Dynamic pressure evolution during the LHC operation

For the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than $-500\mathrm{V}$. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud.

Figure 1

Basic schematic of the electron current measurement with a positive (left) and a negative bias (right) electrode.

Figure 2

Measurements performed in station 4 of the VPS during fill 7319: Energy and intensity of the beam 1 (a), pressure measured with a Bayard Alpert gauge (b), electron current collected with the K11 and EKD electrodes (biased at $+9\mathrm{V}$, the inner grid of EKD being not polarized in this case) (c), positive current collected with the K6517A electrode polarized at $-600\mathrm{V}$ (d); ($\mathrm{INJ}=\mathrm{injection}$, $\mathrm{E}\text{−}\mathrm{R}=\mathrm{Energy}$, Ramp, $\mathrm{FT}+\mathrm{S}=\mathrm{Flat}$, Top, and Squeeze).

Figure 3

Energy and intensity of beam 1 (top), electron current and pressure (bottom) during fills 7105 (a) and 7128 (b), respectively.

Figure 4

Evolution of the number of protons per bunch, pressure and intensity of beam 1 in station 4 as a function of the number of bunches during injection at 450 GeV for fill 7105 (a) and fill 7128 (b), respectively.

Figure 5

Evolution of the number of protons per bunch in station 4 (beam 1) as a function of the number of bunches during injection at 450 GeV for several fills (recorded between May 3 and May 7, 2018). Red symbols correspond to fills for which a pressure increase during injection is observed, blue symbols are related to fills for which no pressure increase occurred.

Figure 6

Evolution of the number of protons per bunch in station 4 (beam 1) as a function of the number of bunches during injection at 450 GeV for several fills (recorded between August 23 and October 9, 2018). Red lines and symbols correspond to fills for which a pressure increase during injection is observed, and blue lines and symbols are related to fills for which no pressure increase occurred.

Figure 7

Variation of pressure with the number of protons per bunch during the p-p collision step with $6.5\mathrm{TeV}/\mathrm{beam}$ for several standard physics fills.

Figure 8

Measurements performed in station 4 of the VPS during fills 7221 and 7328. Beam 1 parameters (a), pressure (b), electron current (c), positive current (d); ($\mathrm{INJ}=\mathrm{injection}$, $\mathrm{E}\text{−}\mathrm{R}=\mathrm{Energy}$, Ramp, $\mathrm{FT}+\mathrm{S}=\mathrm{Flat}$, Top, and Squeeze).

Figure 9

Normalized energy spectrum of electrons $n(E)$ (blue open circle) recorded by the EKD electrode in station 4 during a fill at 6500 GeV (data with courtesy of Elena Buratin). Lines are fit to experimental data with Eq. (6): total spectrum (blue line), low energy part (red line), and high energy part (green line).

Figure 10

Normalized energy spectrum of electrons $n(E)$ as a function of energy. The blue part corresponds to the contribution of electrons to the intensity recorded by an electrode polarized at $V_{\mathrm{bias}}$.

Figure 11

SEY curves calculated for copper using Eq. (10) with different values of ${\delta }_{\max }$ and $E_{\max }$.

Figure 12

Variation of the electron current vs $V_{\mathrm{bias}}$ calculated for several SEY using Eqs. (10) and (11) with normalization.

Figure 13

Energy spectra calculated with different parameters for peak 2 (the position of the peak is displaced at higher energy or its intensity is modified).

Figure 14

Comparison between calculated intensities from different energy spectra and a maximum SEY of 1.7 (lines), and experimental data (purple squares) recorded on the electrode K6517A for a negative bias scanning performed during fill 7328.

Figure 15

Variation of the normalized electrode intensity vs $V_{\mathrm{bias}}$. Experimental data (blue circles and purple squares) were recorded during fills 6640 and 7328, and values were calculated using the experimental electron energy spectrum and several SEY (color lines).

Figure 16

Normalized electrode intensity vs $V_{\mathrm{bias}}$. Experimental data recorded during several fills (red circles) or during fill 7328 (purple squares) are compared to calculated intensities by adding 2.5% of positive ions and taking a SEY of 1.6 (blue line) or without ions and taking a SEY of 1.7 (red line): linear (a) or logarithmic (b) $y$ axis.