CERN Accelerating science

Internal Note
Report number	CERN-ACC-NOTE-2020-0051
Title	Can an extra HOM-like impedance explain the high octupole threshold at $Q′ ∼ 0$ in the LHC?
Related title	Can an extra HOM-like impedance explain the high octupole threshold at Q′ ∼ 0 in the LHC?
Author(s)	Antipov, Sergey ; Biancacci, Nicolo (CERN) ; Metral, Elias (CERN)
Corporate author(s)	CERN. Geneva. ATS Department
Publication	2020
Imprint	17 Sep 2020
Number of pages	13
Subject category	Accelerators and Storage Rings
Accelerator/Facility, Experiment	CERN LHC
Keywords	LHC ; Impedance ; Octupole threshold
Abstract	The 2015 LHC octupole threshold measurement campaign revealed a higher Landau octupole current between the measured octupole threshold and impedance model predictions for chromaticities below few units. The discrepancy was recently reduced by revisiting the emittances of both transverse planes but still exceeds 300 A in some measurements (i.e. 400 A instead of 100 A), although excesses of 50 − 100 A were also measured at multiple $Q′$ between -10 and -5, where the expected octupole threshold should be negligibly small. One possible cause of the difference might be an HOM unaccounted for in the impedance model and recently, after having analyzed other options, it was considered likely. In this note we investigate this possibility via analytical and semi-analytical estimates and compare the findings with existing experimental evidence.
Other source	INSPIRE
Submitted by	[email protected]