Design and Event Handling of Data Acquisition and Power Supply devices for FFMM at CERN

INTRODUCTION 
9 
 
These reasons led to launch the development of a new platform for magnetic 
measurements at CERN. The software Flexible Framework for Magnetic 
Measurements (FFMM) was subsequently developed to handle such platform.[6] 
In this thesis work, design and event handling of several FFMM classes are 
proposed in order to allow the control of two strategic devices: a Data 
AcQuisition (DAQ) board and a low voltage Power Supply. Finally, experimental 
results of measurements carried out on the devices in two different laboratories 
(University of Sannio and CERN) will be described. 
 
  
10 
 
PART 1 – STATE OF THE ART 
 
  
11 
 
CHAPTER 1. CERN  
 
 
 
The European Organization for Nuclear Research (French: Organisation 
Européenne pour la Recherche Nucléaire), known as CERN, is the world's largest 
particle physics laboratory, situated in the northwest suburbs of Geneva on the 
Franco-Swiss border, established in 1954. The organization has twenty European 
member states, and is currently the workplace of approximately 2,600 full-time 
employees, as well as some 7,931 scientists and engineers (representing 500 
universities and 80 nationalities). 
CERN's main function is to provide the particle accelerators and other 
infrastructure needed for high-energy physics research. Numerous experiments 
have been constructed at CERN by international collaborations to make use of 
them. The main site at Meyrin also has a large computer centre containing very 
powerful data processing facilities primarily for experimental data analysis, and 
because of the need to make them available to researchers elsewhere, has 
historically been (and continues to be) a major wide area networking hub.[1] 
1.1 Particle accelerators 
In general, particle accelerators are machines accelerating charged particles to 
high-kinetic energies, by applying electro-magnetic fields. A particle of charge q 
and momentum ��  moving through an electromagnetic field is submitted to the 
Coulomb and Lorentz’s forces: 
�� =
∂�� 
∂��
= �� �� + �� ∧ ��    
Equation 1.1 
 
CHAPTER 
1 
 
PART 1 – STATE OF THE ART 
12 
 
 
where ��  is the electromagnetic force exerted by the electric field ��  and the 
magnetic field ��   on a particle of velocity �� .[2]  
The electric field ��  and magnetic field ��   change both the particle trajectory and 
the velocity, thus the trajectory and the energy can be modified. According to 
Equation 1.1, three fundamental elements are necessary to realize the particle 
accelerator: 
 ξ particle beams have to be accelerated;  
 ξ particles must be guided on the reference “circular” orbit;  
 ξ particle beams must maintain during their revolution a proper intensity 
and size. 
CERN operates a network of six accelerators and a decelerator. Each machine in 
the chain increases the energy of particle beams before delivering them to 
experiments or to the next more powerful accelerator.  
1.1.1 The Large Hadron Collider (LHC) 
The LHC is the world's largest and highest-energy particle accelerator, intended 
to collide opposing beams of protons or lead ions, each moving at approximately 
99.999999% of the speed of light. 
The LHC tunnel is located 150 meters underground. It uses the 27-km 
circumference circular tunnel previously occupied by LEP, closed down in 
November 2000. CERN's existing PS/SPS accelerator complexes will be used to 
pre-accelerate protons which will then be injected into the LHC at the insertion 
points at the energy of 450 GeV. With the LHC, the aim is to continue to push our 
understanding of the fundamental structure of the universe. The results from the 
LHC might shed light on the following topics: Dark energy, Dark matter, Extra 
dimensions, Higgs boson, Supersymmetry. 
The LHC relies heavily on superconducting magnets which are at the edge of the 
present technology. Presently three large accelerators are based on 
superconducting magnets: the Tevatron (FNAL), HERA (Desy) and RHIC (BNL). All 
CHAPTER 1 – CERN 
13 
 
of these make use of classical Nb-Ti superconductors, cooled by supercritical 
helium at a temperature slightly above 4.2 K, with nominal fields below or 
around 5 T. 
The challenge for the LHC magnet system is to obtain the highest possible 
bending strength whilst still making use of the well-proven technology based on 
Nb-Ti Rutherford cables. Performance of Nb-Ti is increased by cooling the 
superconductors to a temperature below 2 K, using superfluid helium. In 
practice, at 1.9 K, an extra 1.5 T is attainable in the LHC dipole, i.e. from around 
6.5-7 T to 8-8.5 T central field, with a corresponding 20 % gain in beam energy. 
The LHC will collide two counter rotating proton beams at a nominal center of 
mass energy of 14 TeV. In addition to protons, also heavy ions will be brought 
into collision.[3] 
The initial particle beams were injected into the LHC August 2008, the first 
attempt to circulate a beam through the entire LHC was on 10 September 2008, 
but the system was taken down for repairs on 19 September 2008 and due to a 
faulty cable fixture it must be reheated, repaired, and recoiled. With this and the 
previously scheduled winter shutoff due to power consumption, no results are 
expected before February 2009. 
 
