Radio frequency noise effects on the CERN Large Hadron Collider beam diffusion

Radio frequency (rf) accelerating system noise can have a detrimental impact on the Large Hadron Collider (LHC) performance through longitudinal motion and longitudinal emittance growth. A theoretical formalism has been developed to relate the beam and rf station dynamics with the bunch length growth [T. Mastorides et al., Phys. Rev. ST Accel. Beams 13, 102801 (2010)]. Measurements were conducted at LHC to determine the performance limiting rf components and validate the formalism through studies of the beam diffusion dependence on rf noise. As a result, a noise threshold was established for acceptable performance which provides the foundation for beam diffusion estimates for higher energies and intensities. Measurements were also conducted to determine the low level rf noise spectrum and its major contributions, as well as to validate models and simulations of this system.

Figure 1

Simplified block diagram of the rf system with beam phase loop.

Figure 2

Beam 1 phase error with beam phase loop open/closed.

Figure 3

Phase noise power spectra for beam 2 cavity sum and VCXO (BPL open).

Figure 4

Beam 2 cavity sum signal with BPL open/closed.

Figure 5

The rf station 6B2 noise spectral density with BPL gain.

Figure 6

The rf station 6B2 noise spectral density with BPL gain.

Figure 7

Beam 1 bunch length with time.

Figure 8

Beam 2 bunch length with time.

Figure 9

Levels of injected noise around $f_{\mathrm{rev}}\pm f_s$. Horizontal axis shifted by $f_{\mathrm{rf}}+f_{\mathrm{rev}}$.

Figure 10

Beam 1 bunch length growth. Data in blue. Linear segments correspond to different noise injection levels.

Figure 11

Beam 2 bunch length growth. Data in blue. Linear segments correspond to different noise injection levels.

Figure 12

Beam intensity.

Figure 13

Accelerating voltage phase power spectral density with controller gain setting from 4B2 (solid lines) and simulation (dashed lines).

Figure 14

The rf feedback block diagram.

Figure 15

Noise PSD at the rf feedback output with different switch positions. The rf input is terminated.

Figure 16

Noise PSD at the rf feedback output as a function of controller gain. The rf input is terminated.