Bias in Internet Measurement Platforms

Network operators and researchers frequently use Internet measurement platforms (IMPs), such as RIPE Atlas, RIPE RIS, or RouteViews for, e.g., monitoring network performance, detecting routing events, topology discovery, or route optimization. To interpret the results of their measurements and avoid pitfalls or wrong generalizations, users must understand a platform's limitations. To this end, this paper studies an important limitation of IMPs, the \textit{bias}, which exists due to the non-uniform deployment of the vantage points. Specifically, we introduce a generic framework to systematically and comprehensively quantify the multi-dimensional (e.g., across location, topology, network types, etc.) biases of IMPs. Using the framework and open datasets, we perform a detailed analysis of biases in IMPs that confirms well-known (to the domain experts) biases and sheds light on less-known or unexplored biases. To facilitate IMP users to obtain awareness of and explore bias in their measurements, as well as further research and analyses (e.g., methods for mitigating bias), we publicly share our code and data, and provide online tools (API, Web app, etc.) that calculate and visualize the bias in measurement setups

Noise effects in the Large Hadron Collider (LHC) and its High-Luminosity upgrade (HL-LHC)

In order to optimize the performance of a high-energy particle collider such as the Large Hadron Collider (LHC) and its high-luminosity upgrade (HL-LHC), a thorough understanding of all the phenomena that can act as a luminosity degradation mechanism is required. A major concern for the transverse single-particle beam dynamics is the presence of noise, a mechanism that can impact the long term stability of the circulating particles. From the plethora of noise sources that are present in the accelerator, this thesis investigates the impact of a modulation in the strengths of the dipole and quadrupole magnets of the lattice due to power converter noise. In the presence of non-linearities, depending on the spectral components of the noise and the nature of the source, such a mechanism can increase the diffusion of the particles in the distribution through the excitation of additional resonances in the vicinity of the ones driven by the lattice non-linearities. As this effect can have severe implications on the beam performance, it is important to understand and control the noise mechanisms that have been observed in the LHC and that are anticipated in the HL-LHC era. To this end, this thesis presents the investigation of the 50 Hz harmonics that have been observed in the beam spectrum since the beginning of the LHC operation. Based on several observations and dedicated experiments, the source of this perturbation is identified and its impact on the beam performance is quantified with single-particle tracking simulations. For the HL-LHC, the reduction of the beam size in the interaction points of the high luminosity experiments will lead to an increase of the maximum beta-functions in the quadrupoles of the inner triplet. The expected increase of the sensitivity to noise, combined with the new hardware that is currently implemented, justifies the need to investigate the impact of tune modulation effects on the beam performance. The modulation that may arise from the power supply ripples in the high beta-function regimes will be combined with the tune modulation that intrinsically emerges from the coupling of the transverse and longitudinal plane for off-momentum particles through chromaticity. The aim of the thesis is to determine whether tune modulation effects will pose a limitation in the luminosity production of the HL-LHC

Φαινόμενα θορύβου στον Μεγάλο Επιταχυντή Αδρονίων και στον αναβαθμισμένο Επιταχυντή Υψηλής Φωτεινότητας

In order to optimize the performance of a high-energy particle collider such as the Large Hadron Collider (LHC) and its high-luminosity upgrade (HL-LHC), a thorough understanding of all the phenomena that can act as a luminosity degradation mechanism is required. A major concern for the transverse single-particle beam dynamics is the presence of noise, a mechanism that can impact the long term stability of the circulating particles. From the plethora of noise sources that are present in the accelerator, this thesis investigates the impact of a modulation in the strengths of the dipole and quadrupole magnets of the lattice due to power converter noise. In the presence of non-linearities, depending on the spectral components of the noise and the nature of the source, such a mechanism can increase the diffusion of the particles in the distribution through the excitation of additional resonances in the vicinity of the ones driven by the lattice non-linearities. As this effect can have severe implications on the beam performance, it is important to understand and control the noise mechanisms that have been observed in the LHC and that are anticipated in the HL-LHC era. To this end, this thesis presents the investigation of the 50 Hz harmonics that have been observed in the beam spectrum since the beginning of the LHC operation. Based on several observations and dedicated experiments, the source of this perturbation is identified and its impact on the beam performance is quantified with single-particle tracking simulations. For the HL-LHC, the reduction of the beam size in the interaction points of the high luminosity experiments will lead to an increase of the maximum beta-functions in the quadrupoles of the inner triplet. The expected increase of the sensitivity to noise, combined with the new hardware that is currently implemented, justifies the need to investigate the impact of tune modulation effects on the beam performance. The modulation that may arise from the power supply ripples in the high beta-function regimes will be combined with the tune modulation that intrinsically emerges from the coupling of the transverse and longitudinal plane for off-momentum particles through chromaticity. The aim of the thesis is to determine whether tune modulation effects will pose a limitation in the luminosity production of the HL-LHC

