Accelerators and Detectors Course Gergana Angelova Hamberg Department

Accelerators and Detectors Course Gergana Angelova Hamberg Department of Physics and Astronomy, Uppsala University gergana. angelova@physics. uu. se tel: 018 471 7644 Reports should be sent electronically to Studentportalen 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 1

Problem Solving from last week 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 2

Production, Acceleration, Focusing q Production Ø Electron Sources Ø Plasma and Positive ion sources Ø Negative ion sources q Injection/Extraction q Acceleration Ø Linear Ø Circular q Focusing q Homework 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 3

Particle sources q How particles are first produced? q How to extract particles with the right properties? q What are the limitations of the sources? The quality of the source is very important. If the particles emitted by the source do not have the right properties, it will be very difficult and/or expensive to rectify it later. 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 4

Production of Electrons § Any plasma has a lot of electrons (e-) § Plasma generated e-, can be used as sources of e- but in many cases the quality and stability of the beam is far from what is needed § Many ion sources Ø include an e- source in their design to give an initial supply of ionizing e or Ø use a high quality e- beam. § To obtain good characteristics, Ø Ø the e-, need to be emitted from a well defined surface in a controlled manner. to know in advance for what the required beam will be used for do computer simulations prior to the construction § In general e- can be produced by: Ø Thermionic emission Ø Photo emission Ø High field emission 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 5

Thermionic Emission § When a metal is heated some e- can populate high energy levels. § Above a certain threshold they e- can break their bound and be emitted § This is called thermionic emission. § To escape from the metal the electrons must reach an energy greater than the edge of the potential well. § The energy that must be gained above the Fermi energy is called the “work function” of the metal. § The work function is a property specific to a given metal and can be expresses as J = AT 2 e (-11160. W/T) J is the maximum current density, A is a constant with a theoretical value of 120 A/cm 2. K, T is the temperature of the metal [K], W is the work function of the metal [e. V] § Example values: Ø Ø 2020 -11 -22 Fe: 4. 7 e. V Cu: ~5 e. V Al: ~4. 1 e. V Cs: ~2 e. V Gergana Angelova Hamberg- Production, Acceleration, Focusing 6

Thermionic Emission § Two parameters are important when considering a thermionic cathode material: § W (work function) § T (operation temperature) As low as possible Preferably high J = AT 2 e (-11160. W/T) Material A W Temp (°K) J (A/cm 2) Tungsten 60 4. 54 2500 0. 3 Thoriated W 3 2. 63 1900 1. 16 Mixed oxides 0. 01 1. 1200 1. Cesium 162 1. 81 - - Tantalum 60 3. 38 2500 2. 38 0. 003 0. 72 1000 0. 35 Cs/O/W - Mixed oxide cathode is commonly found in small radio type valves. Cs/W/O, although not good for thermal emitters, is usually found in photo-tubes heavy metal cathodes are used in high power electron tube devices. § Thermionic emitters are used in Ø electron tubes Ø special electron guns, as for example in klystrons, welding, industrial materials processing Ø accelerators for lepton production. 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 7

Pierce gun (Thermionic DC Gun) • Simplest gun design • Main features: • 2020 -11 -22 - Thermionic cathode which is heated to create a stream of electrons via thermionic emission - Electrodes generating an electric field which focus the beam - One or more anode electrodes which accelerate and further focus the electrons. - A large voltage between the cathode and anode accelerates the electrons - A repulsive ring placed between them focuses the electrons onto a small spot on the anode at the expense of a lower extraction field strength on the cathode surface. - Often at this spot is a hole so that the electrons that pass through the anode form a collimated beam and finally reach a second anode called a collector. - Emission of the beam is controlled by a HV grid. Grid control limits pulse length. Typically >1 ns. Spring 8 SCSS thermionic gun. Gergana Angelova Hamberg- Production, Acceleration, Focusing 8

