Download Beam Position Monitor for a Particle Accelerator Final Report Spring

Beam Position Monitor for a Particle Accelerator
Final Report
Spring 2015
Full Report
by
Mona Elkady
Andrew Noble
Prepared to partially fulfill the requirements for ECE 402
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins, Colorado 80523
Project Advisors:
Sandra Biedron, Stephen Milton
Approved by:
Sandra Biedron
Elkady, Noble
ECE 402 Final Report
Spring Semester 2015
Colorado State University
Abstract:
Particle accelerators are used for many purposes, such as scientific research, high energy
physics, manufacturing, and medicine. These particle accelerators function by generating a beam
of particles within a vacuum. Particles are accelerated through strong electromagnetic fields to
relativistic speeds, and then used for whatever intended purpose. However, when generating such
a small and high speed beam of particles, the importance of accuracy increases to ensure
functionality. This is why a beam position monitor (BPM) is necessary for the use of a particle
accelerator.
Typically BPMs work through the use of four sensors encircling the tube of the particle
accelerator. These sensors measure the x and y coordinates of the electron beam by measuring
the voltage induced by the electron beam on the walls of the particle accelerator tube. In modern
designs each of these sensors has their own channel for collecting data. To improve on this
design, our team is attempting to make a BPM where all of the data inputs are processed through
one single channel. This would undoubtedly reduce the cost of implementing BPMs, and could
potentially increase the accuracy of measurements by removing any manufacturing errors caused
by variations in each measurement channel.
Over the course of the year, the findings of our research have been very positive. We
have found no reason to suggest that implementing a BPM using only one channel isn’t possible,
and we know that it will be more cost effective; however, that doesn’t mean that our design
approach isn’t without complications. These include that using only one processing channel
harms the sampling rate. Also, having each sensor sample at different points in time could
potentially harm the accuracy of the BPM, as the beam may not be completely stationary
between each sample. There is still more testing that needs to be done in this area to ensure that
our design will remain accurate enough to measure the position of beam the on the order of tens
of micrometers.
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Acknowledgements:
We would like to thank Professors Sandra Biedron and Stephen Milton for sponsoring
our project and guiding us throughout it. We would also like to thank Joshua Einstein, Max Van
Keuren, and Olivera Notaros for their assistance throughout the project as well.
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Table of Contents
Title
i
Abstract
ii
Acknowledgements
iii
Table of Contents
iv
List of Figures
v
1.
Introduction
1
2.
Review of Previous Work
3
3.
Hardware Design
4
3.1
Pulse Generator
4
3.4
PCB Design
5
3.3
Sample and Hold
8
3.4
Analog to Digital Converter
11
3.4
Potential Market, Manufacturability and Marketability
12
4.
Conclusions and Future Work
13
References
14
Appendix A – Abbreviations
15
Appendix B – Budget
16
Appendix C - Project Plan Evolution
17
4
List of Figures
Figure 1: Diagram of FEL [2]
1
Figure 2: Picture of Last Year’s Pulse Generator Circuit
3
Figure 3: Picture of Pulse Generator Test Setup
4
Figure 4: Picture of Pulse Generator Pulse [3]
5
Figure 5: Schematic of Pulse Generator Circuit
6
Figure 6: Early PCB Footprint
7
Figure 7: Final PCB Footprint
7
Figure 8: Sample and Hold Schematic/Test Setup
8
Figure 9: Sample and Hold Cadence Test
9
Figure 10: AD585AQ in Unity Gain Mode
10
Figure 11: ADS1115 Interfaced with Microcontroller
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Figure 12: Arduino Code Measuring Voltage
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Figure 13: Table Comparing Arduino Measurements with Voltmeter
12
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Elkady, Noble
ECE 401 First Semester Report
Fall Semester 2014
Colorado State University
Introduction:
Colorado State University currently possesses a particle accelerator used to generate a
free electron laser (FEL). This type of particle accelerator uses electron bunches accelerated
within the tube for the purpose of generating a laser beam with a tunable frequency. This is
achieved by using a laser on a photocathode, which via the photoelectric effect will emit
electrons. Those electrons will then enter an accelerating cavity under vacuum with a large
electromagnetic standing wave. In that chamber the electrons are separated into bunches and
accelerated to relativistic speeds. Those electron bunches then enter a set of “wiggler” magnets.
