Introduction to particle accelerators and their applications - Parte II: Components

Transcription

1 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 (20 hours, 4 CFD) 1

2 The component of an accelerator Source Injection (non treated here) Vacuum Magnets RF Extraction (non treated here) ntroduction to accelerators : Components 2 /54

3 Source Electrons Thermionic emission Photoemission Ions ECR Penning (PIG) Negative Ions ntroduction to accelerators : Components 3 /54

4 Electron sources Vacuum Electrons emitted by a cathode An anode with or without a hole Acceleration potential Focusing structure ntroduction to accelerators : Components 4 /54

5 Thermionic emission Metals heated to temperatures close to incandescence emit a current of electrons out of the surface( hot cathode ) ntroduction to accelerators : Components 5 /54

6 Photoemission The electrons of a metal can receive higher energy than vacuum energy absorbing photons ( photocathode and laser) QE=Quantum efficiency=n electrons/n photons Metals QE=0.01%! Semiconductor QE ~=10-30% The accelerating structure is a RF cavity Planck constant h=6.6e-34 J x s = 4.1E-15eV x s ntroduction to accelerators : Components 6 /54

7 Quantum efficiency 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. ntroduction to accelerators : Components 7 /54

8 Ions source Gas inlet Ionizing power input Plasma Plasma production region Magnetic confinement Ions extractions ntroduction to accelerators : Components 8 /54

9 The 4th state of matter: the plasma It is a rarefied and ionized gas electrically neutral (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 ntroduction to accelerators : Components 9 /54

10 Charged particle motion in a plasma 1 ρ girazione = qb mv 1. Cyclotron motion along B. Radius of tens of micron for electrons and few millimetre for ions. 2 3 v drift = E B 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). ntroduction to accelerators : Components 10/54

11 ECR (Electron Cyclotron f girazione (elettroni) = Resonace) ion source v 2πρ = 1 eb 2π m e f girazione (elettroni) = 28B(T) = f ECR (GHz) condizione di risonananza ECR ECR resonance condition 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. ntroduction to accelerators : Components 11/54

12 real ECR ntroduction to accelerators : Components 12/54

13 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. ntroduction to accelerators : Components 13/54

14 Real PIG real ntroduction to accelerators : Components 14/54

15 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 B ntroduction to accelerators : Components 15/54

16 Production and extraction of negative ions Technique 1: caesium coated cathode (surface phenomena) Technique 2: gas+caesium mixture(bulk phenomena) 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. ntroduction to accelerators : Components 16/54

17 Vacuum systems ntroduction to accelerators : Components 17/54

18 Pressure units Gas law P = nkt n[molecules/m3]=molecular density! Boltzman's constant k=1.38e-23 J/K = 8.6E-5eV/K The vacuum is a force: Pressure x Surface ntroduction to accelerators : Components 18/54

19 Mass flow and Throughput! 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 ntroduction to accelerators : Components 19/54

20 C[litres/s]=Conductance Q=Throughput [litre x mbar / s] P1-P2=pressure drop[mbar] C(hole area A )=11.6A[cm 2 ] C(tube of diameter D and length L ) =12.1D 3 /L[cm 2 ] Conductances in parallel increase Conductances in series decrease It is better short and large tubes ntroduction to accelerators : Components 20/54

21 Effective pumping speed The pumping speed is limited by the conductance of the connecting pipe: Pumping speed of a pump 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. ntroduction to accelerators : Components 21/54

22 Pumping time In a vacuum tight volume V at pressure p without gas leak the pumping time is determined by the Q = S x p Vdp/dt = S x p dp/dt = S x p / V dp/dt/p=s/v where S is the effective pumping speed The pressure follow the exponential decay law with time constant = S/V ntroduction to accelerators : Components 22/54

23 Viscous and molecular flow Molecular mean free path in air λ aria [cm] ~ P[mbar] Viscous flux: dominated by intra-molecule collisions 1 < P < 10 3 mbar < λ aria < cm 10 3 < P < 1 mbar < λ aria < 6.7 cm Molecular flux: dominated by collisions with the wall P < 10 3 mbar 6.7 cm < λ aria 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 ) ntroduction to accelerators : Components 23/54

24 Vacuum classification Medium vacuum: 10-3 <P<1mbar viscous flux High vacuum: 10-7 <P<10-3 mbar molecular flux Ultra high vaccum: <P<10-7 mbar molecular flux ntroduction to accelerators : Components 24/54

