I am a physicist who is currently pursuing a PhD in accelerator physics at the European Organization for Nuclear Research (CERN) together with the University of Hamburg. I am working in the Accelerators and Beams Physics group of the Beams department. My focus is on beam optics measurements and corrections for the Large Hadron Collider.
During my PhD I am working on the optimization of beam optics measurements and corrections for the Large Hadron Collider. This involves the development of improved algorithms for the analysis of measurement data. One approach for increasing the precision and accuracy of the measurement is based on a detailed study of statistical and systematic errors including correlations. Furthermore, I use models and perform simulations for various tasks from the estimate of systematic errors, to the study of the impact of perturbations, and also to test the benefit of a new algorithm.
I performed my Master thesis at the Free-Electron Laser FLASH at DESY, where I was working on a longitudinal beam diagnostic system. My work involved the matching of an optics in a section of the accelerator to the requirements of the diagnostic system. Besides, a new screen station was commissioned and an upgrade of the machine protection system performed. In the measurement analysis I was using image processing and tomographic reconstruction methods.
Ph.D. in physics
CERN / University of Hamburg
Master of Science in Physics
DESY / University of Hamburg
Bachelor of Science in Physics
Optics measurement algorithms have been improved in preparation for the commissioning of the LHC at higher energy, i.e. with an increased damage potential. Due to machine protection considerations the higher energy sets tighter limits in the maximum excitation amplitude and the total beam charge, reducing the signal to noise ratio of optics measurements. Furthermore the precision in 2012 (4 TeV) was insufficient to understand beam size measurements and determine interaction point (IP) $\beta$-functions ($\beta^*$). A new, more sophisticated algorithm has been developed which takes into account both the statistical and systematic errors involved in this measurement. This makes it possible to combine more beam position monitor measurements for deriving the optical parameters and demonstrates to significantly improve the accuracy and precision. Measurements from the 2012 run have been re-analyzed which, due to the improved algorithms, result in a significantly higher precision of the derived optical parameters and decreased the average error bars by a factor of three to four. This allowed the calculation of $\beta^*$ values and demonstrated to be fundamental in the understanding of emittance evolution during the energy ramp.
During LHC Run 1 about 30 % of the potential peak performance was lost due to transverse emittance blow-up through the LHC cycle. Measurements indicated that the majority of the blow-up occurred during the energy ramp. Until the end of LHC Run 1 this emittance blow-up could not be eliminated. In this paper the measurements and observations of emittance growth through the ramp are summarized. Simulation results for growth due to Intra Beam Scattering will be shown and compared to measurements. A summary of investigations of other possible sources will be given and backed up with simulations where possible. Requirements for commissioning the LHC with beam in 2015 after Long Shutdown 1 to understand and control emittance blow-up will be listed.
Optics measurement algorithms which are based on the measurement of beam position monitor (BPM) turn-by-turn data are currently being improved in preparation for the commissioning of the LHC at higher energy. The turn-by-turn data of one BPM may be used more than once, but the implied correlations were not considered in the final error bar. In this paper the error propagation including correlations is studied for the statistical part of the uncertainty. The confidence level of the measurement is investigated analytically and with simulations.
LHC will resume operation in 2015 at 6.5 TeV. The higher energy allows for smaller IP beta functions, further enhancing the optics errors in the triplet quadrupoles. Moreover the uncertainty in the calibration of some quadrupoles will slightly increase due to saturation effects. The complete magnetic cycle of the LHC will take longer due to the higher energy and extended squeeze sequence. All these issues require more precise and more efficient optics measurements and corrections to guarantee the same optics quality level as in 2012 when a 7% peak beta-beating was achieved. This paper summarizes the on-going efforts for achieving faster and more accurate optics measurements and corrections.
After the long shut down of 2013-2014, the LHC energy will be pushed toward 7 TeV. In this range of energy, the main magnets will enter a new regime. For this reason, this paper will present a detailed study of the performance of the FiDeL model that could be critical for the operation in 2015. In particular this paper will study the saturation component and its precision in the model, the errors due to the hysteresis, and an estimate of the dynamic effects for the 7 TeV operation.
The first physics production with asymmetric p-Pb collisions in 2013 required a complex machine commissioning to be done in a very short time after the Christmas technical stop. A substantial part of this was the commissioning of the newly created squeeze process to beta-star = 0.8m at IP2, as well as IP1 and IP5, and beta-star = 2.0m in IP8, the lowest value ever provided at LHCb. It was necessary to make all tests off-momentum in both positive and negative directions to prepare for both p-Pb and Pb-p configurations. Using protons in both rings, the commissioning was carried out successfully in three days.
