Kinetic Transverse Profile Monitors

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Randy Thurman-Keup

Fermilab

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J. J. Thompson

Outline

• Beam Interaction Basics
• Wire Based
• Flying Wires
• Secondary Emission Monitors
• Gas Wire Chambers
• Ionization Profile Monitors
• Residual Gas
• Gas Jet
• Gas Fluorescence
• Auxiliary Beam Deflection
• Electron Beam
• Ion Beam

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R. Thurman-Keup --- USPAS Hampton, VA

2

January 30, 2024

Energy Loss By Charged Particles

• Bethe(-Bloch) equation for energy loss by ionization

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R. Thurman-Keup --- USPAS Hampton, VA

3

January 30, 2024

Energy loss by Bremsstrahlung

Maximum energy transfer

Effective�ionization �potential

Density effect

(Polarization shielding)

Logarithmic�growth w/�energy

(Distance where particle left with 1/e if its energy)

NIST!!

Multiple Scattering

• Particle of charge ze undergoes scattering many times such that the net effect is Gaussian via the central limit theorem
• rms of outgoing angle

R. Thurman-Keup --- USPAS Hampton, VA

4

January 30, 2024

• Increases the angular spread of the beam
• Particles lost at apertures
• Beam lifetime reduction in rings
• Emittance growth

Outline

• Beam Interaction Basics
• Wire Based
• Flying Wires
• Secondary Emission Monitors
• Gas Wire Chambers
• Ionization Profile Monitors
• Residual Gas
• Gas Jet
• Gas Fluorescence
• Auxiliary Beam Deflection
• Electron Beam
• Ion Beam

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R. Thurman-Keup --- USPAS Hampton, VA

5

January 30, 2024

Wire Scanners

• A physical wire(s) is inserted into the beam
• Wire is typically thin to minimize heating from beam energy absorption (dE/dx)
• Interaction with the beam is measured as a function of wire position within the beam
• Interaction is proportional to beam intensity at that location
• Wire may or may not be moving
• Measurement may be based on secondary emission or downstream detection of high energy secondary particles
• In all these cases we must consider the consequences of the beam colliding with a solid target

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R. Thurman-Keup --- USPAS Hampton, VA

6

January 30, 2024

Flying Wire Scanner

• A flying wire is a moving wire scanner used in rings and based on detection of high energy secondary particles
• The wire is usually mounted on a pair of rods or forks which rotate the wire through the beam at high angular �velocity to avoid heating of the wire
• Secondaries can be detected by �scintillator / PMT combination or �a newer radiation hardened �particle detector such as diamond
• Hadrons 🡪 Shower of π,n,p,K,…
• Electrons 🡪 Bremsstrahlung

R. Thurman-Keup --- USPAS Hampton, VA

7

January 30, 2024

Flying Wire Scanner

• New CERN SPS version
• Wire speed of 20 m/s
• Wire is typically <10μm�Carbon fiber
• In-vacuum motor
• ‘3-D printed’ forks
• Optical encoder

R. Thurman-Keup --- USPAS Hampton, VA

8

January 30, 2024

Beam

Flying Wire Scanner

• Tevatron example
• 7 μm carbon wires
• 6.6 m/s wire speed
• Proton beam scattering effects

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R. Thurman-Keup --- USPAS Hampton, VA

9

January 30, 2024

Vertical�Measurement

Horizontal�Measurement

Forks

Pretensioning�Device

Rotation Axes

Scattering

Component

Slow Wire Scanner

R. Thurman-Keup --- USPAS Hampton, VA

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January 30, 2024

Diagonal Wire

🡪 x-y correlation

Secondary Emission

• Beam interacts via dE/dx and some of the�ionized electrons escape the surface

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• Probability of escaping the surface depends only �minimally on the material

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R. Thurman-Keup --- USPAS Hampton, VA

