# Presentation: 476549

## BIW

**Interceptive Beam Diagnostics ****−**** Signal Creation and Materials Interactions**
**Beam Instrumentation Workshop**
**May 3-6, 2004**
**Knoxville, TN **
**by**
**Mike Plum**

## Table of contents

- List of ways particle beams interact with materials.
- A more detailed look into some of the interactions.
- Error analysis of harp and wire scanner profile measurements.
- Example – design of the SNS wire scanner system.
- Example – design of the SNS harp system.

## How particle beams interact with materials

- Scattering -- multiple, large angle Coulomb, and nuclear.
- Energy deposition (heating).
- Secondary electron emission (SEM).
- Optical transition radiation (OTR).
- Ĉerenkov radiation.
- Scintillation, light creation (view screens).
- Gas ionization (residual or injected gas).
- The beam may stop in the material.
- Stripping (e.g. electron stripping from ion beams).
- Secondary particle creation, including delta rays.
- Nuclear interactions.

## Quick examples of each interaction (1)

- Scattering -- multiple, large angle Coulomb, and nuclear.
- Example: wire scanner profile measurement, which measures beam loss as a function wire position.
- Energy deposition (heating).
- Example: harp profile measurement, which measures wire temperature (via resistance change) as a function wire number to determine the beam profile.
- Secondary electron emission (SEM).
- Example: wire scanner profile measurement, which measures wire signal as a function wire position.

## Quick examples of each interaction (2)

- Optical transition radiation (OTR).
- Example: beam profile and emittance measurement by imaging the OTR.
- Ĉerenkov radiation.
- Example: particle identification in particle and nuclear physics experiments.
- Scintillation, light creation (e.g. view screens).
- Example: beam profile measurement by inserting the view screen and imaging the light.
-  Gas ionization.
- Example:  beam profile measurement by measuring the quantity of ions collected as a function of position.

## Quick examples of each interaction (3)

- The beam may stop in the material.
- Example: wire scanner profile measurement, which measures wire signal as a function wire position.
- Stripping (e.g. electron stripping).
- Example: laser profile monitor, which measures the quantity of electrons stripped from an H– beam as a function of laser beam position.
- Secondary particle creation, including delta rays.
- Example:  wire scanner profile measurement using secondary flux (e.g. a loss monitor signal) as a function wire position.
- Nuclear interactions.
- Example:  transmutation, where a beam profile is measured utilizing foil activation.

## Quick examples of each interaction (4)

- Note that almost all the examples were for profile monitors and emittance measurement.
- Why? For other commonly measured beam parameters it is simple to use non-intercepting diagnostics, and of course this is preferred.
- Beam position, beam current, beam energy are all *easy* to measure non-interceptively.
- Beam profile and emittance measurements are *difficult* to do non-interceptively.

## The Bethe-Bloch equation

- The Bethe-Bloch equation is very important for beam diagnostics.
- Describes energy loss in materials by charged particles heavier than electrons.
- It can determine if a material will melt or if the impact on the beam is so large that the choice of material is impractical.
*K* = 0.3071 MeV cm2; *z* = the atomic number of the incident particle;
*Z* and *A* = the atomic number and mass of the material;
*m**e* = the mass of the electron,  ** and **  = the usual relativistic factors,
*I* = the mean excitation energy, and ** = the density effect correction.

## The Bethe-Bloch equation (2)

*T**max* is the maximum kinetic energy that can be imparted to a free electron in a single collision.
- For an incident particle of mass *M*, in the low-energy approximation (*2****m**e**/M** <<1*)

- Higher beam energy means less energy loss

- Material density effect is here

- Ion beams see a lot more energy loss

## Stopping power and range

- Stopping power

- Range

- (From the Particle Properties Data Booklet)

## Multiple scattering

- Why do we care?
- Sometimes scattering is desirable – e.g. wire scanner profile measurement that plots wire position vs. signal on a loss monitor.
- Sometimes scattering is bad – e.g. beam loss, emittance growth.
- Multiple scattering is a series of small angle kicks to a charged particle beam caused primarily by Coulomb interactions, and to a lesser extent by nuclear interactions.

## Multiple scattering (2)

- The scattering angle distribution is roughly Gaussian with a width
*p*, ***c*, and *z* = momentum, velocity, and charge number of the incident particle,
*x/X**0* is the thickness of the scattering medium in radiation lengths.
- The radiation length, *X**0*, is the distance an electron travels before it loses 1/*e* of its energy.