Photo Emission • First observed by Heinrich Hertz in 1887, the phenomenon is also known as the Hertz effect • Photo-electric emission takes place in 3 steps: Ø Absorption of a photon by an electron inside the metal. The energy transferred is proportional to the photon energy. Ø Transport of the photon to the physical surface of the metal. The electron may loose energy by scattering during this process. Ø Electron emission (if the remaining energy is above the work function; If the photon has an energy at least equal to the work function, then electrons will be emitted, i. e. λ < hc/e. W λ - wavelength of the incident light, c - the velocity of light and h - Plank's constant. W - work function of the metal [e. V] 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 9

Quantum Efficiency (QE) 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 10

QUIZZ 1. Which of these materials would give the highest thermionic emission current (at the same temperature)? (a) Iron (Fe); W=4. 7 e. V (b) Gadolinium (Gd); W=2. 90 e. V (c) Cobalt (Co); W=5 e. V 2. Which laser would give the best Quantum efficiency on a Copperbased photo-cathode (W=5 e. V) (a) A 5 GW CO 2 laser (wavelength=10 micrometers) (b) A 10 k. W frequency doubled Nd: YAG laser (wavelength=532 nm) (c) A 3 MW frequency quadrupled Ti-Sapphire laser (wavelength=200 nm) 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 11

High Field Emission § High field emission is the discharge of electrons from the surface of a material subjected to a strong electric field. § In the absence of a strong electric field, an electron must acquire a certain minimum energy, called the work function, to escape through the surface of a given material, which acts as a barrier to electron passage. § If the material is placed in an electric circuit that renders it strongly negative with respect to a nearby positive electrode (i. e. , when it is subjected to a strong electric field), the work function is so lowered that some electrons will have sufficient energy to leak through the surface barrier. § The resulting current of electrons through the surface of a material under the influence of a strong electric field is called field emission. § This effect is utilized in the field-emission electron microscope, which in some instances achieves resolution of atomic dimensions. § The major disadvantage of this type of source is that an excessive current density can destroy the points either by erosion or self heating. 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 12

Production, Acceleration, Focusing q Production ü Electron Sources Ø Positive ion sources Ø Negative ion sources q Injection/Extraction q Acceleration Ø Linear Ø Cicular q Focusing q Homework 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 13

Ion Sources § Very broad field with many applications: Ø Ø Ø § Beams of nanoamperes to hundreds of amperes § Very thin to very broad beams (μm 2 to m 2) § 2020 -11 -22 Material science and technology (e. g. ion implantation) Food sterilization Medical applications Military applications Accelerators. . . Types of Ion Sources Surface ionization Plasma beam Field ionization Duoplasmatron Sputter Hollow cathode Laser Pigatrons Electron beam ionization Multifilament Arc discharge Cyclotron resonance Multipole confinement Surface plasma Pennings Magnetrons Charge exchange RF plasma Gergana Angelova Hamberg- Production, Acceleration, Focusing 14

Plasma Ion Sources • Device that produces a stream of ions, especially for use in particle accelerators or ion implantation equipment. • An electric discharge creates a plasma in which positively and negatively charged ions are present (as well as neutrals). § If such plasma experiences an intense electric field ions will separate in opposite directions. • 2020 -11 -22 This is a rather crude and inefficient (but very simple) way of producing any sort of ions. Gergana Angelova Hamberg- Production, Acceleration, Focusing 15

Electron Beam Ion Sources (EBIS) 2020 -11 -22 • Fast, dense, electron beam interacts with cold ions trapped in an electrostatic well. • Ions are confined radially by the potential well in the electron beam and axially by electrostatic mirrors. • Ions accumulated in the trap can be expelled by lowering the potential of one end of the trap. • As the interaction time between hot electrons and ions depends on the electron energy and the source length, for highly charged ions this time is necessarily short. • Thus high density, and hence high current density, electron beams are required. In practice of the order of 1000 A/cm 2 is needed. Gergana Angelova Hamberg- Production, Acceleration, Focusing 16