Figure 1.1 Injection of the first beam into the LHC 
PART 1 – STATE OF THE ART 
14 
 
1.2 Particle detectors 
In experimental and applied particle physics and nuclear engineering, a particle 
detector, also known as a radiation detector, is a device used to detect, track, 
and/or identify high-energy particles, such as those produced by nuclear decay, 
cosmic radiation, or reactions in a particle accelerator. Modern detectors are 
also used as calorimeters to measure the energy of the detected radiation. They 
may also be used to measure other attributes such as momentum, spin, charge 
etc. of the particles.[4] 
Six experiments (CMS, ATLAS, LHCb, TOTEM, LHC-forward and ALICE) are 
currently being built, and will be running on the collider; each of them will study 
particle collisions under a different point of view, and with different 
technologies.  
 
Figure 1.2 CERN Accelerators 
CHAPTER 1 – CERN 
15 
 
1.2.1 CMS 
One of two large general-purpose particle physics detectors built on the LHC is 
the Compact Muon Solenoid (CMS) experiment. 
CMS is designed as a general-purpose detector, capable of studying many 
aspects of proton collisions at 14 TeV, the mean energy of the LHC particle 
accelerator. It contains subsystems which are designed to measure the energy 
and momentum of photons, electrons, muons1, and other products of the 
collisions. The innermost layer is a silicon-based tracker. Surrounding it is a 
scintillating crystal electromagnetic calorimeter, which is itself surrounded with a 
sampling calorimeter for hadrons. The tracker and the calorimetry are compact 
enough to fit inside the CMS solenoid which generates a powerful magnetic field 
of 4 T. Outside the magnet are the large muon detectors, which are inside the 
return yoke of the magnet.[5] 
 
Figure 1.3 The set up of the CMS. In the middle, under the so-called barrel there is a man for 
scale (HCAL=hadron calorimeter, ECAL=electromagnetic calorimeter). 
                                                     
1
 The muon (from the letter μ used to represent it) is an elementary particle with negative 
electric charge and a spin of 1/2 
PART 1 – STATE OF THE ART 
16 
 
1.3 The LHC magnets 
A bending dipole field ��   of 8.33 T is required by a protons beam to reach the 
energy of 7 TeV on a circular trajectory with a curvature radius of 2803 m. The 
LHC quadrupole magnets are designed for a gradient of 223 Tm−1 and a peak 
field of about 7 T. These high-intensity magnetic fields can be achieved efficiently 
and practically with superconducting magnets only. The LHC will contain a total 
of 8.400 magnets, including the 1232 (15 m long) dipoles, 360 (3.25 m long) 
quadrupoles, and the various families of corrector magnets up to dodecapole 
orders. The high strength of magnetic field to be achieved in the LHC magnets 
leads to the choice of the superconducting technology that allows very high 
density currents to flow producing negligible joule heating.[6] 
In storage rings like the LHC, stable beams have to run as long as possible on the 
circular orbit (for several hundreds of millions of turns), in order to increase the 
number of collisions between the counter-rotating beams. This imposes strong 
constrains on the tolerable field perturbations along the trajectory. Deviations 
from the dipole and quadrupole fields, even if short in both space and time, can 
induce instabilities reducing the beam life-time. Higher-order multipoles 
correctors are required to compensate the unavoidable imperfections of dipole 
and quadrupole magnets. Ideally, a pure n-pole field could be produced by a 
current flowing along an infinitely thin cylindrical shell, with a cosine like 
distribution, 
�� θ = ��0 cos ��θ  
Equation 1.2 
where θ is the azimuthal angle.[7] 
CHAPTER 1 – CERN 
17 
 
 
Figure 1.4 Generation of pure dipole (a), quadrupole (b) and sextupole (c) fields by I(θ) current 
distributions with n=1, 2, and 3, respectively. 
Figure 1.4 shows schematically the current distributions that produce pure 
dipole, quadrupole, and sextupole fields. The current distribution that can be 
practically achieved is only an approximation of the ideal one that would 
produce a perfect multipole field magnet. 
The LHC dipoles are 15 meters long with a beam aperture of 50 mm in diameter, 
giving the possibility to consider the coils as infinitely long, and evaluate the 
magnetic field in the x-y complex plane by neglecting the z component. This 2-
dimensional approximation is very convenient to describe ��   in terms of a 
complex variable z. In the central part of the dipole taking into account the 
properties of the analytical functions, it can be postulated that the magnetic field 
generated ��   can be expanded in the complex plane in a power series:[8] 
�� �� = ��1 ������������
��−1
��1
∞
��=1
 
��
��������
 
��−1
= ��1 ����
∞
��=1
 
��
��������
 
��−1
∙ 10−4 
Equation 1.3 
where Cn is in units of �� ∙ ��1−��  while  ���� =
������������
��−1
��1
 are the multipoles normalized 
respect to the main dipole field and referred to a reference radius �������� =
17����. In this way, all the series coefficients cn result dimensionless and are 
expressed in so called units of the main field at the reference radius.[8] They are 
then multiplied by the scaling factor 10-4 that is the order of the ratio between