## Multiple scattering (3)

- Examples:
*X**0* = 18.8 cm for carbon
*X**0* = 1.43 cm for copper
*X**0* = 0.35 cm for tungsten
- Main points for us:
- the scattering angle increases as the incident particle velocity decreases,
- as the charge of the incident particle increases, and
- as we move to heavier elements in the periodic table.
- To keep scattering small, need:
- Materials that are thin, and
- Materials with long radiation length (low atomic number).

## OTR

- Optical transition radiation (OTR).
- The use of OTR for beam diagnostics is relatively new, and it has quickly grown in popularity and range of application in the last 10 years.
- Created by a charged particle traveling across the boundary between two media with different dielectric properties (e.g. a foil in a vacuum).
- OTR radiation has an angular distribution with a strong peak at angle ** = 1/**.

- light

- foil

- beam

- (Fiorito & Rule,
- 1993 BIW)

## OTR (2)

- The OTR process is **inefficient**.
- Intensity proportional to **2.
- Practical applications have been limited to electron beams with kinetic energy > 10’s of MeV, and protons with kinetic energy > 10’s of GeV.
- OTR is also **polarized**.
- By separately measuring the parallel and perpendicular polarization components, the horizontal and vertical beam divergences can be measured.
- Together with the beam spot size measurement (also using OTR), and by focusing the beam down to a waist at the OTR foil, this provides a nice measurement of beam emittance.
- OTR is **fast**.
- E.g. it is a good way to measure sub-picosecond bunch lengths.

## Optical Transition Radiation Interferometry - OTRI

- OTR from 2 foils interferes to create fringe patterns.
- Allows a more sensitive measurement of the beam divergence, and therefore a more accurate beam emittance measurement.
- The interference pattern fringe positions also provide a measurement of the beam energy.
- Example: divergence error of 30% and a beam energy measurement error of 1% was obtained by Fiorito and Rule with a 107-MeV electron beam.

- (Figs. from Fiorito & Rule,
- 1993 BIW)

## OTR - summary

- Useful for higher energy beams (>10 MeV *e*’s, >10 GeV *p*’s)
- Almost non-interfering with beam (very thin foil).
- Polarized light, max at ** = 1/** .
- Good for beam size and emittance.
- Limitations:
- Beam scattering in the foil which may interfere with the accuracy of the OTR-interference method or with beam operations.
- Foil melting in high-intensity beams.
- Low OTR intensity, particularly for non-electron (e.g. proton) beams with less than a few GeV kinetic energy.

## Scintillation & fluorescence 

- Particle beam loses energy as it passes through the gas.
- Some of this energy causes electrons to be promoted to excited states.
- When the electrons fall to lower energy orbits, photons are emitted.
- Scales with dE/dx – the more energy the beam deposits in the material, the more light you get.
- Plastic scintillators are commonly used in nuclear and particle physics experiments, but not used much in intercepting beam diagnostics.
- View screens (phosphor screens) *are* commonly used .
- Gas scintillation, from residual or injected gas in the beam pipe, is another common beam diagnostics used to measure beam profiles.

## Scintillation & fluorescence (2)

- If the molecules have not traveled too far from the points where the interactions with the particle beam took place, the photons can be collected to produce an accurate measure of the beam profile.
- If the emission lines are in the visible spectrum, simple glass lenses can be used to focus the light onto detectors.
- The perfect gas will have large cross sections in the visible region and short excited-state lifetimes.
- If gas injection into the beam pipe is necessary, the gas should also be easily pumped by the vacuum system.

## Scintillation & fluorescence (3)

- Nitrogen gas is commonly used for gas scintillation profile monitors because of its large cross sections in the visible range, the short (60 ns) excited state lifetimes, and because most vacuum systems efficiently pump N2 gas.
- For the case of N2 gas, it takes about 3.6 keV energy loss to create a visible photon.
- From the Bethe-Bloch stopping power equation we see that for a given set of beam parameters, the beam will deposit significantly more energy into a gas at low beam energies, and it is for this reason that most gas scintillation profile monitors are found at the low energy ends of particle accelerators.

## Gas ionization

- Gas ionization is closely related to gas scintillation.
- Particle beam loses energy as it passes through the gas.
- Some of this energy ionizes residual or injected gas in the beam pipe.
- By guiding the ions and/or electrons to detectors using electric and magnetic fields the beam profile can be measured.
- It takes between 25 and 35 eV of energy loss to ionize a gas molecule, and the number of ions and electrons created can be simply calculated using the Bethe-Bloch equation.