These magnets are aligned along the beam axis set in an alternating dipole pattern. The magnetic
forces created by these magnets will cause the electron bunches to “wiggle” in a sinusoidal
fashion causing the electron bunches to emit photons. This phenomenon can be characterized by
this equation.
[1]
λr is the wavelength of the emitted radiation, λu is the undulator wavelength (wiggler
wavelength), 𝛾𝛾 is the relativistic Lorentz factor and K is the wiggler strength parameter. As you
can see by this equation FELs allow you to tune the wavelength of emitted electromagnetic
radiation by varying the parameters the characteristic equation.
Figure 1 Diagram of FEL
[2]
This brings us to the purpose of our senior design project, the BPM. For a particle
accelerator like this to function properly, the position of the electron beam within the accelerator
tube must be centered, which is achieved through the measurement of a BPM and through
corrector magnets aligning the beam. The BPM works by measuring the induced voltage on the
walls of the accelerator tube. The bunches of electrons shooting through the particle accelerator
generate brief spikes of electric fields perpendicular to the accelerator tube that can be measured
as a voltage pulse outside of the particle accelerator. Using several (typically four) of these
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voltage pulse sensors around a section of the particle accelerator tube and using the peak voltage
value measured from each sensor, it is possible to determine where within the accelerator tube
the beam is positioned. The corrector magnets along can then adjust the position of the beam
until it is centered within the tube.
Our work is attempting to reduce the cost of the implementation of BPMs by reducing the
circuitry required for the acquisition of data from the sensors. The voltage captured by each of
the sensors of a BPM has to go through a processing channel which typically consists of a pulse
stretcher, sample and hold circuit and an A/D (analog to digital) converter. In modern designs of
a BPM each of the sensors has its own processing channel for the sampling of data, which isn’t a
cost effective solution. In our project, we will use only one processing channel for all of the data
acquisition to reduce cost. We intend to do this by multiplexing the sensor inputs of the BPM and
by ensuring that our single processing channel is quick enough to process the multiple sensor
inputs without losing accuracy. The rest of this paper will cover what we have accomplished and
have been working on to complete this project.
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2. Review of Previous Work:
Last year’s team worked on creating a pulse generator. The pulse generator is essentially a circuit
that will replicate the pulses generated from the electron bunches inside the particle accelerator.
They achieved this through the use of the avalanche mode in transistors. They designed a circuit
that builds the voltage on the collector side of a bipolar junction transistor (BJT) until it reaches
roughly 90 volts. At this point the electromagnetic force inside the transistor is so large that the
transistor stops blocking the flow of current and the charge stored on the collector side of the
BJT is quickly dissipated, causing a quick pulse similar to that of a pulse caused by the electron
bunches in a particle accelerator.
Figure 2 Last Year's Pulse Generator Circuit
[3]
At the beginning of the year our team worked primarily on researching the topic of
particle accelerators, BPMs, and we met with part of last year’s team to familiarize ourselves
with their work. Then we proceeded with picking up where they left off. We spent time testing
their pulse generator circuit to ensure its functionality. Once we verified it’s functionality we
began redesigning it using expressPCB software to implement it on a PCB board. We proceeded
to work with the previous year’s circuit after the re-evaluation; our work is discussed below.
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3. Hardware Design
3.1 Pulse Generator
Last year’s senior design group had successfully completed designing and building a
pulse generator and stretcher to simulate a fast electron bunch, and this year we expanded on
their design. The first thing our team worked on was reviewing that the schematic and logic used
to create the pulse generator was accurate. We met with last year’s team to fully ensure we knew
exactly what it was they did and the reasoning, and ultimately we decided to stick with the
design they had used. Then we tested their circuit to ensure that it was still functional.
