Introduction to particle accelerators and their...
Transcript of Introduction to particle accelerators and their...
Introduction to particle accelerators and their applications
- Parte II: Components
Gabriele Chiodini Istituto Nazionale di Fisica Nucleare
Sezione di Lecce
PhD lessons in Physics for Università del Salento
2015-16 (20 hours, 4 CFD)
1
/54Introduction to accelerators : Components G. Chiodini - May 2015
The component of an accelerator
• Source
• Injection (non treated here)
• Vacuum
• Magnets
• RF
• Extraction (non treated here)
2
/54Introduction to accelerators : Components G. Chiodini - May 2015
Source
3
Electrons Thermionic emission Photoemission
Ions ECR Penning (PIG) Negative Ions
/54Introduction to accelerators : Components G. Chiodini - May 2015
Electron sources
• Vacuum
• Electrons emitted by a cathode
• An anode with or without a hole
• Acceleration potential
• Focusing structure
4
/54Introduction to accelerators : Components G. Chiodini - May 2015
Thermionic emission
5
Metals heated to temperatures close to incandescence emit a current of electrons out of the surface( hot cathode )
/54Introduction to accelerators : Components G. Chiodini - May 2015
Photoemission
6
Planck constant h=6.6E-34 J x s = 4.1E-15eV x s
The electrons of a metal can receive higher energy than vacuum energy absorbing photons ( photocathode and laser)
Metals QE=0.01%!
Semiconductor QE ~=10-30%
The accelerating structure is a RF cavityQE=Quantum efficiency=N electrons/N photons
/54Introduction to accelerators : Components G. Chiodini - May 2015
Quantum efficiency
7
QE is determined by a three step process: !1. Electron excitation from the valence band to conductive band 2. Electron-phonon and electron-electron scattering thermalise the
electron approaching the surface 3. The electron affinity with respect to the conduction band
minimum determine the probability of emission in vacuum of electron in conduction band and near the surface
!
• Step 2 and 3 is less favorevol for metal than semiconductor. • The semiconductor photocathode have high QE but low
stability in time.
/54Introduction to accelerators : Components G. Chiodini - May 2015
Ions source
• Gas inlet
• Ionizing power input
• Plasma production region
• Magnetic confinement
• Ions extractions
8
Plasma
/54Introduction to accelerators : Components G. Chiodini - May 2015
The 4th state of matter: the plasma
• It is a rarefied and ionized gas e lectr ica l ly neutra l (molecules, ions+, ions-, electrons).
• It screens electrically a charged object placed inside by the formation of a charge layer
• It is created by heating, electric discharge, microwave absorption, laser absorption
9
/54Introduction to accelerators : Components G. Chiodini - May 2015
Charged particle motion in a plasma
1. Cyclotron motion along B. Radius of tens of micron for electrons and few millimetre for ions.
2. Drift motion orthogonal to E and B equals for electrons and ions (no charge, no mass dependence).
3. Magnetic mirror or bottle (not homogeneous B push charged particles versus low field region).
10
ρgirazione =qBmv
vdrift =EB
1
2
3
/54Introduction to accelerators : Components G. Chiodini - May 2015
ECR (Electron Cyclotron Resonace) ion source
11
fgirazione (elettroni) = v2πρ
=1
2πeBme
fgirazione (elettroni) = 28B(T) = fECR (GHz) condizione di risonananza ECR
•The ECR plasma production region ( discharge region ) corresponds to the region where the electron cyclotron frequency is equal to the frequency of the input microwave ( resonance). •The electrons are trapped by the magnetic mirror and heats up to keV and even to MeV energy level. •The ions slip-out the magnetic mirror because heavier than electrons, and form the ion beam. •No thermionic filament is used and then it is a very robust source.
ECR resonance condition
/54Introduction to accelerators : Components G. Chiodini - May 2015
real ECR
12
/54Introduction to accelerators : Components G. Chiodini - May 2015
Penning ion source or Philips Ionization Gauge(PIG)
• Pressure = 1E-3 Atm
• B=0.1 T
• Hot or cold cathode
• Electrons are accelerated in arc discharge due to the high voltage between anode and cathode (V=1kV and I=0.1-50A )
• Electrons rotate around B (tens of micron radius) and ionise the gas very effectively before to reach the anode.
• The ions slip-out from the hole and form the ion beam.
13
/54Introduction to accelerators : Components G. Chiodini - May 2015
Real PIG real
14
/54Introduction to accelerators : Components G. Chiodini - May 2015
Negative ion source
The negative ions are generated from positive ions or neutral atoms by electron transfer from substance with low electron affinity, such as an alkali metal ( for example cesium ).