## Gas ionization - example

- Assume a 1 GeV, 10 mA proton beam passing through 10-7 Torr of N2 gas at 20 oC that has 35 eV ionization energy, and a detector length of 10 cm.
- The ionization current (i.e. ion pairs created per second) is
- From the Bethe-Bloch equation, the stopping power is 1.8 MeV-cm2/g, so...

*I* = ionization energy,
- = mass density of the gas,
*t* = thickness or length of the gas in the sensitive volume of the detector,
*i**beam* = beam current.

## Gas ionization – example (2)

## Gas ionization or gas scintillation?

- I believe that the residual gas ionization profile monitor is better because:
- 1) the number of ion pairs created is higher than the number of photons created. Both scale with dE/dx, but the ion pair creation mechanism is more efficient than the visible photon creation mechanism (35 eV/ion vs. 3,600 eV/photon for nitrogen -- i.e., we get about 100 x less photons for a given gas density and detector length).
- 2) Almost all the ions are collected in an IPM, but only a small fraction (<10%) of the photons are typically collected in a gas scintillation profile monitor.
- 3) Ion and electron detectors are more efficient than photon detectors.
- In other words IPM’s are at least three orders of magnitude more efficient.
- For most applications a gas scintillation profile monitor would be preferred over an IPM only at low beam energies where copious photons are produced.

## Electron stripping

**Electron stripping**

- Electron stripping is an important issue for ion beams.
- It takes very little material in the beam to strip electrons from an ion beam.
- In synchrotrons and long linacs just the residual gas pressure can be enough to causes excessive electron stripping.
- For example, for H− ions,
- Ions stripped per second:

-  per gas atom, carbon or nitrogen or oxygen.

**  = cross section [m2],
***N* = numerical density [1/m3],
*F* = flux [1/s], and
*L* = length of the material [m].

## Electron stripping (2)

- For 1000 MeV H– beam, it takes a carbon foil of just 400 g/cm2 (2.2 microns thick) to strip 98% of the H– ions to H+.
- At 2.5 MeV it takes just 15 nm of carbon to strip the electrons.
- Stripping just one electron from an H– ion takes even less material.
- Just about any material used for an interceptive beam diagnostic device will strip electrons from an ion beam.
- Sometimes stripping can be a good thing – e.g. laser profile monitor.

- Examples...

- Toroid 2

- Toroid 1

## Electron stripping (3)

- Of course electron stripping can also be a bad thing...
- For example, in the CERN PS, introducing even a small amount of gas for a gas scintillation profile monitor strips the Pb ion beam enough to prevent operations.
- In high intensity machines such as the SNS, if the residual gas pressure rises above approximately 10-8 Torr the beam loss due to stripped H– beam will rise above acceptable levels.
**WARNING:** An often unappreciated aspect of electrons in ion beams is the amount of energy the stripped electrons can deposit in a material.
- At beam energies above a few MeV the protons in an H– beam pass through thin materials without depositing much energy.
- But the electrons stripped from the H– beam can stop in the material and deposit all their energy.

## H+ and H- beams incident on 25 micron tungsten foil

**H****+**** and H****-**** beams incident on 25 micron tungsten foil**

- H− beam

- H+ beam

-     6x more energy deposition

- (Shafer ’02 tech note)

## Electron stripping (4)

- The electron range, and the distance a stripped electron will travel in a material, can be estimated with Feather’s rule suitably modified for materials other than aluminum:

*A*, *Z* = atomic mass and number,
*E** = *electron energy in MeV.

## Range and stp pwr of electrons in carbon

*(from physics.nist.gov)*

## Secondary electron emission

- Why do we care? SEM is a very common signal creation mechanism in beam diagnostics. Used for wire scanners, harps, current measurements, bunch shape monitors, etc.
- Secondary electrons can also interfere with the performance of a beam diagnostic system if they are not properly controlled (e.g. gas ionization profile monitor being built by BNL for the SNS ring).
- When a beam enters or exits a material secondary electrons are created.
- This is a surface phenomenon, and the amount of secondary electrons created is very sensitive to the condition of the surface.