Penning Ion Source 2020 -11 -22 • The Penning-source establishes a discharge to generate plasma and ions. • Usually two cathodes facing each other are located in a magnetic dipole or solenoid field. • The material which must be ionized is located in the anode and will be sputtered into the ionisation region. Due to the extraction slit, the resolution of the dipole spectrometer is increased. • The electrons emitted gyrate around the magnetic field lines and diffuse slowly in anode direction. • Due to the magnetic field the effective travel length of the electron in the plasma chamber is increased Gergana Angelova Hamberg- Production, Acceleration, Focusing 17

Electron Cyclotron Resonance IS (ECRIS) 2020 -11 -22 • Vapor held in a cavity with high magnetic field • Microwaves with frequency that coincides with eˉ cyclotron frequency in the field heat the electrons (and only electrons). • No electrodes, no arc discharge – very reliable, high currents • 14 GHz, 0. 5 T @ GSI, Dubna, LBNL, CERN Gergana Angelova Hamberg- Production, Acceleration, Focusing 18

Sputter Ion Source 2020 -11 -22 • Cesium vapor, hot anode, cooled cathode • Some of the vapor gets condensed on the cathode, some gets ionized on the anode and accelerated towards the cathode where it sputters atoms from the cathode • Produces negative ions of all elements that form stable negative ions Gergana Angelova Hamberg- Production, Acceleration, Focusing 19

Production, Acceleration, Focusing q Production ü Electron Sources ü Positive ion sources Ø Negative ion sources q Injection/Extraction q Acceleration Ø Linear Ø Circular q Focusing q Homework 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 20

Negative Ion Sources • Ions with a negative charge have gained popularity in the accelerator field. • They were originally used to double the effective energy of electrostatic machines by Ø stripping the excess electron Ø then the natural one at the high potential electrode Ø re-accelerating the resultant positive ion. • Although often used in accelerators for charge exchange injection from linear to circular machines, negative ions are also used for • • The physical processes in negative ion sources are still poorly understood but three types of source are generally recognized; • • • 2020 -11 -22 fusion plasma heating directed energy weapon research semiconductor processing surface volume charge exchange. • It should not be forgotten that electrons and negative ions have the same charge and thus both will be extracted from the source • Elimination of this unwanted electron component is one of the major technological problems in negative ion source design. Gergana Angelova Hamberg- Production, Acceleration, Focusing 21

Negative Ion Sources § Historically, negative hydrogen ion sources were modifications of existing proton sources such as duoplasmatrons with the ions extracted from the anode plasma off axis. Insertion of a floating electrode into the channel of the plasma chamber improved the yield of negative ions but the addition of caesium to the discharge dramatically increased the ion current (and electrons). • The increase in source efficiency from the addition of caesium accelerated the development of higher intensity devices based on the cold cathode magnetron geometry • There is still no clear evidence as to what is happening inside a caeseated discharge. All or some of the following processes may be involved: 1) Dissociation of plasma produced caesium hydride Cs + H -> Cs. H -> Cs+ + H 2) Sputtering of lightly bound ions from the surface 3) Attachment of an electron after scattering from the surface • However, it is known that the surface coverage is important (about 0. 7 monolayer) and that the energy of the incident ion must be low (> a few hundred e. V). • However, these sources can work in the steady state mode and the Fig shows such a source Cross section of a steady state magnetron negative ion source 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 22

Negative ions in plasmas § Negative ions are present in plasmas when using electronegative gases: H 2, O 2, Cl 2, C 2 F 6… § Negative ion in are usually produced in plasma volume by electron dissociative attachment on molecules: Ø e + H 2(v) -> H- + H Ø e + O 2(v) -> O- + O … Negative ions can also be created on surfaces: Ø Hx+ + surface -> H- Ø H + surface -> H- 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 23

