Problem Solving from last week 2020 12 01

Problem Solving from last week 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 1

BEAM DIAGNOSTICS I (MS 3) 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 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 2

BEAM DIAGNOSTICS (MS 4) Introduction Beam Charge / Intensity Beam Position Homework Diagnostics I Introduction Normalized Beam Emittance Measurement of Emittance Homework Diagnostics II 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 3

CONTENT (Diagnostics I) • Introduction • Current measurements – Faraday Cup – Fast current transformer – DC current transformer • Position measurements - BPMs - OTR screens - Wirescanners • Homework 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 4

Introduction q Accelerator performance depends critically on the ability to carefully measure and control the properties of the accelerated particle beams q In fact, it is not uncommon, that beam diagnostics are modified or added after an accelerator has been commissioned q This reflects in part the increasingly difficult demands for high beam currents, smaller beam emittances, and the tighter tolerances place on these parameters (e. g. position stability) in modern accelerators q A good understanding of diagnostics (in present and future accelerators) is therefore essential for achieving the required performance q An accelerator is as good as its diagnostics ! 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 5

Diagnostic device and quantity measured 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 6

Diagnostics at FLASH at DESY 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 7

CONTENTS ü Generalities about beam diagnostic devices • Current measurements Ø Faraday Cup Ø Fast current transformer Ø DC current transformer • Position measurements Ø BPMs Ø OTR screens Ø Wirescanners • Homework 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 8

Faraday Cup • First method for beam current measurements • Act as a collector for electrons in a vacuum Ø Electrons simply hit the cup and the accumulated charge is converted into current • Electrode: 1 mm stainless steel • Only low energy particles (down to 1 p. A) can be measured • From 10 -100 W , forced air cooling is used • For higher beam intensities H 2 O cooling is needed • Design factors: beam energy, beam power, high frequency response (up to 2 GHz available in industry), secondary electron emission 9 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 9

Faraday Cup • Creation of secondary e- of low energy (below 20 e. V) Electrons liberated from the collector surface escape into the surroundings and thereby contributing uncontrollably to the current flowing thought the meter • In order to keep secondary electrons within the cup a repelling voltage is applied to the electrode • With increasing repelling voltage the electrons do not escape the Faraday Cup any more and the current measured stays stable • At 40 V and above no decrease in the Cup current is observed any more 2020 -12 -01 Current μA • Since the electrons have energies of less than 20 e. V some 100 V repelling voltage is sufficient Repealing voltage V Gergana Angelova Hamberg- Beam Diagnostics 1

Wall current monitor • Device for bunch signal observation (number and amplitude of bunches) • The beam current in the beam pipe is accompanied by an equal “wall current” on the inner surface of the beam pipe, flowing in opposite direction. • To avoid perturbation, resistors are connected at several points symmetrically around the circumference of the beam pipe cross • The voltage developed across the resistor due to the flow of wall current is taken out by a coaxial cable for further processing and observation. • The ceramic ring in the beam pipe diverts the current to flow through the resistors connected across the ceramic ring. • Conducting shield is be placed around the wall current monitor so the electromagnetic radiation from the beam will not leak out. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 1

Current transformers • Widely used device which allows one to determine the electric current that a beam constitutes • Current transformers are available both for single pass machines (pulse transformer) and for circular machine (DCCT) • The CT is based on the transformer principle, • the beam is the primary circuit and • the windings on the magnetic core the secondary Relativistic bunch with length σs containing N particles of elementary charge e, travelling past an observer at time t 0, produces a beam current pulse A magnetic field is induced around the beam which varies very rapidly with time. If we surround the beam with a ring shaped iron core with radius Rcore then the produced field inside the coil is There are n winding around the iron core with cross section area A. The induced voltage is 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 12

DCCT current transformer • Device for measuring stored beam current circulating in the storage ring. • In a storage ring, however, the beam may circulate for hours (999 h or 42 days is the longest a beam has circulated uninterrupted in the Antiproton Accumulator at CERN). No integrator can cope with that, DCCT current transformer are used • Modulator sends a current at several 100 Hz through the excitation coils, which are excited in opposite directions. • The pick-up coils mounted on the rings are connected in series, their sum signal, Vs, will thus be zero. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 11