Φαινόμενα θορύβου στον μεγάλο επιταχυντή αδρονίων και στον αναβαθμισμένο επιταχυντή υψηλής φωτεινότητας

In order to optimize the performance of a high-energy particle collider such as the Large Hadron Collider (LHC) and its high-luminosity upgrade (HL-LHC), a thorough understanding of all the phenomena that can act as a luminosity degradation mechanism is required. A major concern for the transverse single-particle beam dynamics is the presence of noise, a mechanism that can impact the long term stability of the circulating particles. From the plethora of noise sources that are present in the accelerator, this thesis investigates the impact of a modulation in the strengths of the dipole and quadrupole magnets of the lattice due to power converter noise. In the presence of non-linearities, depending on the spectral components of the noise and the nature of the source, such a mechanism can increase the diffusion of the particles in the distribution through the excitation of additional resonances in the vicinity of the ones driven by the lattice non-linearities. As this effect can have severe implications on the beam performance, it is important to understand and control the noise mechanisms that have been observed in the LHC and that are anticipated in the HL-LHC era. To this end, this thesis presents the investigation of the 50 Hz harmonics that have been observed in the beam spectrum since the beginning of the LHC operation. Based on several observations and dedicated experiments, the source of this perturbation is identified and its impact on the beam performance is quantified with single-particle tracking simulations. For the HL-LHC, the reduction of the beam size in the interaction points of the high luminosity experiments will lead to an increase of the maximum beta functions in the quadrupoles of the inner triplet. The expected increase of the sensitivity to noise, combined with the new hardware that is currently implemented, justifies the need to investigate the impact of tune modulation effects on the beam performance. The modulation that may arise from the power supply ripples in the high beta function regimes will be combined with the tune modulation that intrinsically emerges from the coupling of the transverse and longitudinal plane for off-momentum particles through chromaticity. The aim of the thesis is to determine whether tune modulation effects will pose a limitation in the luminosity production of the HL-LHC.Ο Μεγάλος Επιταχυντής Αδρονίων ολοκλήρωσε με επιτυχία το δεύτερο κύκλο λειτουργίας του έπειτα από τρία χρόνια πειραμάτων (2015-2018), επιτυγχάνοντας πρωτοφανείς τιμές ολοκληρωμένης φωτεινότητας και ενέργειας κέντρου μάζας. Για τα επόμενα δύο χρόνια προβλέπονται εκτεταμένες τροποποιήσεις με σκοπό την προετοιμασία του επιταχυντή για τη μελλοντική του αναβάθμιση σε επιταχυντή Υψηλής Φωτεινότητας. Η βελτιστοποίηση της απόδοσης του επιταχυντή απαιτεί, παράλληλα, την κατανόηση όλων των μηχανισμών που μπορούν να συντελέσουν στη μείωση ή υποβάθμιση της τιμής της φωτεινότητας. Ένας μηχανισμός που μπορεί να επηρεάσει την εγκάρσια κίνηση των σωματιδίων της δέσμης πρωτονίων, οδηγώντας σε χαοτικές τροχιές και κατ' επέκταση σε απώλειες πρωτονίων, είναι τα φαινόμενα θορύβου στη δυναμική των φορτισμένων σωματιδίων. Ανάμεσα στις πολλαπλές πηγές θορύβου που υπάρχουν σε έναν επιταχυντή, η παρούσα διδακτορική διατριβή επικεντρώνεται στις επιπτώσεις αρμονικών διαταραχών στην τάση των πηγών τροφοδοσίας των μαγνητικών διπόλων και τετραπόλων, ένα φαινόμενο που οδηγεί σε διαμόρφωση συχνότητας του μαγνητικού πεδίου. Ανάλογα με τις συχνότητες και το πλάτος των φασματικών συνιστωσών του θορύβου, που εξαρτώνται από το είδος του μαγνήτη και τον τύπο της πηγής τροφοδοσίας, οι επιπτώσεις στην εγκάρσια κίνηση των σωματιδίων της δέσμης ποικίλουν. Ο θόρυβος από τις πηγές τροφοδοσίας, σε συνδυασμό με τα μη-γραμμικά μαγνητικά πεδία του επιταχυντή, που είτε είναι απαραίτητα για τον έλεγχο της χρωματικότητας και την απόσβεση της συλλογικής κίνησης των σωματιδίων είτε απορρέουν από τη σύγκρουση των δύο δεσμών, μπορεί να αυξήσει τη διάχυση της κατανομής των σωματιδίων, μέσω της εμφάνισης και διέγερσης των επιπλέον συχνοτήτων συντονισμού πέρα από τις προβλεπόμενες λόγω της μη-γραμμικότητας των επί μέρους μαγνητικών πεδίων. Η ύπαρξη ενός τέτοιου φαινομένου είναι ιδιαιτέρως κρίσιμη για τη διατήρηση του συνολικού χρόνου ζωής της δέσμης. Η παρούσα έρευνα παρουσιάζει τη μελέτη αυτών των φαινομένων πρώτα στα δίπολα, ένα φαινόμενα που έχει παρατηρηθεί στον επιταχυντή, και στη συνέχεια στα τετράπολα, ένα είδος θορύβου που είναι ιδιαίτερα κρίσιμο για τον επιταχυντή Υψηλής Φωτεινότητας