Figure 3 Pulse Generator Test Setup
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Figure 4 Pulse Generator Pulse (about 20v peak with 7.68 ns width)
After finding that the circuit was still functional, we began to consider different ways to
improve the design that we currently had. For weeks we explored the possibility of using a
MOSFET instead of a BJT since MOSFETs generally have less noise and thus might yield a
better result for a pulse, but ultimately we decided that since the BJT was faster and had the
avalanche capability, it would be the best to use. After deciding this, there wasn’t much else to
change in the design since everything else was built around the transistor, so we moved on to the
next phase. This was finding ways to improve and solidify our design by producing a PCB of it.
3.2 PCB Design
After deciding that the design wasn’t going to be significantly modified, we began
working on translating our circuit to a PCB. A PCB is desirable for many reasons. A PCB
enables circuits to be smaller which reduces any stray capacitances and inductances that might be
created between the parts on the board. Since the pulse we are trying to create is very high
frequency, we are more concerned with the elimination of the stray capacitances, as having a
capacitance on the circuit board could potentially short any high frequency signals and thus
destroy the high frequency pulse that we are trying to generate. A PCB also enables wires to be
directly etched into a surface to prevent any overlap between connections, to solidify those
connections, and to make the overall appearance of the circuit neater and easier to understand.
Furthermore, with a solid circuit product in hand, we will be able to move it around easily and
use it on many platforms for testing or for future applications without worrying about the circuit
breaking due to loose wires. As a result of these many reasons we began working on producing
the printed circuit board.
The first step in doing so was to find the best program that would allow for us to design
our own unique components (specifically the BJT and the DC-DC converter). We experimented
with the programs Eagle, MakePCB, and ExpressPCB. The process of creating components in
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Eagle was very complicated, so we moved on to MakePCB. While theoretically MakePCB
would have been great since it allowed for the capability of importing the desired parts directly
from the DigiKey website, it did not work well for parts that were not sold through DigiKey and
as a result we moved on to working with ExpressPCB. ExpressPCB was slightly better than the
previous two programs, but was still quite challenging to use.
There are two parts to creating a PCB with ExpressPCB. The first part is to create a
schematic with ExpressSCH, and then linking that to a new PCB file. Creating a schematic was
not difficult and was extremely straightforward. It involved selecting the desired parts then
placing them onto the schematic and assigning them a value. For parts that were not available in
the provided library, we had to create them (such as for our DC-DC converter). During the
overall process of creating the schematic we would continuously revise our design and check to
see if there were more components we wanted to add on (since we were continuously
researching ways to improve our design), and ultimately decided to add a few test points to our
circuit design and some connectors. Since these changes did not take much effort and should not
affect the performance of our circuit, we were able to proceed to the next phase, which was to
design the footprints.
Figure 5 Schematic of Pulse Generator Circuit
The next step was creating a new PCB file and linking to the previously created
schematic file. We needed to import all the components into the tool and then arrange them in a
way to prevent wire overlap. We also had to make the footprint for the DC-DC converter with
the correct sizing (accurate to the millimeter) for the pins and the boundaries, as well as the
correct locations of the pins. This particular issue took a significant amount of time to solve since
the details of the location of the pins and their sizing require a significant amount of attention.
The main reason for this is that the ExpressPCB tool sizing available for pins is different than the
pin sizing required for the converter. Even now, this issue isn’t completely resolved (we are
awaiting a response from the makers of the software for some input on how to proceed), but we
found a temporary workaround which will have our pin size slightly smaller, but we can use a
drill to expand them.
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Figure 6 Initial PCB Footprint
After creating our footprint for the converter, all that was left was to wire all the
components together. This process was difficult and took quite some time to figure out because
of all the large components we had. Eventually we were able to find different parts that would
take up less space and then rearrange everything to make sure that none of the wires were
crossing until we got the final product.
Figure 7 Final PCB Footprint after all changes have been implemented
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3.3 Sample and Hold
A sample and hold circuit is an essential component to the BPM. This component of the
BPM does exactly what its name would suggest; It takes a sample of a voltage (the voltage pulse
of the electron bunch) and holds the voltage value until it can be processed and digitized. A
sample and hold circuit is a device generally consisting of two voltage follower op-amps
(operational amplifier), with a capacitor and a transistor switch between them.