## Secondary electron emission (2)

- Oxidation, surface cleanliness, and monolayers of contaminants can significantly change the secondary electron emission (SEM) coefficient.
- Beam exposure can also change the SEM coefficient.
- For example, Ferioli and Jung observed a 50% decrease in the SEM coefficient for aluminum, gold plated aluminum, and gold plated titanium (1997 DIPAC).

## Theory of secondary electron emission

- Sternglass’s theory of secondary electron emission

*Y* = electron yield
*P* = probability that electron will escape (~0.5)
*d* = average depth from which electrons escape (~1 nm)
** = average amount of kinetic energy lost by an ion per                  ionization in material (~25 eV)

- Example: For an 800 MeV proton beam, dE/dx is about 1.8 MeV/g/cm2. With C wire, *Y* = 0.0079.

- Secondary electron emission is like a 1-nm thick ion chamber fastened to the surface of the wire (or foil), with a ionization energy of 25 eV (much like a gas) and a collection efficiency of 50%.

## Stopping, or just passing through?

- 

- H
- H

- 

- High energy H+

- 

- Low energy H+ or H

- Med. energy H

- High energy H

- Convoy electrons

- Secondary electrons

## Wire signal levels for H− beam

## Accuracy of a beam size measurement with a harp or wire scanner

- Some of the more common intercepting beam diagnostics are the wire scanner and the harp, typically used to measure beam sizes and shapes.
- But how accurate are these measurements?
- There will often be an accuracy requirement that must be addressed when designing a WS or HP system.

## Accuracy of a beam size measurement with a harp or wire scanner (2)

- The error on a beam size measurement, when measured with a wire scanner, depends on several parameters:
- the error in positioning the wire,
- the absolute and relative errors on the signal amplitude measurement,
- the number of positions used for the scan,
- the beam jitter, and
- the data analysis method, e.g., if a fitting function is used or if the width is directly computed from the data, as in an rms measurement.
- In most real-life situations it is best to fit the data with a function, such as a Gaussian, that accurately describes the data.

## Accuracy of a beam size measurement with a harp or wire scanner (3)

- As an example of one way to determine the effect of the various parameters, let’s take a look at the SNS wire scanner system.

-     Simulation of beam distribution at the exit of DTL tank 1 from a 40,000 particle PARMILA model of the SNS.

## Accuracy of a beam size measurement with a harp or wire scanner (4)

- Simulate a wire scan by sampling the simulated beam with a variable number of equally spaced wire positions between ±1 cm.
- The signal, *y*, for each wire is assigned to be all the particles within ±0.01 cm of the wire center.
- An error is assigned to each *y* value, where ***abs* is the absolute error (e.g. electronic noise), and ***rel *is the relative error (e.g. non-linearity in an amplifier circuit).
- The wire position error ***x* is folded into the overall error ***i* by computing ***x* = *f *’***x*, where *f’* is the derivative of the fitting function, and then

## Accuracy of a beam size measurement with a harp or wire scanner (5)

- First take a look at the error due to the number of wire positions across the beam.
- Fix the other error contributions:
*eabs* = 2% of the peak number of counts,
*erel* = 0.01, and
***x* = 0.01 cm.
- Fit the simulated data with a Gaussian distribution.
- Define the error in the fitted width to be *abs[(**true width**) – (**fitted width**)] + (**error in fitted width**)*, where *true width* is the rms beam size of 1.80 mm.
- Plot error as function of number of wire scanner samples across the beam.

## Error in beam size vs. number of wire positions

- 40,000 particle PARMILA model of SNS at 7.5 MeV at D-plate wire scanner.  rms width = 0.180 cm.
- Absolute error = 2% of max.
- Relative error = 1%
- Gaussian fit to data.

*error = abs[(true width) *
*– **(fitted width)] **+ (error in fit width)*

- 12 positions corresponds to
- 1 sample per rms beam size

## Error in beam size vs. error in wire position

- Same signal level errors:
*eabs* = 2% of the peak number of counts,
*erel* = 0.01.
- Number of wire positions fixed for each beam size.
- Plot beam size error as function of wire position error.

## Error in beam size vs. error in wire signal

- Fix number of wire positions (15), error on wire position (0).
- Plot error in beam size vs. error in absolute and relative signal levels.