Electron affinity = Energy released from electron transfer
15
B
/54Introduction to accelerators : Components G. Chiodini - May 2015
Production and extraction of negative ions
16
NEGATIVE ION EXTRACTION • The extraction of negative ions favours also the
extraction of electrons because they have the same charge .
• By means of a dipole you can separate the electron current from the negative ion beam.
Technique 1: caesium coated cathode (surface phenomena)
Technique 2: gas+caesium mixture(bulk phenomena)
/54Introduction to accelerators : Components G. Chiodini - May 2015
Vacuum systems
17
/54Introduction to accelerators : Components G. Chiodini - May 2015
Pressure units
18
The vacuum is a force: Pressure x Surface
P = nkTGas law
n[molecules/m3]=molecular density !Boltzman's constant k=1.38E-23 J/K = 8.6E-5eV/K
/54Introduction to accelerators : Components G. Chiodini - May 2015
Mass flow and Throughput
19
!
The quantity of gas flowing through a piping element is described by: • Mass Flow=Mass/Time (Kg/s) • Throughput Q = pV/Time (mbar x litre / s ) .
NB 1: In a perfect gas pV=M/mkT NB 2: The outgassing is measured in mbar x litre /s like Q
/54Introduction to accelerators : Components G. Chiodini - May 2015
C[litres/s]=Conductance
20
Q=Throughput [litre x mbar / s] P1-P2=pressure drop[mbar]
Conductances in parallel increase
Conductances in series decrease
C(hole area A )=11.6A[cm2]
C(tube of diameter D and length L ) =12.1D3/L[cm2]
It is better short and large tubes
/54Introduction to accelerators : Components G. Chiodini - May 2015
Effective pumping speed
21
The pumping speed is limited by the conductance of the connecting pipe:
Example: a turbo molecular pump of 8 keuro have a pumping speed S=400 l/s. If I connect a pipe of diameter d=10 cm and length L=2m I have C=60l/s then Seff~60l/s. In this case is better to buy a cheaper pump with S=60l/s which cost half and have Seff=30l/s.
Pumping speed of a pump
/54Introduction to accelerators : Components G. Chiodini - May 2015
Pumping time
22
Q = S x p Vdp/dt = S x p dp/dt = S x p / V dp/dt/p=S/V
In a vacuum tight volume V at pressure p without gas leak the pumping time is determined by the
where S is the effective pumping speed
The pressure follow the exponential decay law with time constant = S/V
/54Introduction to accelerators : Components G. Chiodini - May 2015
Viscous and molecular flow
23
λaria[cm] ~6.7 ⋅10−3
P[mbar]
1 < P < 103mbar 6.7 ⋅10−6 < λaria < 6.7 ⋅10−3cm
10−3 < P < 1 mbar 6.7 ⋅10−3 < λaria < 6.7 cm
P < 10−3mbar 6.7 cm < λaria
Molecular flux: dominated by collisions with the wall
Viscous flux: dominated by intra-molecule collisions
Molecular mean free path in air
•The two regimes differ each other completely for calculations and vacuum components. •The molecular regime is the true vacuum technology and is dominated by the nature of the surface walls which continuously release molecules ( Outgassing )
/54Introduction to accelerators : Components G. Chiodini - May 2015
Vacuum classification
• Medium vacuum: 10-3<P<1mbar viscous flux
• High vacuum: 10-7<P<10-3mbar molecular flux
• Ultra high vaccum: 10-12<P<10-7mbar molecular flux
24
/54Introduction to accelerators : Components G. Chiodini - May 2015
Outgassing and cleaning• In a vacuum system the final pressure is given by
the OUTGASSING of the surface walls.
• The outgassing depends on the nature, treatment, cleaning, temperature and pumping time
• Methods of cleaning
• Remove residues by chemical products
• Fire under vacuum at 9500C to extract hydrogen from stainless steel
• Electrical discharges to remove gas and atomic metal
• Heating to 1500C to remove water molecules ( bake- out )
25
Pfinale =Qoutgas sin g
Seff
Use ONLY metals NEVER plastics:
Qplastics=5000Qmetals
/54Introduction to accelerators : Components G. Chiodini - May 2015
Metals vs Plastics
26
Pfinale =Qoutgas sin g
Seff=qoutgas sin gASeff
/54Introduction to accelerators : Components G. Chiodini - May 2015
Vacuum measurements 1: 1E-4mbar<P<1 mbar
27
PIRANI GAUGE below 1E-4mbar gives wrong values
The reading is done by zeroing a resistive bridge by setting the current value necessary to maintain constant the temperature of a THERMISTOR placed in vacuum . Lower is the pressure, less cooling on the thermistor and less current is needed to keep it warm.