11

January 30, 2024

Yield: Number of secondaries�per primary

Sternglass formula

1 MeV

1 MeV

δ-ray: ionized electron with enough�energy to cause further ionizations

Secondary Emission Monitors

• A SEM in a basic configuration is just a conductor�placed in the beam and instrumented to detect�the secondary emission
• This example also shows bias�voltage planes to prevent �re-absorption and increase yield�
• In practice SEMs usually have an array of either wires or thin foils and may have thin foil bias planes
• Wires can be Tungsten, Titanium, Carbon Fiber
• Typically 10’s of µm
• The goal is to maximize surface area (emitting area) and minimize volume (dE/dx)

R. Thurman-Keup --- USPAS Hampton, VA

12

January 30, 2024

Secondary Emission Monitors

• Example SEMs from FNAL

R. Thurman-Keup --- USPAS Hampton, VA

13

January 30, 2024

Secondary Emission Monitors

• Example SEMs from FNAL
• Avoid intercepting beam with support structure

R. Thurman-Keup --- USPAS Hampton, VA

14

January 30, 2024

Clearance for

beam

SEM Issues

• Scattering of the beam
• Increased losses downstream
• Heating of the wires or foils
• Melting of the wires
• Ti 1660°C
• Monofilament C 3545°C
• Surface changes and decrease in �secondary emission yield with �integrated beam
• Cross-talk between adjacent signal wires
• Guard wires between

R. Thurman-Keup --- USPAS Hampton, VA

15

January 30, 2024

Fast�decrease�early

100 °C

Slow�decrease�later

Temperature Increase with 120 GeV p on 25 μm Ti wires

Example

• The Mu2e experiment at FNAL has a transfer line that will supply 6 x 10¹² protons per second. Let’s say we put Tungsten wire SEMs along the transfer line. The wires are 10 μm diameter and 75 mm long. The beam size is 2 mm rms. The dE/dx for 8 GeV protons in Tungsten is 24.3 MeV/cm.
• How much energy is deposited in a wire per second at the center of the beam?
• Using the heat capacity of Tungsten, and assuming the entire wire is heated uniformly (use the whole wire for the volume of material), how fast does the temperature rise assuming no loss of heat from the wire?
• How fast does the temperature rise if the heating is only over a 10 mm long section of the wire? Is there a risk of it melting?
• Determine the secondary emission yield from the plot in the lecture (8 GeV protons). The units are per proton per surface, and the wire has two surfaces (entry and exit). How many secondaries per second are emitted from a wire located at the center of the beam?

R. Thurman-Keup --- USPAS Hampton, VA

16

January 30, 2024

Ion Chambers

• Related to SEMs, ion chambers are SEMs surrounded by a gas
• Signal is generated by ionizations in the gas
• An electric field between the signal wires and bias wires/foils separate the ion pairs generating a signal
• Various modes of operation depending on the magnitude of the electric field near the anode
• Ionization Mode (Gain = 1)
• Proportional Mode (Gain > 1)

R. Thurman-Keup --- USPAS Hampton, VA

17

January 30, 2024

Ionization Chamber Signals

R. Thurman-Keup --- USPAS Hampton, VA

18

January 30, 2024

F. Sauli, CERN-77-09

Outline

• Wire Based
• Flying Wires
• Gas Wire Chambers
• Secondary Emission Monitors
• Ionization Profile Monitors
• Residual Gas
• Gas Jet
• Gas Fluorescence
• Auxiliary Beam Deflection
• Electron Beam
• Ion Beam

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R. Thurman-Keup --- USPAS Hampton, VA

19

January 30, 2024

Ionization Profile Monitor (IPM)

• The beam interacts with a gas of some sort and produces electrons and molecular ions
• Either the electrons or the ions are collected via an electric clearing field
• The electric field, possibly in combination with a magnetic field, must preserve the spatial distribution of ionization products to accurately reproduce the profile
• The ions are heavy and less�influenced by the beam fields�and the clearing field
• Electrons can be constrained by�a magnetic field parallel to the�electric field

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R. Thurman-Keup --- USPAS Hampton, VA

20

January 30, 2024

IPM Detectors

• Generally not enough signal to directly observe ionization charges without amplification
• Common amplifier is a Micro (or Multi) Channel Plate
• Assembly of many, many tiny photomultiplier tubes (without the photo part)