-     (abs =150  corresponds to 10% of peak signal)

## Example: How to design a wire scanner system

- Let us now take what we’ve been learned and apply it to a real- life example – the SNS wire scanner system.
- Non-intercepting profile monitors expensive and take up real estate. OTR not an option at these low energies.
- Settled on a wire scanner system for most of our measurements.
- Define the requirements.
- Beam energy, intensity, & pulse structure.
- Beam aperture.
- Beam profile measurement accuracy.
- Need for position measurement as well?
- Actuator cycles expected over lifetime.
- Radiation levels.
- Machine protect interface?

## Example: SNS linac wire scanner system

- SNS wire scanner system requirements

| Beam energy | 2.5 MeV to 1.3 GeV |
| --- | --- |
| Beam current | 5 to 38 mA pk, <1.4 mA avg. |
| Pulse length | 50 – 1000 us |
| Rep rate | Up to 60 Hz |
| Radiation levels | Up to 7 Mrad in 30 y |
| Beam size | 0.9 to 4.3 mm rms |
| Beam size accuracy | 10% |
| Beam pipe aperture | 2.5 to 20 cm |
| Actuator cycles over lifetime | 30,000 |
| Machine protect interface? | Yes |
| Position measurement too? | Preferred |

## SEM or beam loss?

- First question: use secondary emission or beam loss to measure profile?
- At low beam energies less than about 10 MeV, using beam loss to measure profiles is not an option. Radiation caused by wire scanner does not penetrate the vacuum chamber.
- Need to use secondary emission signal.
- We chose to use SEM signal at all energies, except in the ring, where we are concerned about rf structure of the beam interfering with the measurement.
- Another advantage to SEM is that we can have more than one wire in the beam at a time. This means we can reduce the stroke requirement. Good thing at SNS, where the beam apertures are large.
- We did choose to offset the wires so that only one wire at a time is within ±2 rms of the center of the beam.  Helps with cross-talk. (Exception: wires in the 2.5 MeV MEBT are not crossed.)

## What kind of signal wire?

- From the beam energy, current, and beam size we see that the SNS beam is very intense.
- At low beam energies, where the beam fully stops inside a wire, the energy deposition is very high, and it is impossible to find a wire that will survive a full pulse length.
- As a compromise, we chose a wire that will maximize our pulse length.
- Detailed thermal analysis shows that carbon wire is best.
- We chose the 32 micron diameter wire – the largest diameter available on the market.

## Wire signal levels

- Wires at 2.5 MeV get the hottest

## Wire temperature

- 2.5 MeV H− beam, 1 x 1.5 mm2 rms 2 Hz40 us26 mA.
- Max temp for a carbon wire in SEM mode is about 1325 oC.
- Just barely makes requirements at 40 us, 2 Hz.

## An SNS wire scanner fork

- Wires offset so that only one wire at a time is within ±2 rms of the beam center.
- 32 micron diameter carbon wires mounted under tension and electrically isolated.

## SNS wire scanner fork

- tooling holes
- for alignment if req’d

- 2 rms beam size

## Wire mount

- It is a good idea to mount the signal wires under tension.
- This keeps them nice a straight, and also prevents sagging if they get hot and expand.

- collet

- collet holding ring

- spring

- Macor insulating block

- retaining
- ring

## The actuator

- We now have our wire, wire mount, and fork design.
- For the actuator itself, the design should allow a stroke large enough to pass all the signal wires completely through the beam aperture.
- At the SNS, we have 3, 6, 8, and 12-inch stroke actuators.
- For the present case, where we measure the wire signal, it is desirable to connect each end of each signal wire to the outside world. One end for the signal, the other end to allow us to test the wire.
- Therefore need a six-pin vacuum feedthrough.
- Note: no vacuum feedthrough is necessary in wire scanner systems that measure secondary radiation rather than wire signals.

## The actuator (2)

- We also need limit switches.
- One in-limit and one out limit for the motion control electronics.
- Extra limit switches can be added for interlocks and redundancy.
- For the SNS actuators we have 5 limit switches. Two for the motion control electronics, two more (1 in, 1 out) for the interlocks, and one more for redundancy in the out limit interlock. (Running the wire during high duty cycle operations will melt the wire!)
- We also use brakes on our actuators so the forks will not get sucked in by the vacuum when the stepper motors are not energized.