Particle Production Summary • We have discussed how to produce the particles used in accelerators. • The requirement of the next generation of electron accelerators impose strong constraint on the quality of the beam produced by electron sources. • Ion machine are limited by the beam intensity that can reliably be extracted form the source. • For both electrons and ions the quality of the particle source has a strong impact on the overall accelerator performance. 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 24

Production, Acceleration, Focusing ü Production ü Electron Sources ü Positive ion sources ü Negative ion sources q Injection/Extraction q Acceleration Ø Linear Ø Circular q Focusing q Homework 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 25

Single Injection / Extraction q Fundamental problem of injection: Ø Taking particle beam with well defined energy from pre-accelerator and inject it into circular accelerator WITHOUT significant lost of particles Ø The injected beam would cross the orbit of the already injected beam and hit the wall. WHAT DO WE DO? ? ? Ø Install bending magnet immediately after the injection point. Ø The properties of this magnet are carefully chosen so that the injected beam will be deflected as precisely as possible to the designed orbit Ø We CAN’T use static magnet because once the beam had made one revolution it will encounter this field again and will be deflected through the same angle away into the wall ü Instead we USE rapidly pulsed magnet called kicker magnet Ø The incoming beam is deflected only once, as it enters the circular accelerator Ø By the end of the 1 st revolution the field of the kicker magnet has died away, and the beam is not deflected second time 2020 -11 -22 26

Multi-turn Injection / Extraction q Previous slide was how to inject particles in storage ring ONLY ONCE. What do we do if we need continuous injection in order to reach the desired high currents ? ? ? q The system must inflect the beam into the ring with an existing beam circulating without producing excessive disturbance or loss to the circulating beam. Fundamental rule of injection It is not possible to inject particles into an already occupied volume of phase space without losing the particles already present q The circumference of the ring L is divided in to n equal sections with length ΔL, ΔL = τ0ν where τ0 is the time taken by a particle with speed v to travle a distance q We have n empty sections to fill with particles Ø The beam is slightly shorter than ΔL so it can fit perfectly in the space Ø In the first injection we fill only one section n of all possible Ø In the next injection the time is shifted by τ0 so that the neighboring volume can be filled and so on n times untill all the volume is filled 2020 -11 -22 27

Multi-turn Injection q Fundamental problem of multi-turn injection: If we use kicker magnet, the magnet should produce pulse no longer than τ0 and should NOT desturb the particles sirculating in the two neighboring phase volumes. What do we do? ? ? q Use kicker magnets and add a magnet called septum magnet. q In the 1 st injection the strength of the first kicker magnet is chosent so that the beam is bent exactly onto the orbit, which it then follows q In the 2 nd injection the strength of the two kickers is reduced so the resulting orbit bump brings the beam as close as possible to the septum without loosing particles q With the beam in this position, 2 nd beam can now be injectedthrough the septum, very close to the first beam 2020 -11 -22 28

Production, Acceleration, Focusing ü Production ü Electron Sources ü Positive ion sources ü Negative ion sources ü Injection q Acceleration Ø Linear Ø Circular q Focusing q Homework 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 29

Acceleration- DC voltage § To accelerate particles, some force has to be applied- the electromagnetic force => only charged particles like e- or p+ can be accelerated. § When entire atoms need to be accelerated, first they have to lose part of their electrons (they have to be ionized); thus they become positively charged and can be accelerated § In earlier accelerators, strong electric fields were used to accelerate the particles (similar to the electrons in cathode-ray-tube). § The simplest acceleration method: DC voltage § As there is a limit to the strength of electric fields because of spontaneous discharge, this simple technology soon reached its limits. Ø breakdown voltage at ~10 MV 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 30