Figure 8 Basic Sample and Hold Schematic/Test Setup
When a voltage arrives at the sample and hold circuit and the switch is closed (sample
mode), the first op-amp tracks the input voltage and charges a capacitor to match the input value.
When the switch is open (hold mode), the capacitor holds that value until you are ready to
process the data, at which point the sampled value is digitized by an A/D converter.
There are many design parameters of the sample and hold to be considered for us to meet
our design requirements. The first is, of course, accuracy. The open loop gain of the amplifiers
inside must be large enough that it can track minute differences in voltage. Specifically, if we
want to measure tens of micrometers differences in position we should be able to measure tens of
microvolts differences in pulses, which would suggest that we need to have an open loop gain of
at least 10,000 V/V (but we want to have as high a loop gain as possible for greatest possible
accuracy). There is also a concern of bandwidth. We are measuring fast pulses, so the bandwidth
of our sample and hold circuit must be large enough to compensate. However, it isn’t reasonable
due to cost and availability to purchase a sample and hold circuit that would have the bandwidth
necessary to measure a pulse, with a pulse width on the order of picoseconds. To resolve this
problem we have implemented a pulse stretcher to stretch pulse over a longer period of time
allowing for a lower bandwidth sample and hold circuit while keeping a proportional signal.
Considering that this sample and hold circuit will only be sampling at a 10 Hz rate as defined by
our design requirements, bandwidth isn’t really a design concern, as we can stretch the voltage
pulse over as long a period of time as we could possibly need.
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Figure 9 Sample and Hold Cadence Test
Figure 9 is of an early sample and hold circuit simulation of a custom design in cadence,
while it helped us get a grasp of the concepts and functioning of sample and hold circuit this
design was not implemented for several reasons. The first is that the design was custom, and
regardless of whether or not the design met our specifications, it would likely not be the ideal
implementation. This is because producing circuits with custom parts is far more expensive than
using an off-the-shelf part, as the fabrication of custom circuits is costly. As such, this option
isn’t viable as long as there are op-amp/sample and hold circuits that can be purchased to meet
our specifications. As a result we decided on a dedicated sample and hold IC, specifically the
AD585AQ shown in figure 10. This IC provides the performance required to meet our
specifications. It has a 200,000 V/V open loop gain, giving it a closed loop gain of .999995,
which is close enough to unity to allow us to sample voltages with the accuracy required to meet
our accuracy specifications. The sample and hold also has a bandwidth of 2 MHz, which far
exceeds our 10 Hz sample rate requirement and allows us to work with a less stretched pulse.
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Figure 10 AD585AQ in Unity Gain Mode
[4]
Despite this sample and hold meeting all of our specifications, there were many issues
that arose with its implementation. The AD585AQ was very finicky when being implemented
with other devices, probably due to some interaction with the internal capacitances of the other
devices. For this reason, implementation was difficult and time consuming. Even when tested
alone with an oscilloscope, the interaction between the sample and hold chip and the
oscilloscope caused the output of the sample and hold to stay at its maximum value regardless of
the input. The sample and hold IC didn’t even appear to be functioning at all until we began
placing buffers between it and the devices interfacing with it. Eventually, we found that placing a
2 KOhm resistor in series between the sample and hold and other devices resolved the issue and
allowed the sample and hold to function as intended. Once testing on the sample and hold had
finished and the IC was working as expected, the next step in the BPM design was to digitize the
sampled data through the use of an A/D converter.
3.4 Analog to Digital Converter
The A/D converter is an important component of our design. It allows the voltage pulses
measured by the sample and hold circuit to be interfaced with a computer for further processing.
The main criteria we looked at in our choice of A/D converter was the number of bits of
accuracy. First we had to find out how many bits of accuracy the A/D converter would need to
meet our specifications. To do this, specifications on the area where the electron beam could be
found within the particle accelerator tube were detailed by the advisors to be within an inch
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radius from the center of the particle accelerator tube. Using this information we were able to
find how many bits would be required to discretize that area into 10 micrometer increments. The
number of bits required to meet was found to be 12 bits at the very least; however, considering
that no device is perfect, and that the last couple bits of any A/D converter we bought would be
essentially useless, we decided that a 14 bit A/D converter would be our minimum specification.
