part 1 lecture 1 ax hcmuns 2013.pdf
TRANSCRIPT
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Introduction for Mo dern ccelerators
Pr inciple and its pplication
Takahisa I tahashi
Visiting Professor,
UNI VERSITY of SCIENCE-VNUHCM,
Faculty of Physics and Nuclear Technology
Part 1, Accelerators, Principle and Components
Part 2, Applications of Accelerators
Lecture guide; appeared in several slides beforehand,
such as references, required courses, and credits ----
You will have occasionally small exams, after the end
of class and you will have to present reports
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Chapter 1. Accelerator and its principle
1.1 I ntroduction
Accelerators were developed along with a realization of scientific curiosity in
Nuclear Physicsand in Elementary Physics
Scientific innovations have been brought about with advanced technologies inAccelerator Science and Technologies.
These works have been more widely applied to various scientific subjects such as
Biology, Archeology, Astrophysics, Medical Science, Environmental Science and so
on.
Developments of compact and cost-effective accelerator are really crucial for these
studies
In Particle Physics new particles and ideas have been brought about by the
development of accelerators and various detectors You should remember these developments to cope with the difficulties in existing
technologies; super-conducting magnet, RF cavities, ion source, and so on.
Realized ideas have been appeared in the several lectures in undergraduate courses;
such as Analytical Mechanics, Electricity and Magnetism.
Today cost effective and compact accelerators are needed in various scientific fields;
medical applications and nano-technologies.
This lecture includes the fundamental aspects of accelerator principle and I will focus
the physics ideas appeared in the specific performances of applied fields.
The main parts are concerned with Cyclic Accelerators and the other also with
Static Accelerators
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The Linear Acceleratorwill be briefly mentioned about, and the references will be
listed up.
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Various words appeared in this lecture are:
What is a charged particle?
Ionization Figure Ions
What is an accelerator?
A machine for which a charge particle can be accelerated and transported by using the
Coulomb and /or Lorentz forcesto increase the particle energy
Components of the accelerator
Production Ion source
Transport Beam line
Acceleration >>> Transport>>> Beam line
Irradiation Target
Motion in the Electric Field
Motion in the Magnetic field
What is an ion beam?
An ion beam is a flow of energetic ions, similar to the light.
Figure
Unit of the Energy,
Energy-Mass Equivalence Priciple
Through their Electromagnetic Interaction, Particles are accelerated.
Only electrically charged particles that are stable may be readily accelerated.
All accelerators must have a source of charges, a means for accelerating the charges, andan evacuated ----.
Besides the particle species,the two parameters the energy and the particle flux.
The particle flux in an experiment is often referred to as the luminosity.
From the definition of cross section the rate(R) of event for a parti cular process,the
luminosity (L ) times the cross section () for that process:
R = L
The units of l uminosity are m-2s
-1
x
For proton, try the same ways ?
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1.2 Birth of accelerator
It was based on the following background
Curiosity for nature -> invisible things-> The discovery of natural radio-activities
Chemical properties (periodic table) -> logical ways ->Matter->molecule-> atom
Observation of Cosmic ray (Hess received Nobel Prize in 1935) -> other direction from
accelerator->has become an interesting field of science
Natural particle sources such as Alpha-, beta-, gamma-, decays ->(means, expectation)
existing of internal structure, tiny constituents
As a tool,
Statistics and systematic would be not enough compared with laboratory
measurements, it will be experienced if you start the some studies
Reproduction and confirmation would be not so easy to realize the same
experimental condition
Model would be so sophisticated as not to elucidate the phenomena happened
in the studies
We need accelerators
I on source, acceleration, potenti al, high voltage,
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The acceleration principles are based upon
the Classical Electricity and Magnetism
By considering the acceleration, there appeared two types of accelerators such as
DC-accelerator and AC-accelerator
1.3 I ntroduction of DC-Accelerator
DC accelerator has a severe limit
to the obtainable energy due to the
applied voltage
The motion of Charged Particles
in the Electric Field
Acceleration of charged Particles
Deflection of Charged Particles
Focusing of Charged Particles
F
= qE
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1.3.1 Energy Units
The energy units of modern physics are derived from the unit of potential
difference in electricity, the volt(V). The definition of the volt is
1 J
=
1 eV = The electric force constant kin Coulombs law is measured
to be 8.99 x 109J m/C
2. Calculate the quantityke
2in units of (Jm)
and (eVnm).
ke2=
= 2.30x10-28
J
And k = = 1.44 eVnm
Calculate the strength of the electric field at a distance of 0.1 nm frm a proton
E=
=144 v/nm = 1.44x1011
V/m this electric field is an extraordinaly big compared
with to fields accessible in the laboratory.
1.3.2Mass Energy
Cathode ray tube
The apparatus is a kind of accelerator
By the way, do you know G.P. Thomson ?