/54Introduction to accelerators : Components G. Chiodini - May 2015
Vacuum measurements 2: 1E-10mbar<P<1E-5 mbar
28
PENNING GAUGE
M e a s u r e t h e discharge current of a P e n n i n g c e l l b e t w e e n c o l d cathode and cold anode in magnetic field.
/54Introduction to accelerators : Components G. Chiodini - May 2015
Vacuum measurements 3: 1E-12mbar<P<1E-5 mbar
29
BAYARD-ALPERT GAUGE
Measure the current of the electrons emitted in vacuum from a hot filament and rotating in a magnetic field
/54Introduction to accelerators : Components G. Chiodini - May 2015
Pumps to create the vacuum
30
Primary rotational pump ( dry or oil ) used to pump from environmental pressure to 1E - 2 mbar . S = m3 / h . It works in viscous regime creating a pressure drop between input and output . Often it prepares the vacuum for the turbomolecular pump.
/54Introduction to accelerators : Components G. Chiodini - May 2015
Pumps to create the vacuum
31
Turbo-molecular pump used to pump from 1E-2 mbar up to 1E-11mbar and then it can be removed. S = 10 to 3000 l / s . It works in molecular regime by momentum transfer : when a molecule touches the blades rotating at a very high speed comparable to the molecule thermal speed the molecule is removed from the volume .
/54Introduction to accelerators : Components G. Chiodini - May 2015
Pumps to keep the vacuum
32
!Ion sputtering pump is used to maintain the vacuum and can work from 1E- 5 mbar up to 1E-11mbar. S = 1-500 l/s . It is a Penning cell where the emitted electrons ( 6kV ) ionize residual molecules and sputter the titanium covering the cathode. The sputtered titanium can chemically bond the residual gases or bury the not reactive o n e ( n o b l e g a s e s a n d hydrocarbons ) carrying them on the metal walls where they are absorbed .
/54Introduction to accelerators : Components G. Chiodini - May 2015
Vacuum components
33
BelowCooper
pipe
Collar,gasket, L-pipeFlange
Section valves
/54Introduction to accelerators : Components G. Chiodini - May 2015
Normal conductive magnets
34
/54Introduction to accelerators : Components G. Chiodini - May 2015
Magnet components
35
/54Introduction to accelerators : Components G. Chiodini - May 2015
Normal conductive magnet
• PRO IRON
• Less Ampere x Nturn
• Less dissipated power
• Guide and shape the magnetic field
• CONV IRON
• Saturate at about 2 Tesla (all magnetic domains are oriented along B)
36
Btraferro =µ0NIh
= BNI(1+ χferro )
χferro ~ 103 Microscopic currents (oriented magnetic domain)
Macroscopic currents (Coil)
/54Introduction to accelerators : Components G. Chiodini - May 2015
Coil
37
The standard coil is made by rectangular wires of copper or aluminium with cooling water going through. The wires are isolated and glued together by glass and epoxy resin.
2 Layers
4 Layers• Maximise NI (Ampere x turn) • Choose conductor area A and number of turn N
• Current density J= NI/A
Many N: low current (less losses), small terminal (easy and cheap connection), more isolation, more assembly cost, higher voltage Low J : less losses, less consumption, less cooling High j : smaller coil, cheaper, smaller magnets
/54Introduction to accelerators : Components G. Chiodini - May 2015
The limit of the normal conductive magnets
38
The iron saturates at T = 1.5 -1.8T and in ramping regime as in a synchrotrons can not exceed a magnetic field of 1-1.2T . !
You need to switch to the superconducting magnets (zero electric resistance) which are free of iron, but just coils with very high currents (10,000 A) and no power dissipation. !
But the superconductive magnet coil must be kept at temperatures close to absolute zero and therefore require cryogenics cooling systems.