Acceleration techniques: RF field § More modern construction uses a cascade of electric fields, which yields much higher acceleration energies. § The modern solution is cavity resonators. § They produce a special kind of electromagnetic wave. Particles can "surf" on this wave and are accelerated in this way. § The particles gain energy by surfing on the electric fields of well-timed radio oscillations (in a cavity like a microwave oven) § The challenge is of course (just as for surfers on the ocean's waves) to be at just the right position in the wave § Otherwise, there is no acceleration but deceleration. 2020 -11 -22 31

RF Cavity Modes q Cylindrical waveguide with TM 01 wave Ø Electric field is purely longitudinal Ø Magnetic field lines are only transverse Ø The electric field runs parallel to the cylinder axis and can accelerate charged particles as they travel through the waveguide q Only well defined waveelngths canbe present in the cavity, called resonant wavelengths q Near the resonant wavelength the cavity acts as a oscillator q For the accelerating mode (TM 010), the resonant wavelength is: x 1 = 2. 40483 (first root of the 0 th order Bessel function) λr is the resonant wavelength q mode number (for TM 01, q = 0) Electric Fields D is the diameter of the cavity 2020 -11 -22 Magnetic Fields 32

Method of Acceleration: Linear • Simplest example is a vacuum chamber with one or more DC accelerating structures with the E-field aligned in the direction of motion. – Limited to a few Me. V • To achieve energies higher than the highest voltage in the system, the E-fields are alternating at RF cavities. SLAC linear accelerator – Avoids expensive magnets – No loss of energy from synchrotron radiation (q. v. ) – But requires many structures, limited energy gain/metre – Large energy increase requires a long accelerator 2020 -11 -22 SNS Linac, Oak Ridge Gergana Angelova Hamberg- Production, Acceleration, Focusing 33

Method of Acceleration: Linear Structure 1: q Travelling wave structure: particles keep in phase with the accelerating waveform. q Phase velocity in the waveguide is greater than c and needs to be reduced to the particle velocity with a series of irises inside the tube whose polarity changes with time. q In order to match the phase of the particles with the polarity of the irises, the distance between the irises increases farther down the structure where the particle is moving faster. But note that electrons at 3 Me. V are already at 0. 99 c. Structure 2: 2020 -11 -22 q A series of drift tubes alternately connected to high frequency oscillator. q Particles accelerated in gaps, drift inside tubes. q For constant frequency generator, drift tubes increase in length as velocity increases. q Beam has pulsed structure. Gergana Angelova Hamberg- Production, Acceleration, Focusing 34

Method of Acceleration: Circular § Use magnetic fields to force particles to pass through accelerating fields at regular intervals § Principle of frequency modulation but in addition variation in time of B-field to match increase in energy and keep revolution radius constant. § Magnetic field produced by several bending magnets (dipoles), increases linearly with momentum. For q=e and high energies: § Practical limitations for magnetic fields => high energies only at large radius e. g. LHC • E = 8 Te. V, B = 10 T, r = 2. 7 km Cyclotron – Vary B to compensate and keep f constant. – Leads to construction difficulties. • Synchrotron – Modulate frequency f of accelerating structure instead. – In this case, oscillations are stable 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 35 35

Summary of Circular Machines Machine RF frequency f Magnetic Field B Orbit Radius Comment Cyclotron constant increases with energy Particles out of synch with RF; low energy beam or heavy ions Isochronous Cyclotron constant varies increases with energy Particles in synch, but difficult to create stable orbits Synchro-cyclotron varies constant increases with energy Stable oscillations Synchrotron varies constant Flexible machine, high energies possible FFAG varies constant in time, increases with energy Increasingly attraction option for 21 st century designs varies with radius 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 36

Production, Acceleration, Focusing ü Production ü Electron Sources ü Positive ion sources ü Negative ion sources ü Injection ü Acceleration ü Linear ü Circular q Focusing q Homework 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 37