DCCT current transformer • When a beam current Ib passes through the rings, it introduces a bias in the excitation of the cores, Vs will no longer be zero and the second harmonic of the modulator frequency will appear in it • Which the demodulator converts into a dc voltage. • This controls a power supply, sending a current Ic through a compensating winding on the two rings • Equilibrium is reached when the compensating current Ic cancels the beam current Ib. • The final measurement is that of Ic. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 14

Ideal transformer • An ideal transformer is a metal ring with windings connected to it on two sides. • The number of turns in the windings is Np on the primary side of the transformer and Ns on the secondary side. • An alternating voltage (Vp) applied to the primary side produces an alternating voltage (Vs) in a secondary circuit. • The relationship between the voltages and the number of turns in the windings is given by Vs/Vp = Ns/Np. • A real transformer is not exactly 100% efficient, however, its efficiency, ε , is given by ε = Vs. Is/Vp. Ip • If Rs is the resistance in the secondary circuit, then an effective resistance (Reff) can be defined as Reff = ε (Np/Ns)2 Rs. Based on these relationships, Vp=Ip. Reff. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 15

CONTENTS ü Generalities about beam diagnostic devices ü Current measurements Ø Faraday Cup Ø Fast current transformer Ø DC current transformer • Position measurements Ø BPMs Ø OTR screens Ø Wirescanners • Homework 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 16

Beam position monitors (BPM) (BP • Used in high intensity machines with short bunches. Synchrotron light sources • Picks up the wall currents at several positions • The BPM consists of four metal strips on the inside of the accelerator structure, connected to wires that extend outside the structure and are grounded. • The entire apparatus is electrically isolated from the accelerator structure itself. • The information from the four strips can be used to measure the number of electrons in the bunch and determine the position of the bunch • If more signal on the left (A+D) compared to (B+C) then the beam is further to the left. A D B C 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 17

Stripline BPM • Stripline has 4 strips running along the axes of beam, parallel to the vacuum chamber wall • The length of the strip is usually longer than the characteristic bunch length and equal to quarter wavelength of fundamental RF. • The electromagnetic field of the beam induces signal on the strip line. • The amplitude of signal is a function of • its solid angle subtended on the beam and • distance of the conductor from the beam. • Two ends of the stripline are taken out of the chamber. These are called upstream and downstream ports respectively with reference to the beam direction. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 16

Cavity BPM • Consists of (usually) a cylindrically symmetric cavity, which is excited by an off-axis beam: • Off-axis beam excites transverse mode • Used for very short intense bunches http: //learn. hamamatsu. com/tutorials/streakcamera/

Shoebox BPM • Can measure horizontal and vertical position at once • Has 4 electrodes: a b c d 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 20

BPMs at FLASH in DESY, Hamburg Hambu • The signals from two opposite pickups of a BPM (left-right, or up-down) are sent to a BPM electronics board. • The signal is then sampled and digitized by an ADC and sent to the control system. • After applying the calibration coefficients, the data is provided to the operators or to other middle layer servers for further calculations. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 21

Optical Transition Radiation Screens • Optical Transition radiation occurs when a charged particle crosses the boundary between two media with different dielectric constants. • According to the characteristics of transition radiation, if the electron beam incidence to the boundary at 45° the transition radiation appears at 90° to the electron beam direction. • The wavelength range between 400 nm and 800 nm OTR can be used for transverse beam profile measurements. • Silicon screens + Ag or Al coating 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 22

Optical Transition Radiation Screens • Often beams are far from Gaussian especially in linacs • Camera must be protected from radiation requiring a complex optical lines • Filters are needed to avoid saturating the camera 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 23

OTR assembly for ORS at FLASH 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 24

Wirescanners • Wirescanners are commonly in use at storage rings and linacs. • A thin wire is moved transversely across the path of the beam. • Detects secondary emission current or high energy secondary particles • The electrons catch on the wire generate photons which both can be detected downstream the scanner. • Thus, a scan across the beam yields the intensity profile. • The resolution is limited by the smallest wire diameter and the accuracy of the wire translation. • Carbon fibers down to 4 μm diameter have been used, resulting in a rms-resolution of about 1 μm. • Positioning accuracy in the sub-micron regime is achieved using precise stepping motors in combination with optical encoders. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 25

