80 likes | 90 Views
This study evaluates the effect of ground motion on beam movements at the CNGS target using data from LEP orbits. The analysis estimates the amplitude and correlations of the ground movement, as well as the resulting beam movement. The study also considers the impact of resonances and amplification factors. The results suggest that the interlock limit at the CNGS target is reached within a week, highlighting the importance of evaluating and mitigating these effects in beam stability.
E N D
Ground motion induced beam movements at the CNGS target J. Wenninger AB-OP-SPS Estimates for beam movements @ CNGS target based on LEP orbit drift data. CNGS Comm. Comm. / J. Wenninger
Ground motion • The effect of ground motion on the beam is in principle easy to evaluate … provide you know how ground and girders (!) move. • Both amplitude and correlations of the ground movement over the ring/line are important. • For rings resonances may occur when ground motion wavelengths become comparable to or smaller than betatron wavelengths. At the LHC amplification factors ~ 10-100 are possible at higher frequencies ( 5-10 Hz). Amplification = orbit amplitude/GM amplitude. • For slow movements (time 10-60 seconds) a conservative estimate can be obtained assuming that the movement of the quadrupoles are un-correlated. CNGS Comm. Comm. / J. Wenninger
Ground motion effects • Assumption : • The ground movement is characterized by an r.m.s. motion sG. • Every quadrupole is subject to the same motion. • Quadrupole movements are NOT correlated. • It is possible to estimate analytically the resulting orbit r.m.s. for a ring and the resulting r.m.s. beam movement at a ‘target’ location in a line CNGS Comm. Comm. / J. Wenninger
LEP orbits A large data sample of LEP orbits from 1997 to 2000 is available to estimate long(er) term ground motion effects (time 30 seconds). The LEP orbit data sample was analyzed in the following way : • For every fill the first orbit with stable colliding beams is taken as reference. • The r.m.s. drift (normalized by b) with respect to the reference is reconstructed for each fill. Orbit corrections are unfolded. • For the vertical plane, anomalous drifts from the low-beta quadrupoles are removed. • The data of all fills of a given run is averaged in time bins (from start of fill time): typical sample size ~ 30’000 orbits. • The ground motion rms sG is obtained from the drifts by correcting for kR. Data from different years is very consistent. CNGS Comm. Comm. / J. Wenninger
LEP Data / 1 H plane : sG versus time from start of fill rms spread The LEP data is ~ consisted with : mean C 50-60 nm/s½ V plane : sG versus time from start of fill sG2 versus time
LEP Data / 2 • The time dependence of the LEP orbit data is consistent with a random walk (Brownian motion) of the quadrupoles. • It is also consistent with the ‘T’ (but not necessarily the ‘L’) of the ATL law that postulates that the relative movement D between 2 points depends on time T, on the distance between the points L and on a constant A A depends on the ground structure. • Strictly speaking the LEP data is only valid over ~ 10-12 hours, but it is likely that it can be extended to longer time periods. • Extrapolated to 1 year : sG ~ 0.3 mm • Observed (~ 1 year) : sG ~ 0.1 mm • SPS estimates (<< statistics) yield C values that are ~ factor 3-4 smaller. CNGS Comm. Comm. / J. Wenninger The r.m.s. drift (normalized by b) with respect to the start of the fill was reconstructed for each LEP fill. Orbit corrections were unfolded. For the vertical plane, anomalous drifts from the low-beta quadrupoles were removed. The data of all fills of a given run were averaged in time bins (from start of fill time): typical sample size ~ 30’000 orbits. The ground motion rms sG is obtained from the drifts
CNGS Target • At the CNGS target, the effect of the TT40 & TT41 transfer lines is kL = 5-6 for H & V plane • The estimated beam position drift due to the TL (using LEP numbers) : takes ~ 30 days to reach 0.5 mm. • The effect of the ground motion on the SPS orbit and the resulting ‘imported’ drift on the target is ~ factor 2 larger. But the position interlock in the SPS will trigger first (± 0.5 mm @ b = 100 m). • It should be noted that the fluctuations around the average drift are very large (> 30%) the time span gives ~ order of magnitude ! CNGS Comm. Comm. / J. Wenninger
Summary • An estimate of the beam movement at the CNGS target due to ground motion based on LEP data implies that the interlock limit of ~ 0.5 mm is reached on the time scale of ~ week ! • This estimate does not include ground settlement after civil engineering work. • The short time beam movements (cycle to cycle) will probably be dominated by PC ripple (dominated by extraction septum, see TI8). • Additional ‘warm/cold’ effects (after access, MD…) must be evaluated experimentally. • For the commissioning I propose to perform stability runs of 1 shift with low or moderate intensity (criterion is good BPM data quality) to obtain more information on ripple(s) and ground motion see TI8 stability analysis based on MIA techniques (Model Independent Analysis). CNGS Comm. Comm. / J. Wenninger