Weak Focusing q Let us consider that we have a particle moving on a trajectory close to the ideal and a magnet with bending angle 180 degree. q The particle has a displacment from the ideal orbit +x when it enters the magnet (point A) q The particle trajectory approaches the ideal orbit and crosses it at (point B) q It then runs inside the orbit and has an eventual displacment –x when it exits the magnet (point C) q If we plot the trajectory as a function of the distance traveled along the ideal orbit, we see immediately that the particle is always bent in towards the orbit. q In other words IT IS FOCUSED q For that reason the focusing effect caused by dipole magnets is called weak focusing q Since the dipole magnets dont bend the beam in the vertical plane, the do not cause any focusing in that plane. B 0 is the magnetic field in the gap B 0 = µ 0 n. I/h 1/R = e. B 0/p = eµ 0 n. I/ph 2020 -11 -22 1/R is the dipole magnet bending strength h is the gap distance p is the momentum of the particle µ 0 is relative permability n is number of turns per coil I is the current

Strong Focusing • Quadrupoles focus horizontally, defocus vertically or vice versa. Forces are linearly proportional to displacement from axis. • But two successive elements, one focusing the other defocusing, can focus in both planes. • A succession of opposed elements (focusing , defocusing quads) enable particles to follow stable trajectories • Technological limits on magnets are high. 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 39

Weak and Strong Focusing DIPOLS § All accelerators built before 1952 used so called weak focusing magnets. § The slight focusing effect caused by dipole magnets is called weak focusing § Dipoles have such weak focusing only in vertical plane because only in that plane they bend the beam QUADRUPOLS 2020 -11 -22 § The concept of strong focusing can be understood from the fact that a pair of optical focusing and defocusing lenses can be arranged in such a way that the total system is focusing. § Magnetic quadrupoles act for charged particles as optical lenses. § Certain arrangements of quadrupoles with alternating magnetic gradients (AG) can be used to achieve a stable particle motion. § The focusing power of this system is much stronger than that of weakly focusing magnets. § One was able to built accelerators with energies 3 orders of magnitude higher than could have been envisaged with the old technology. Gergana Angelova Hamberg- Production, Acceleration, Focusing

Outline of the Accelerator part ü Production ü Electron Sources ü Positive ion sources ü Negative ion sources ü Injection ü Acceleration ü Linear ü Circular ü Focusing q Homework 2020 -11 -22 1. Please take a copy of the exercises. 2. Solve the problems and send a written report via mail. 3. Next week before the lecture we will discuss the solutions. 4. Deadline for submission of the report 18 April Gergana Angelova Hamberg- Production, Acceleration, Focusing 41

Magnet requirements § Magnets required for injection and extraction systems. i) Kicker magnets: Ø pulsed waveform; Ø rapid rise or fall times (usually << 1 ms); Ø flat-top for uniform beam deflection. ii) Septum magnets: Ø Ø Ø pulsed or d. c. waveform; spatial separation into two regions; one region of high field (for injection deflection); one region of very low (ideally 0) field for existing beam; septum to be as thin as possible to limit beam loss. Septum magnet schematic

Multi-turn injection solutions § Beam can be injected by phase-space manipulation: a) Inject into an unoccupied outer region of phase space with non-integer tune which ensures many turns before the injected beam re-occupies the same region (electrons and protons): septum 0 field x deflect. field turn 1 – first injection b) x’ turn 3 turn 2 Inject into outer region of phase space - damping coalesces beam into the central region before re-injecting (leptons only): dynamic aperture stored beam a) 2020 -11 -22 turn 4 – last injection injected beam next injection after 1 damping time Inject negative ions through a bending magnet and then ‘strip’ to produce a p after injection (H - to p only) 43

RF Gun • The high voltage of a DC gun can be replaced by a RF cavity. • This can provide much higher accelerating gradients and hence limit the space charge. • RF guns are often coupled with a photo-cathode. • RF gun can generate shorter bunches (using short laser pulses). • Principle of a RF Photo-gun 2020 -11 -22 Gergana Angelova Hamberg- Production, Acceleration, Focusing 44