Types of Wirescanners • Secondary emission • Good for low energy beams (no high energy secondary) • Small signal If the wire becomes too hot it can start to emit thermionic electrons spoiling the measurement • High energy secondary • No problem with wire heating • Strong signal • Detection may be non homogeneous leading to distorted profiles • Fast scanners • Present limit is around 20 m/s • Usually rotational mechanism • Acquire profile snapshots during acceleration • Reduce wire heating (short scan time) • Slow scanners • High wire position accuracy • Possibly thinner wires (low accelerations) • More reliable mechanisms • Long(er) measurement time • Tighter intensity limits 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 26

Conclussions q Good Diagnostics define well operating accelerator q Preferably remotely controlled diagnostics with no interruption of the accelerator operation q Before designing the diagnostics for an accelerator one should get to know well the accelerator itself and its mode of operation for example q When one design a new accelerator the diagnostics should be operatable for the first day of the beam operation q Good documentation 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 27

CONTENTS • Generalities about beam diagnostic devices • Current measurements Ø Faraday Cup Ø Fast current transformer Ø DC current transformer ü Position measurements ü BPMs ü OTR screens ü Wirescanners • Homework 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 28

Homework (written report) 1. What types of beam loss monitors are used in particle accelerators. Pick two and compare them. 2. Imagine you are writing an introductory section of the Design Report. i. The physics motivation and uniqueness of the European XFEL to carry out high-precision studies in biology, chemistry, material sciences etc. ii. The types of beam diagnostics planned to be used. iii. The beam requirements for the electron beam and the photon beam. Deadline 25 April. max 4 A 4 pages. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 29

Electro-optical sampling (EOS) • The method is based on the detection of the co-traveling electric field of a relativistic electron bunch when it passes close to a non-linear optical crystal. • The electric field induces double reflection which is probed by a synchronized femtosecond Ti: Sapphire laser pulse. • The initial linear polarized laser pulse becomes elliptic polarized, whose ellipticity is measured using a combination of a quarter wave plate, a Wollaston prism and a balanced detector. • The achievable time resolution of about 100 to 200 fs is limited by the excitation of phonon resonances in the nonlinear crystal which distort and attenuate the electric field pulse. • The method can be extended to a single-shot measurement. The temporal distribution of the electron bunch is obtained using a diffraction grating and a CCD camera. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 30

Electro-optical sampling at FELIX FEL in the Netherlands • The longitudinal electron bunch profile has been measured at the entrance of the undulator inside the vacuum chamber. • A 0. 5 mm thick Zn. Te crystal has been used as an electrooptic sensor. • The crystal was placed in a distance of 6 mm perpendicular to the propagation of the electron beam. • A 12 fs Ti: Sapphire laser was synchronized to the accelerator rf. • Electron pulses down to 1. 7 ps (FWHM) have been measured with a resolution of better than 800 fs. 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 31

Streak Camera • Device for a direct (single-shot) determination of the longitudinal bunch charge distribution. • The photoelectrons generated by the photocathode pass between the pair of sweep electrodes • The applied sweep voltage steers the electron away from the horizontal direction at different angles, depending upon their arrival time at the electrodes. • The amplified electrons reach the phosphor screen and form an image oriented in the vertical direction according to the arrival time at the sweep electrodes. • The earliest pulse is arranged in the uppermost position and the latest pulse is in the bottom most portion of the phosphor image. • The resulting streak image has space as the x-axis and time as the y-axis. • The present resolution limit of commercial available streak camera is 200 fs and shortest bunch length measured is 440 femtoseconds (FWHM). http: //learn. hamamatsu. com/tutorials/streakcamera/

LOLA 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 33

LOLA • Measurement of bunch length distributions on a time scale (fs time regime) • This is well beyond the reach of conventional streak camera. • Consists of • RF transverse deflecting cavity operating at the same frequency as the main linac • The cavity is used to sweep the beam transverse to the beam direction and operated at the zero-crossing phase of the field, where the time derivative is maximum, so that the bunch is given a strong correlation between longitudinal z-coordinate and transverse position. • Standard OTR screens then allow for measurements of the particle distribution in the longitudinal-horizontal plane. • The screen is off axes so LOLA measurements can be taken parasitically ORS (black line), LOLA (blue dots) 2020 -12 -01 Gergana Angelova Hamberg- Beam Diagnostics 34

Wirescanner

LOLA 2020 -12 -01 Gergana Angelova Hamberg. Beam Diagnostics 36