## The actuator (3)

- We would also like a position read back on our actuator.
- This allows the actuator to be tested and the profile data to be checked for consistency.
- Optical encoders, potentiometers, and linear-variable differential transformers (LVDT’s) are all commonly used.
- For the SNS actuators we chose the LVDT’s because the optical encoders are not rad hard, and potentiometers are not as accurate.
- Even with the spec of ±0.25% of full scale on our LVDT’s the position read back is not as accurate as just counting pulses issued to the stepper motor, so at SNS we plot the wire signal as a function of stepper motor pulses, and use the LVDT as a check.

## The actuator (4)

- To get an accurate beam profile measurement we must of course have an actuator that can travel in well-defined steps through the beam.
- The accuracy of the step size directly impacts the accuracy of the beam profile measurement.
- Using the graphs shown earlier, to meet the SNS requirement of a 10% or less error on the rms beam size (assuming a ±2% absolute and ±1% relative error on the signal processing), our actuator should have a positioning accuracy better than ±0.15 to ±0.25 mm (depending on beam size).

## Pos’n error in SNS 3-inch actuator

- Measurements made with 4.3 kg weight suspended from shaft.
- rms deviation from straight line fit is 0.14 mm.
- Wobble in ball lead screws (reduced in the latest version) adds another 0.066 mm to horiz. and vert. signal wires.
- Total position error is 0.15 mm.
- Bottom line for this case is a 9% beam size error.

## Motion control

- Every wire scanner system needs motion control.
- Typical solutions range from “flying” wires swept through the beam using servo motors to “crawling” wires using stepper motors.
- At SNS we chose the stepper motor solution with off-the-shelf motion control based on National Instruments units.
- There seems to be some debate in the diagnostics community about linear driver modules vs. stepper driver modules.
- Linear drivers are inherently electrically quieter but also run hotter. However, a well-designed chopper driver can meet electrical noise requirements.
- For SNS we tried each type and finally settled on the chopper driver solution, since the electrical noise was below detectable limits.

## Signal processing

- Our wire scanner system detects signals from the wires, so our signal processor will have three channels (one for each signal wire).
- The signals are weak, so the signal cables should be well shielded.
- At SNS we use RG-223 coax cables run in cable trays with solid metal shielding.
- We chose to make the bandwidth of the signal processor front end as low as we could and still meet requirements.
- The minimum beam pulse length is 50 s (30 s to ramp up to full current, and 20 s to stabilize for a profile measurement), so we chose a bandwidth of 36 kHz.

## Signal processing (2)

- There is a wide range of signal strengths in the SNS wire scanner system, so we chose to design a signal processor with three gain ranges.
- Specifications of the SNS wire scanner electronics:

| Minimum signal | 16 nA at high gain |
| --- | --- |
| Maximum signal | 2.1 mA at low gain |
| No. gain ranges | 3, separated by 18 dB |
| Linearity | <0.1% |
| Rise/fall time | 8 s |

## Self test

- Every beam diagnostic system needs a diagnostic. This is especially important in a facility where a large number of units are deployed.
- Earlier we talked about bringing both ends of each signal wire out through the vacuum interface. This allows us to easily check for a broken wire with a continuity test.
- One can also build a self-test feature into the electronics.
- At SNS we chose to add a 180 k-ohm resistor to ground at one end of each signal wire, down at the actuator.
- The signal processor circuit has the ability to apply a high voltage bias to the signal wire, and if the wire breaks the read back voltage increases to the supply voltage and indicates a fault.

## SNS actuator, driver, signal proc.

## New example – how to design a harp

- Designing a harp is a lot like designing a wire scanner system.
- The main difference is that the wires are fixed and many processing channels are required.
- Why use a harp instead of a wire scanner?
- Fast profile acquisition in just one beam pulse.
- Motion control may not be necessary. If it is not, then it is simpler.
- So why use a wire scanner?
- Just one signal to look at per wire plane (or if beam-loss based, just one signal for all planes).
- Harps systems have to deal with possible gain variation from one electronics channel to the next.
- Harps cause a lot more beam loss.
- Harps have more complicated construction.
- With wire scanners have the option to measure profile with secondary radiation.

## Defining the requirements

- Start by defining the requirements:
- Beam energy, intensity, & pulse structure.
- Beam profile measurement accuracy.
- Active size of harp.
- Need for position measurement as well?
- Insertable or fixed?
- Number of wire planes.
- Actuator cycles expected over lifetime.
- Radiation levels.
- Machine protect interface (if insertable)?

## Example – the SNS target harp

- Designed to measure the profile and position of the beam on the SNS target.
- Must be very rad hard due the close proximity of the 1.4 MW proton beam hitting the mercury target.
- Must stand up to intense beams.
- High reliability is important.