25 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 ) P finale = Q outgas sin g S eff Use ONLY metals NEVER plastics: Qplastics=5000Qmetals ntroduction to accelerators : Components 25/54

26 Metals vs Plastics P finale = Q outgas sin g S eff = q outgas sin g A S eff ntroduction to accelerators : Components 26/54

27 Vacuum measurements 1: 1E-4mbar<P<1 mbar 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. ntroduction to accelerators : Components 27/54

28 Vacuum measurements 2: 1E-10mbar<P<1E-5 mbar 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. ntroduction to accelerators : Components 28/54

29 Vacuum measurements 3: 1E-12mbar<P<1E-5 mbar BAYARD-ALPERT GAUGE Measure the current of the electrons emitted in vacuum from a hot filament and rotating in a magnetic field ntroduction to accelerators : Components 29/54

30 Pumps to create the vacuum 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. ntroduction to accelerators : Components 30/54

31 Pumps to create the vacuum 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. ntroduction to accelerators : Components 31/54

32 Pumps to keep the vacuum! Ion sputtering pump is used to maintain the vacuum and can work from 1E- 5 mbar up to 1E-11mbar. S = 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. ntroduction to accelerators : Components 32/54

33 Vacuum components Below Cooper pipe Flange Collar,gasket, L-pipe Section valves ntroduction to accelerators : Components 33/54

34 Normal conductive magnets ntroduction to accelerators : Components 34/54

35 Magnet components ntroduction to accelerators : Components 35/54

36 B traferro = µ 0NI h χ ferro ~ 10 3 Normal conductive = B NI (1+ χ ferro ) 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) Microscopic currents (oriented magnetic domain) Macroscopic currents (Coil) ntroduction to accelerators : Components 36/54

37 Coil 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. 4 Layers 2 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 ntroduction to accelerators : Components 37/54

38 The limit of the normal conductive magnets The iron saturates at T = T 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. ntroduction to accelerators : Components 38/54

39 Comparison normal and super conductive magnets 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) ntroduction to accelerators : Components 39/54

40 Radio-Frequency accelerating structure ntroduction to accelerators : Components 40/54

41 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 It is necessary to abandon drift tubes with uniform fields for accelerating structures with distributed fields ntroduction to accelerators : Components 41/54

42 Electromagnetic spectrum ntroduction to accelerators : Components 42/54

43 Dispersion relation in a pipe Cylindrical pipe Vacuum Waveguide ntroduction to accelerators : Components 43/54

44 Dispersion relation in a RF cavity Waveguide with obstacles RF cavity ntroduction to accelerators : Components 44/54

45 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) ntroduction to accelerators : Components 45/54

46 Disk-loaded guide waves with traveling waves (TW) 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 Guide wave IRIS-loaded: f=2.856 GHz (S band) 86 acceleration cells Coupling input/output Accelerating file 30 MV/m ntroduction to accelerators : Components 46/54

47 RF resonator with standing wave (SW) Traveling wave Standing wave V viaggiante (x, t) = V 0 sin(2πft 2π λ x) V stazionaria (x, t) = V 0 sin(2πft)sin( 2π λ x) The boundary conditions decide if TW o SW 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. Reentrant Nose-cone Disk-loaded Coaxial ntroduction to accelerators : Components 47/54

48 Figure of merit of a RF cavity E=Accelerating field =V/L (Inside the Cavity) Q=Total Energy/ Dissipated Energy =U/P=f 0 /Δf 0 Dissipated energy P due to surface resistivity of the RF conductor wall at high frequency (skin effect) = V x I = V 2 /R = I 2 R (In Cavity Wall). 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 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) ntroduction to accelerators : Components 48/54

49 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 R C Sending at the input a fraction of the output signal triggers a positive feedback loop with frequency f RF =1/(RC). Feed-back: Positive=self sustained oscillation Negative=stable gain amplifier ntroduction to accelerators : Components 49/54

50 RF solid state amplifiers Many amplifiers in parallel: f=325 MHz P=190 kw 4.7x4.7x2.3m3 ntroduction to accelerators : Components 50/54

51 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. Tetrodo 50<f<1000MHz, 200kW/tube 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 ntroduction to accelerators : Components 51/54

52 Klystron Speed modulation Position modulation (bunches) ntroduction to accelerators : Components 52/54

53 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 ntroduction to accelerators : Components 53/54

54 Real Klystrons ntroduction to accelerators : Components 54/54