R. Thurman-Keup --- USPAS Hampton, VA

21

January 30, 2024

Micro Channel Plate

• 1 mm glass plate with ≈10 μm holes
• Thin Cr-Ni conductive layer on surfaces
• Voltage ≈1 kV/plate
• e− amplification of ≈10³ per plate
• Resolution ≈0.1 mm (2 MCPs)
• Suffers from aging proportional to total charge extracted from channel walls

🡪 Cannot leave MCP on when not actively measuring

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R. Thurman-Keup --- USPAS Hampton, VA

22

January 30, 2024

RESISTIVE

IPM Detectors

• Generally not enough signal to directly observe ionization charges without amplification
• Common amplifier is a Micro (or Multi) Channel Plate
• Assembly of many, many tiny photomultiplier tubes (without the photo part)
• Exception: Single particle pixel detectors from particle physics do not need MCP (e.g. CERN Timepix)
• Final ‘digitizer’ is one of…
• Phosphor screen with camera system
• Anode strips parallel to beam

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R. Thurman-Keup --- USPAS Hampton, VA

23

January 30, 2024

FNAL Main Injector

120 Anode Strips, 0.5 mm period

Residual Gas IPM

R. Thurman-Keup --- USPAS Hampton, VA

24

January 30, 2024

Ionization cross section

Number density of gas

Average energy�to produce ion pair

Beam current

amu

Residual Gas IPM

• Signal can be extracted by collecting either electrons or ions
• The space charge field of the beam depends on the amount of charge and the transverse and longitudinal sizes

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• Electrons, since they are light, generally need a magnetic field to protect the spatial distribution from the beam space charge as they are accelerated in the electric field
• Ions are too heavy to be constrained by any reasonable magnetic field

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R. Thurman-Keup --- USPAS Hampton, VA

25

January 30, 2024

Consider a cylinder of beam �of radius R (Gauss’ Law)

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Peak space charge electric�field varies as 1/R

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Electron Collection (FNAL)

R. Thurman-Keup --- USPAS Hampton, VA

26

January 30, 2024

Magnet with vertical B field

Cathode

Field shaping electrodes

Secondary Emission Suppression Grid

Wire mesh gate

MCP(s)

Anode strips

Beam

(into page)

Ions

Electrons

Ionization Happens

RF Shield

Electric Field

Electron Collection (FNAL)

• Main Injector

R. Thurman-Keup --- USPAS Hampton, VA

27

January 30, 2024

Field shaping electrodes

Wire mesh gate

RF Shield

Secondary Emission Grid Resistor

Entire structure within beam vacuum

Electron Collection (FNAL)

R. Thurman-Keup --- USPAS Hampton, VA

28

January 30, 2024

Magnet

Ion Collection

R. Thurman-Keup --- USPAS Hampton, VA

29

January 30, 2024

Cathode

Field shaping electrodes

MCP(s)

Anode strips

Beam

(into page)

Electrons

Ions

Ionization Happens

RF Shield

Electric Field

No Magnet

Electron Collection

R. Thurman-Keup --- USPAS Hampton, VA

30

January 30, 2024

Magnetic Field

IPM Gating

• Goal is to prevent ionization products from reaching the MCP
• Reduce aging in the MCP
• Create crossed E and B fields to produce drift along beam
• Drift past end of MCP

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R. Thurman-Keup --- USPAS Hampton, VA

31

January 30, 2024

Out of the page

Gated Off

Gated On

IPM Devices

• CERN PS
• Electron collection
• Better secondary suppression�from ions
• Hybrid pixel detector for�readout (Timepix)
• Time-of-arrival
• Location (x and y)
• Time-over-threshold (energy)

R. Thurman-Keup --- USPAS Hampton, VA

32

January 30, 2024

Traditional secondary suppression

New ion trap

~10%

Beam

IPM Devices

• JPARC; problems with sextupole moment of magnet

R. Thurman-Keup --- USPAS Hampton, VA

33

January 30, 2024

Gas Jet

• Same principle as residual gas
• Higher signal intensity
• Can control gas density
• Possibility of measuring halo of beam
• Pressurized gas is passed through multiple slits or a slit with very thin and long aperture
• Gas molecules exit with low divergence (beam effect)
• Remaining gas, which is rejected by the slits that form the gas sheet, must be pumped out