For the control and optimization of electron beam parameters at modern free-electron lasers (FEL), transverse deflecting structures (TDS) in combination with imaging screens have been widely used as robust longitudinal diagnostics with single-shot capability, high resolution and large dynamic range. At the free electron laser in Hamburg (FLASH), a longitudinal bunch profile monitor utilizing a TDS has been realized. In combined use with a fast kicker magnet and an off-axis imaging screen, selection and measurement of a single bunch out of the bunch train with bunch spacing down to 1µs can be achieved without affecting the remaining bunches which continue to generate FEL radiation during user operation. Technical obstacles have been overcome such as suppression of coherent transition radiation from the imaging screen, the continuous image acquisition and processing with the bunch train repetition rate of 10Hz. The monitor, which provides the longitudinal bunch profile and length, has been used routinely at FLASH. In this paper, we present the setup and operation of the longitudinal bunch profile monitor as well as its performance during user operation.
The first high-luminosity p-Pb run at the LHC took place in January-February 2013 at 4 Z TeV energy per beam. The RF frequency difference of proton and Pb is about 60 Hz for equal magnetic rigidities at that energy, which means that beams move to slightly off-momentum, non-central, orbits during physics when frequencies are locked together. The resulting optical perturbations (“beta-beating”) restrict the available aperture and required a special correction. This was also the first operation of the LHC with low beta function in all four experiments and it required a specific collimation set up. Predictions from offline calculations of beta-beating correction are compared with measurements during the optics commissioning and collimator set-up.
During 2012 the LHC was operating at 4TeV with beta star at ATLAS and CMS interaction points of 0.6 m. During dedicated machine studies the nominal LHC optics was also setup with beta star of 0.4 m. A huge effort was put into the optics commissioning leading to a record low peak beta-beating of around 7%. We describe the correction procedures and discuss the measurement results.
This paper reports on the first direct measurement of amplitude detuning using AC dipoles. The only means in the LHC at high energy to excite large betatron oscillations is using AC dipoles. In the linear regime a perfect AC dipole does not excite the natural tune of the machine. This seriously challenges the measurement of the amplitude detuning by having to rely on imperfections in order to observe the natural tune. The measurements were carried out at β*=0.6 m and at flat-top during two MD sessions in 12-10-2012 and 27-11-2012, respectively.
Chromatic coupling was regularly measured in the LHC throughout 2012. A first beam-based correction of chromatic coupling was applied during a dedicated MD. In this note we summarise the measurements and results showing a significant reduction in the chromatic coupling for both beams.
Errors in the range of 1% have been observed for the MQY magnets in beam-based measurements. Furthermore, inconsistencies have been observed when comparing previous magnetic measurements to the LHC LSA database. After a revision, new calibration data have been extracted and were compared to the optics corrections that have been obtained from beam-based measurements. In 27-11-2012 a MD session has been performed to test these calibration data. This paper reports on the experimental verification of the new calibration data for the MQY quadrupole magnets.
The LHC is currently operating with a proton energy of 4 TeV and beta functions at the ATLAS and CMS interaction points of 0.6 m. This is close to the design value at 7 TeV (beta-star = 0.55 m) and represented a challenge for various aspects of the machine operation. In particular, a huge effort was put into the optics commissioning and an unprecedented peak beta beating of around 7% was achieved in a high energy hadron collider.
This paper reports on the first successful attempt to measure and correct the on-momentum optics close to the half integer resonance in LHC at injection energy. This tune working point is traditionally preferred by colliders for providing the largest resonance-free space in the tune diagram [1, 2]. It is also considered as an option for the HL-LHC upgrade .
At present, the LHC operates with a different fractional tunes at injection and at collision energy due to improved dynamic aperture indicated by tracking studies. Therefore, a tune swing crossing the 10th order resonance is needed during the beta-squeeze. A new proposal to alter the working point to collision tunes already at injection and during an energy ramp is foreseen to avoid the tune jump. Simulations and measurements of the optics along with the beam emittances and lifetime are compared to the nominal injection tunes. Feasibility for a working point close to the 1/2 integer is also attempted.
Two methods for deriving the beta-functions were studied, as a discrepancy occurred when approaching the half integer resonance. Several simulations have been performed to study the behavior of the two methods under different controlled circumstances. An ideal accelerator without any errors of the magnetic components was simulated before magnetic field errors were added to the simulation. A new algorithm was developed in order to improve the computation of the beta-function.