/54Introduction to accelerators : Components G. Chiodini - May 2015
Comparison normal and super conductive magnets
39
T=4.2K At LHC (Large Hadron Collider) all the magnets are made of NbTi superconductive and cooled down to less than 2 K. !The cryogenic fluid at LHC is the superfluid He II (PERFECT HEAT CONDUCTOR): • zero viscosity • second sound (heat waves)
/54Introduction to accelerators : Components G. Chiodini - May 2015
Radio-Frequency accelerating structure
40
/54Introduction to accelerators : Components G. Chiodini - May 2015
What is a RF system• A charged particle could be accelerated
only electric field parallel to the direction of motion
• Variable fields can increase energy without voltage built-up
• Wideroe’s drift tube can work only at low frequency because at high frequency the drift tube are not anymoe equipotential:
• low energy
• low filed gradient
41
It is necessary to abandon drift tubes with uniform fields for accelerating structures with distributed fields
/54Introduction to accelerators : Components G. Chiodini - May 2015
Electromagnetic spectrum
42
/54Introduction to accelerators : Components G. Chiodini - May 2015
Dispersion relation in a pipe
43
Vacuum
Cylindrical pipe
Waveguide
/54Introduction to accelerators : Components G. Chiodini - May 2015
Dispersion relation in a RF cavity
44
RF cavity
Waveguide with obstacles
/54Introduction to accelerators : Components G. Chiodini - May 2015
Main RF components• RF oscillator
• RF power amplifier
• Coupling amplifier-cavity
• Accelerating cavity
• IN and OUT beam
• IN and OUT RF wave
• Power meter (Antenna)
45
/54Introduction to accelerators : Components G. Chiodini - May 2015
Disk-loaded guide waves with traveling waves (TW)
46
Guide wave IRIS-loaded: •f=2.856 GHz (S band) •86 acceleration cells •Coupling input/output •Accelerating file 30 MV/m
Wave guide allows to create a longitudinal component to the electromagnetic field and the discs to reduce the wave phase velocity less than the velocity of light in vacuum in such a way it can accelerate particles
/54Introduction to accelerators : Components G. Chiodini - May 2015
RF resonator with standing wave (SW)
47
Reentrant Nose-cone Disk-loaded Coaxial
The resonant cavities are characterised by stationary resonant modes that oscillate in time with frequency f and in the space with a wavelength λ without propagate ( Standing Wave ) . The standing wave is the sum of two waves traveling in the opposite direction and completely interfering at the boundaries.
Vviaggiante (x, t) = V0 sin(2πft −2πλx)
Vstazionaria (x, t) = V0 sin(2πft)sin(2πλx)
Traveling waveStanding wave
The boundary conditions decide if TW o SW
/54Introduction to accelerators : Components G. Chiodini - May 2015
Figure of merit of a RF cavity
48
• E=Accelerating field =V/L (Inside the Cavity)
• Q=Total Energy/ Dissipated Energy =U/P=f0/Δf
• Dissipated energy P due to surface resistivity of the RF conductor wall at high frequency (skin effect) = V x I = V2/R = I2R (In Cavity Wall).
R can be drastically reduce by a super-conducing RF cavity (Rs=8nΩ one million time lower than Copper at room temperature but not zero)
��
0
The penetration of an alternating current is not complete in a c o n d u c t o r c r o s s section due to the “Skin Effect”
/54Introduction to accelerators : Components G. Chiodini - May 2015
RF oscillator• A transistor is a current amplifier
• The output signal is in opposite phase with the input ( inverting )
• Two inverting stages are not inverting
• Sending at the input a fraction of the output signal triggers a positive feedback loop with frequency fRF=1/(RC).
Feed-back:
• Positive=self sustained oscillation
• Negative=stable gain amplifier
49
R C
/54Introduction to accelerators : Components G. Chiodini - May 2015
RF solid state amplifiers
Many amplifiers in parallel:
• f=325 MHz
• P=190 kW
• 4.7x4.7x2.3m3
50
/54Introduction to accelerators : Components G. Chiodini - May 2015
Tetrode as RF power amplifier
• In thermionic valves the conduction of current is possible only for electrons emitted from the hot cathode and collected by the anode (diode) .
• In the triode the control grid modulates the passage of the electrons between the cathode and the anode amplifying the signal of the grid on the anode.
• In a tetrode a constant potential grid shields the control grid from the anode an reduce the electric capacitance between control grid and anode allowing to work at higher frequency than a triode.
• The frequency limit is due to the transit time of the electrons from cathode to anode which can not reduced too much reducing the distance otherwise the increases of the electrical capacitance reduces the working frequency
51
Tetrodo
50<f<1000MHz, 200kW/tube
/54Introduction to accelerators : Components G. Chiodini - May 2015
Klystron
52
Speed modulation
Position modulation (bunches)
/54Introduction to accelerators : Components G. Chiodini - May 2015
• The klystron works on the principle of speed modulation
• An electron gun generates an electron beam .
• The gap1 ( Buncher ) is fed by a RF wave at high frequency
• Electrons arriving in Gap 1 are accelerated differently depending on the location ( speed modulation) and continue towards the Gap 2 ( Catcher )
• The faster electrons arrive in GAP2 in advance and the slower ones are late ( position modulation)
• In GAP2 the electrons travel in dense bunches and emit an intense electromagnetic wave at the output
53
/54Introduction to accelerators : Components G. Chiodini - May 2015
Real Klystrons
54