Remember J.J.Thomson experiment fordiscovery of electron
With C.J. Davissonhe receivedNobel-Prize
foran electron diffraction
Note the polarity of electron source, cathode
potential
I suppose E-field would be more
easier and familiar to deal with at
that period, 1930
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Consider the b particles (electrons) from the spontaneous decay of a heavy nuclei first
observed by Becqurel. These electrons did not exist before the nuclei dcayed. The
electrons are created in the decay process. The electrons are produced with a typical
kinetic energy of an MeV.
Where does this energy come from ?
The energy that the electron acquires was stored in the form of mass energy of the
decaying nucleus.
The total energy (E) of any particle is defined to be the sum of two parts: energy due
to motion, called the kinetic energy (Ek), and Energy stored in mass, called the mass
energy (E0). Energy is defined in this manner,
Because this quantity is observed to be conserved in all particle interactions.
m/s
The electron mass energy is
= x
eV
1.
Similarly, the proton mass energy is ????2. Calculate the electron mass in units of MeV/c
2
3. Show explicitly the MeV/c2has units by expressing it in kgs
4. The atomic number of hydrogen is A=1.00078. Calculate the mass energy of the
carbon atom (A=12) in electron volts. Estimate firstly the atomic mass unit, 1 u.
1.3.3 Binding Energy
Consider an electron and proton bound togather to form an atom of hydrogen. The energy(E) required to separate the electron and proton to a large distance, work done against
the Coulomb force, is measured to be
E = 13.6 eV
What happens to this energy ?
The energy needed to ionize the hydrogen atom has been converted into mass energy.
The mass energy of the hydrogen atom (mHc2) is smaller than the sum of the mass
energies of the electron(mec2) plus (mpc
2) by an amount equal to 13.6 eV:
mHc2+ 13.6 eV = mec
2+ mpc
2
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The difference between the mass energy of the components (the electron and the proton)
and the mass energyof the composite object (the hydrogen atom) is called the binding
energy (Eb):
Eb= mec2+ mpc
2mHc
2= 13.6 eV
The binding energy is the amount of energy that must be provided inorder to break the
atom into its components. When this happens, the fractional change in mass (mH/mH) of
the atom is
For most processes, we may neglect this change in the mass of the atom.
The experiment of J.J.Thomson
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1.4 Cockcroft-Walton Type Accelerator
DC accelerator has been still used as an injector of the big accelerators, One of the
most valuable dc accelerator, well developed components such as capacitors, resistors,
and various protection parts
Q. Why do we use these old ideas until now ?
Ans. Good beam quality
Well established techniques
Simple assembly
Easy maintenance
Old times 1930
1990 at PSI
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With the first proton accelerator, Cockroft and Waltonobserved the reaction
p +7Li -> +.
Make an order-of-magnitude estimate of the proton energy required to initiate the
reactionp +7Li -> +.
First consider conservation of energy. If the prton has zero kinetic energy, then the Q
value of the decay is
Q = mpc2+ mLic
2mc
2mc
2
or
Q = 938.3 MeV + 6533.9 MeV3727.4 MeV3727.4 MeV ~ 17 MeV
The Qvalue is positive, positive, so conservation of energy does not place a limit on
the proton energy threshold. The energy threshold limit comes from the Coulomb
barrier presented by the7Li nucleus. If R is the nuclear radius, the energy threshold
for the proton to surmount this barrier is
= be careful for h= h/2here.
If we estimate R to be about 2 fm and use Z=3 for lithium
Ek= = 2 MeV
Thus, the order of magnitude of the proton kinetic energy needed to cause the nuclear
reaction is 2 MeV.
Wave Nature-----
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1.4.1 The structure of Cockcroft-Walton Accelerator
Fig 3.1 appeared in pp. 243 (a) schematic
representation of a cascade of C-W double circuits
(J.B.A. England )
The network A0, A1, B0, B1, C0, C1, D0and D1
constitutes a single-voltage doubling stage and by
operating several such stages in cascade a
corresponding increase in voltage is obtained at the
points A in these stages. Initially all the capacitors
are uncharged so that the voltages of all points -----.
1.4.2 The pri nciple of C-W accelerator
In this single voltage doubling stage during the
first cycle of operation, assuming that no further
stages are coupled to it.
Suppose that in the first half-cycle the point B0is
negative with respect to A0.
The point B1attempts to follow point B0and, since C0is uncharged, it becomes negative
with respect to A0so that D0conducts and point B1stays close to the voltage of point A0.
The capacitor C0is thus charged to a total charge Q at the voltage V, where Q = C0V.
This charging occurs so long as point B1 is slightly negative with respect to A0, that is,
during the first quarter of the cycle.Once the voltage of B0passes the peak negative voltage, -V, and starts to rise towards
zero volts, B1rises above A0, because C0 is now charged to a voltage +V, and D0cuts off.