Impact of Power Supply Ripple on the Beam Performance of the Large Hadron Collider and the High-Luminosity LHC

Harmonics of the mains frequency (50 Hz) have been systematically observed in the form of dipolar excitations in the transverse beam spectrum of the Large Hadron Collider (LHC) since the beginning of its operation. The power supply ripple, consisting of both fundamental and higher frequency components, is proven not to be the result of an artifact of the instrumentation systems with which they are observed. Potential sources of the perturbation have been identified through systematic analysis and experimental studies. Single-particle tracking simulations have been performed including a realistic power supply ripple spectrum, as acquired from experimental observations, to demonstrate the impact of such noise effects on beam performance

Impact of the 50 Hz harmonics on the beam evolution of the Large Hadron Collider

Since the beginning of the Large Hadron Collider (LHC) commissioning, spectral components at harmonics of the mains frequency (50 Hz) have been observed in the transverse beam spectrum. This paper presents an overview of the most important observations, collected during the latest physics operation of the LHC in 2018, which clearly indicates that the harmonics are the result of a real beam excitation rather than an instrumental feature. Based on these findings, potential sources of the perturbation are discussed and a correlation with power supply ripple originating from the magnets' power supplies is presented

Tune modulation effects in the High Luminosity Large Hadron Collider

Several transverse noise sources, such as power supply ripples, can potentially act as an important limiting mechanism for the luminosity production of the Large Hadron Collider (LHC) and its future High-Luminosity upgrade (HL-LHC). In the presence of non-linearities, depending on the spectral components of the power supply noise and the nature of the source, such a mechanism can increase the diffusion of the particles in the distribution through the excitation of sideband resonances in the vicinity of the ones driven by the lattice non-linearities. For the HL-LHC, due to the reduction of the beam size in the Interaction Points (IP) of the high luminosity experiments (IP1 and 5), increased sensitivity to noise effects is anticipated for the quadrupoles of the inner triplets. The modulation that may arise from the power supply ripples will be combined with the tune modulation that intrinsically emerges from the coupling of the transverse and longitudinal plane for off-momentum particles through chromaticity. To this end, the aim of this paper is to study the impact of tune modulation effects on the transverse beam motion resulting from the interplay between quadrupolar power supply ripples and synchro-betatron coupling. A power supply noise threshold for acceptable performance is estimated with single-particle tracking simulations by investigating the impact of different modulation frequencies and amplitudes on the Dynamic Aperture. The excitation of sideband resonances due to the modulation is demonstrated with frequency maps and the higher sensitivity to specific modulation frequencies is explained. Finally, a power supply noise spectrum consisting of several tones is considered in the simulations to determine whether the presence of power supply ripples in the quadrupoles of the inner triplet will limit the luminosity production in the HL-LHC era

Development of Computational Tools for Noise Studies in the LHC

Noise can have a significant impact on the beam dynamics in the LHC, enhancing diffusion processes and leading to emittance blowup. In order to study the details of such effects with computer simulations, a new set of tools is being developed. In particular, a demonstrator GPU-based particle tracker has been built profiting from the technology provided by the NVRTC Cuda library. Its performances for short term beam dynamic simulations in presence of many macro particles are highly promising. In addition, the Numerical Analysis of Fundamental Frequencies (NAFF) algorithm has been thoroughly inspected. Several alternatives to its fundamental steps have been investigated in a modern C++ implementation. The method was also used to produce Frequency Maps and benchmark these tools with other simulations