Aperture measurements
Background
Aperture measurements are used to verify the settings of the LHC collimators required to protect the machine. The global aperture bottleneck shifts to the superconducting triplet magnets in the experimental insertions, once the beams are squeezed. One of the limiting factors for small β∗ values and thus for high luminosity is the available aperture in the triplets.
Background on measurement techniques
Orbit bump (local measurement)
Local aperture measurements are realized by means of a local orbit bump shaped such that the location of interest can be touched with the beam. The bump amplitude is successively increased until losses are measured with the BLMs. Typically the bump amplitude is increased in step sizes between 0.25 σ and 0.5 σ. The beam envelope is defined by exciting the beam with the ADT until losses are measured at the primary collimator. The beam then fills the space between the collimator gaps and its normalized beam size is defined by the opening N_p of the primary collimator. The remaining collimator settings are not relevant for the test as long as the bump is not applied in a region where collimators are present. Otherwise the corresponding collimators have to be retracted. The normalized aperture A_{loc} is given by the sum of the beam envelope N_p and the bump amplitude x_b at which the losses occur for the first time:
An intrinsic advantage is that possible asymmetries in the aperture can be identified by measurements with both signs of the bump. A permanent bump can then be deployed to center the beam and gain aperture. In the analysis of the measurement results, the expected orbit is compared to the orbit measured with the beam position monitors (BPM).
Collimator scan (global measurement)
The collimator scan method is an iterative measurement in which the global bottleneck is exposed to beam losses in a controlled manner. The measurement is prepared by retracting all collimators but leaving one dedicated collimator in place. The collimator is retracted in steps of 0.5 σ and the beam is blown up at every step. When the collimator setting is such that the aperture at the global bottleneck becomes unprotected, the highest measured loss peak in the machine moves from the collimator to the aperture bottleneck. The measurement is continued until all losses have moved to the bottleneck. The aperture is deduced from the collimator half gap at which the normalized BLM signal at the bottleneck is higher than at the collimator.
Fast collimator scan (global measurement)
From 2015 on, a semi-automatic, and therefore faster method for aperture measurements with the collimator scan method was established in 2015. Details on the measurement approach and normalization can be found in [2].
The ADT is set up to excite the beam continuously over an extended period of time. The collimator is programmed to open or close automatically in steps of 0.5 σ at a defined frequency. Once the final setting (in the shown example 9 σ) is reached, the measurement is stopped. The aperture is calculated in the same way as the CS method, but the method is significantly faster. In practice, the FCS method is applied to confirm the measurement result by the CS, to first identify the global aperture bottleneck in a controlled and secure way.
Beam based alignment (global measurement)
In this type of measurement all collimators are retracted and the beam is excited, such that losses are measured at the global aperture bottleneck. The beam has then been scraped by the aperture bottleneck which defines the beam envelope. The extension of the beam envelope is measured by performing a beam-based collimator alignment with the primary collimator. In this type of alignment, the collimator is closed until the BLM signal at the collimator exceeds a previously defined threshold [15] (see Fig. 2). Once it is ensured that both collimator jaws have individually touched the beam, the normalized collimator gap can be directly translated into the available aperture at the global bottleneck.
The aperture can be directly deduced without the requirement for post-processing. The method is particularly suited for measurements at the triplet magnets, where the required settings of the local TCT collimators may be concluded from the measurement. On the contrary, the beam edge is not sharply defined and the measurement result depends on the setting of the BLM signal at which the BBA is stopped. The method thus risks to underestimate the available aperture.
AC Dipole
A new method based on the AC dipole, used for optics measurements in the LHC, has been tested for the first time in 2017. This method consists in exciting large coherent beam oscillations without blowing up its emittance, allowing thus to explore large transverse amplitudes. The ultimate goal of this study would be that the aperture measurements at top energy could be combined with optics measurements or with any other beam activity requiring individual low-intensity bunches saving commissioning time. In addition, the optics measurements can be performed with the largest possible amplitude of the AC dipole kicks without causing losses on the aperture, thus making the measurements more precise. Details can be found in [3].
Relevant Publications
The contents of this page are mostly carried over from the following publications:
[1] R. Bruce, et. al, Detailed IR aperture measurements, CERN-ACC-NOTE-2016-0075
[2] P. Hermes, et. al, Improved Aperture Measurements at the LHC and Results from their Application in 2015, Proceedings of IPAC 2016, Busan, Korea.
[3] N. Fuster-Martinez, et. al, Aperture Measurements with AC Dipole at the Large Hadron Collider, Proceedings of IPAC 2018, Vancouver, Canada.