However, because A1is initially at the same voltage as A0, B1is now rising above A1in
voltage and so D1 conducts and part of the charge Q on capacitor C0flows to A1so that
C1is now charged D1continue to conduct so long as B1is at a higher voltage than A1,
that is, from the end of the first quarter of the cycle to the end of the third quarter. Once
B0passes the peak positive voltage, +V, and starts to fall towards zero volts, D1ceases to
conduct and B1follows B0down until the voltage of B1passes zero volts when D0againconducts and C0is then recharged to the voltage +V. The process is repeated and after a
few cycles the points A1, B0and B1 with time over the first few cycles is shown in figure.
Figure 3.1 (b).
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1.5 The Van de Graaff generator
The electrostatic generator was conceived by Load Kelvin in 1890 and put into useful
practice in essentially its modern form by R. J. van de Graaff in 1931. Later this has been
changed into the Pelletron Accelerator by Herb (NEC)
charge
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1.5.1 Principle based on Electricity and Magnetism
Gausss Law
=
=
(VrVR ) = ( q
Vr= VR Vr- VR = 0
=
1.5.2 Negative ion and positive ion; charged parti cles
What are they?
How do you make them?These basic ideas are based on the collision of atoms with electrons
Neutral atoms collides with energetic electrons, bound electrons may easily stripped off,
or attached to and become charged particles, either positive or negative.
This is positive ion source
The other is negative ion source
The principle of positive ion source
Consider the electron of Hydrogen atom
The centrifugal force due to the Coulomb interaction between the electron and Hydrogen
nuclei, proton
mv2/r = e
2/4e0r
2
Bohrs theory for Quantum Theorem,
mvr= nh/2, n = 1,2,3, ----
Discrete radius of electron orbit is described as
rb= (e0h2/me
2) n2= a0n2
r1= a0= e0h2/me
2= 0.529 [Ao] Bohr radius
V_R
++++++
Q
+q+q+qq
Two conducting spheres
Connect each other equi potential not zero
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High voltage
Basic principle of the charging systems
The method for transferring charge to the final
high-voltage terminal in a Van de Graaff type of
electrostatic generator is a mechanical way;
three different versions of this mechanical
transport systems are currently in operation.
The most widely used method is the insulating endless belt.
Molded rubberized fabric.Uniform insulating properties
The belt and pulley system serves the dual purpose of transporting charge to the
terminal and of providing a source of low voltage power inside the terminal for auxulially
or ancillary equipment.
The normal method of placing the charge on the belt is by Corona discharge
This charge placed on the moving belt is then transported to the high voltage terminal
electrode, where it is removed by a second set of corona points or by a wire brush
rubbing on the belt.The discharge in the gas or in air can provide either positive or negative charge on the
belt.
Charge currents of up to about 1 mA can be transported in this way at a belt speed of
about 25 ms-1
.
The belt charging system in Van de Graaff generators are shown in fig. Fig. 3.3 appeared
in page 255.The pellet system developed later by Herb has more advanced technologies and
advantages,
Induction not discharge
Single ended Van de Graaff
Paschen
Law
Discharge in gas
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1.5.4 Tandem accelerators
(1) A terminal voltage V, the final energy {V(1+n)}eV
(2) The ion source and all its controls are outside the pressure vessel and at or near
ground potential.
1.5.5 Tr iple acceleration
Further increasing the final energy of the particles in a purely electrostatic accelerator
system
Only realized is triple three stage Van de Graaff
{V+ V(1+n)}eV
(1) Negative ion produced and accelerated with V
Injected into Tan dem van de Graaff with Vand V(1+n)
In total
(2) positive ion produced and accelerated with small single ended machine at about
500 keV
and neutralized and inject into second tandem pass through adder canal in the negative
terminal and becomes negative ions which are then accelerated.
problem of negative helium ion
Positi ve or Negative ion source
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1.5.6 Tan-dem Van de Graaff and negative ions
This is a fur ther development of the Van de Graaff generator.The new feature is to
use the electrostatic potential twice over. First an extra electron is attached to a neutral
atom to create anegative ion. In recent years, there has been considerable development
and it is now possible to obtain negative ion sources for almost all elements (?). The
negative ion beam is injected at ground potential into the tandem. At the high-voltage
terminal the beam passes through a thin foil which strips at least two electrons from each
Charge
From negative
to positive
Stripper foils or
gas stripper
SF6high pressure
Gas circulation
Purification
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ion converting them to positive ions. They are then accelerated a second time back to
earth potential. The present-day record is held by the tandem at Oak Ridge, which
operates with 24.5 MV on the high-voltage terminal, but a machine called Vivitron is
under construction at Strasburg with a design voltage of 35 MeV.
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6.VI VITRON in France
K-22 HCMUNS Dec.2012 47
1.3.4 World highest voltage for a static
accelerator VIVITRON in France35 MV design
but operating voltage is 25 MV
DC-Accelerator