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R. Thurman-Keup --- USPAS Hampton, VA

34

January 30, 2024

Gas Jet Profiler

R. Thurman-Keup --- USPAS Hampton, VA

35

January 30, 2024

Beam Induced Fluorescence

• Beam hits gas molecules (e.g. N₂) inside the �beam pipe
• Molecules are excited by interaction
• Gas may or may not be ionized
• 1N is excitation following ionization

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• Gas fluoresces when electrons fall to a lower energy state
• Pattern of fluorescence light is proportional to �profile of beam that excited the gas
• Fluorescence light exits beampipe through viewport �and is detected by an optical system

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R. Thurman-Keup --- USPAS Hampton, VA

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January 30, 2024

N₂ + Ion → (N₂⁺)*+ Ion → N₂⁺ + γ + Ion

Beam Induced Fluorescence

R. Thurman-Keup --- USPAS Hampton, VA

37

January 30, 2024

from D. Vilsmeier

Molecules not

Stationary!

Beam Induced Fluorescence

• Gas molecules are in constant motion
• Average kinetic energy of kT
• For N₂, the average velocity at room temperature is

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• In the 60 ns of its decay time it travels

25 μm 🡪 Intrinsic resolution of BIF using N₂

R. Thurman-Keup --- USPAS Hampton, VA

38

January 30, 2024

Beam Induced Fluorescence

• Fluorescence rates are less than ionization rates
• Collection efficiency of photons is much less than ionization products
• All ionization products collected via electric field
• Only small (≈10^-3) fraction of photons exit the window

R. Thurman-Keup --- USPAS Hampton, VA

39

January 30, 2024

‘Single photon counting’:

GSI-Linac

Gas Jet Profiler

R. Thurman-Keup --- USPAS Hampton, VA

40

January 30, 2024

O. Sedlacek, IBIC2023

Electron Lens Beam

LHC 6.8 TeV Beam

CERN�LHC

Gas Sheet

Profile

Collects Photons

Used for overlapping beams of protons and electrons of electron lens

Outline

• Beam Interaction Basics
• Wire Based
• Flying Wires
• Secondary Emission Monitors
• Gas Wire Chambers
• Ionization Profile Monitors
• Residual Gas
• Gas Jet
• Gas Fluorescence
• Auxiliary Beam Deflection
• Electron Beam
• Ion Beam

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R. Thurman-Keup --- USPAS Hampton, VA

41

January 30, 2024

Electron Beam Profiler

• A probe beam of some type is directed through a charge distribution which for our purposes is another particle beam
• The probe beam is deflected by the electric and magnetic fields of the target beam
• The deflection as a function of impact parameter, encodes information about the charge distribution of the target beam
• That information can be converted to a profile

R. Thurman-Keup --- USPAS Hampton, VA

42

January 30, 2024

Electron Beam Profiler

R. Thurman-Keup --- USPAS Hampton, VA

43

January 30, 2024

x profile

~0

Electron Beam Profiler

• FNAL Main Injector

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R. Thurman-Keup --- USPAS Hampton, VA

44

January 30, 2024

Electron Beam Profiler

• SNS Accumulator Ring
• Pulsed electron gun
• 60 keV @ 1μs
• Electrostatic deflector for impact parameter�sweep
• Duration of 20 ns
• Phosphor screen readout

R. Thurman-Keup --- USPAS Hampton, VA

45

January 30, 2024

Electron Beam Profiler

• SNS Accumulator Ring

R. Thurman-Keup --- USPAS Hampton, VA

46

January 30, 2024

Vertical Profile Sweep

No Beam

Beam

Ion Beam Profiler

• CERN SPS
• Tested an ion beam (Xe)
• Ions are heavy and slow
• Average over the beam bunches

R. Thurman-Keup --- USPAS Hampton, VA

47

January 30, 2024

Pencil Beam Scan

Deflection�of sheet beam

Extras

R. Thurman-Keup --- USPAS Hampton, VA

48

January 30, 2024