available at CESR-TA makes it possible to measure effects of FII on individual bunches rather than the beam as a whole.…”