## SNS target harp system requirements

| Beam energy | 1.0 to 1.3 GeV |
| --- | --- |
| Beam current | 10 mA to 24 A pk, <1.4 mA avg. |
| Pulse length | 700 ns |
| Rep rate | up to 60 Hz |
| Radiation levels | 6,000 rad/h when inserted |
| Beam profile accuracy | Measures tails down to <4% of peak. |
| Active area | 34.3 x 17.1 cm2 |
| Signal planes | Horiz., vert., and diagonal (15 deg.) |
| High voltage planes | 4, to sandwich each signal plane |
| Machine protect interface? | Yes, during insertion |
| Position measurement too? | Yes, ± 2 mm  |

## How many wires shall we use?

- Obviously the more wires the better the beam profile measurement. But how many is enough?
- Like the wire scanner, we can run simulations to determine how the wire spacing affects the beam profile measurement.
- It’s simplest to interpret the beam profile measurement if the wires are uniformly spaced, but this is not necessary.
- If the wire count becomes excessive, one may consider spacing the wires further apart out at the tails of the beam and/or near the edges of the beam aperture.
- Ideally the wires will span the entire beam aperture so that the profile can be measured even if the beam is off center, but again this may not be necessary and it may be the victim of a design optimization.
- For the SNS harp, we use a wire spacing of 1.18 cm to measure a beam size of 17.0 cm FWHM, and a spacing of 0.589 cm to measure a beam size of 4.84 cm FWHM.

## Profile accuracy vs. number of wires

## Select signal wire type

- What type of wire shall we use?
- Carbon is best for high thermal loads, but metal wires will work in our application.
- Carbon and SiC wires will also scatter the beam less, so have less beam loss during a profile measurement. But this is not an issue for this application, since shortly after the SNS harp we have a vacuum window.
- Carbon and SiC wires are also more difficult to handle and mount.
- My opinion: use metal wires unless there is a clear reason to use carbon or silicon carbide wires.
- For the SNS harp we chose 100 micron dia. tungsten wires.

## SNS harp wire heating simulation

- Max temp= 586 oC

- Max temp= 586 oC

## Wire mounting method

- We expect our wires to get hot, so to avoid sagging due to thermal expansion we mount our wires with springs on one end.
- This harp will be in a very radioactive environment – dose rate when harp is inserted is estimated to be about 6,000 rad/h.
- Mount the wires on a ceramic PC card. Use traces made of non-hazardous materials (platinum or moly-manganese).
- Also use lead-free solder to avoid mixed wastes.

- Example: LANSCE 13-inch harp

## Reliability

- Harps are typically built with all the signal wires in a flat plane.
- But this can cause problems if a wire breaks in either the signal plane or in an adjacent high voltage plane.
- The broken wire can fall across several signal wires and short them out. If the broken wire is from the high voltage plane it can saturate the signal levels on the signal wires.
- To mitigate this problem we stagger the wires on the signal wire plane, so that the signal wires are on two different levels. Then only one level of wires is likely to short out, leaving the other signal wires fully functional.

- vs.

## Harp insertion mechanism

- To improve the lifetime of the harp, it will be insertable with a ball screw jack driven by an electrical motor behind shielding.
- Signals will be brought out on Kapton insulated solid core wire.
- The whole assembly will be buried under shielding.

## Signal processing

- Harp signal acquisition systems can be of two basic types.
- The wire signals can be sampled either simultaneously or sequentially.
- The cost per channel is often low enough that simultaneous sampling is not a problem.
- For the SNS harp we plan to use simultaneous sampling with custom-designed analog front ends and off-the-shelf ADC’s.

## Self test

- As in the case of our wire scanner system, every diagnostics system needs a diagnostic.
- For the SNS harp we capacitively couple a test signal to each signal wire through 1 pF capacitors mounted on the wire plane near the signal wire, on the opposite end that we take the signal from.

- ...

- ...

- test pulse

## Images of the SNS harp

- Harp assembly

- Harp box and actuator assembly

## Summary

- We’ve talked about the many ways particle beams interact with materials.
- We’ve talked about the accuracy of beam size measurements made with harp and wire scanner systems.
- We’ve run through design examples for two different intercepting beam diagnostics systems built for the SNS – the wire scanner system and the harp system.
- I hope you’ve found this talk to be useful and informative.