Using this information we began looking for A/D converters to implement into the BPM design.
Unfortunately, many of the A/D converters with 14 bits of accuracy were in a package that
would have been hard to test and implement in our current circuit and the ones that were in a
convenient package were excessively expensive. For ease of implementation and cost, the A/D
converter chosen was the Adafruit ADS1115, despite it being a 16 bit device. The last step in the
A/D converter design was a way to interface it with a computer.
To interface the A/D converter with a computer, a microcontroller capable of using I2C is
required, as shown in figure 11. The microcontroller that was ultimately chosen was the Arduino
Leonardo. This was chosen for a couple of reasons. The first was that the Arduino ecosystem
was familiar to both of the team members while also being fast enough to work with our circuit
(Arduino Leonardo runs at 16MHz). The second was that the Arduino Leonardo was a cost
efficient microcontroller that was still capable of I2C communication.
[5]
Figure 11 ADS1115 Interfaced with Microcontroller (GPIO port not used)
After connecting the ADC to the sample and hold circuit and to the Arduino, the final
thing to do was to write a program that would read the output of the sample and hold circuit and
display the results on a computer screen. Using open source code from the Adafruit library, we
were able to program the Arduino to read the output voltage. We checked our results by using a
voltmeter to ensure that our code was working correctly, and found that it was off by a factor of
5430. So we changed our code to reflect that.
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Figure 12 Arduino Code Reading in Voltage
Figure 13 Table Comparing Results of Arduino Measuring Code with Voltmeter Results
3.5 Ethical Concerns
Our circuit at the moment is purely for research purposes and to be used by Colorado
State University’s Advanced Beam Laboratory. Everything related to this project is going to
benefit CSU (and anyone who comes across the information we may post online). As a result of
this, there doesn’t seem to be many ethical issues with our project currently. All of the funding is
provided by CSU and is well under $1000, so there are no ethical issues regarding how much we
are spending, and furthermore we are spending the right amount to purchase quality parts to
ensure that the circuit functions correctly. The research regarding beam position monitors has no
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controversy, so there are no ethical issues there. The only possible issue that may arise for our
circuit is that it uses voltages anywhere from 90V – 120V which is quite high, higher than what
many students use, however this isn’t harmful. Furthermore our circuit will not cause any power
outages around other electronics, so there are no ethical issues from that side.
There is potentially an ethical concern for what this circuit will be used in later,
specifically that there may be some ethical issues from a medical aspect. Lasers are commonly
used in laser eye surgery or laser hair removal, and for these issues some people can regard it as
unethical to use lasers (specifically for eye surgery) due to the potential complications that can
result from surgery; however that concern has more to do with surgeries in general and human
error in general, as opposed to lasers from a pure research stance, which is what CSU is using the
laser for.
3.6 Potential Market, Manufacturability, and Marketability
While we didn’t begin this project with a goal of selling the idea (we are just building for
Colorado State University’s FEL), there is some potential for our design to be marketable. If we
manage to maintain the accuracy of typical BPMs already available in the marketplace while
decreasing the cost as we intend to, this could definitely be a sellable product. The potential
market for BPMs is relatively small, consisting mainly of colleges around the country and
governmental/military research, but it still has some market potential. In terms of
manufacturability, our design is very easy to manufacture. Most of the parts we are using are
common and could easily be standardized into a single circuit board for production. If this
project was to be continued and manufactured onto a single printed circuit board, it could be
marketed later on to other universities or research labs in need of our design to add to their beam
position monitors.
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4. Conclusions and Future Work
This past year we have met all of our deliverables and requirements and have
successfully designed the beginnings of a beam position monitor. We ensured that the
components of the BPM design would be able to measure pulses on a scale of tens micrometers
at a sample rate of 10 Hz, although more testing needs to be done to show that the completed
design will function at these specifications. Furthermore, everything was done using one channel
as required. However, there is still much work to be done for this to be an actual beam position
monitor. For the continuing team we recommend that they continue testing and improving upon
our BPM design, and eventually implement everything onto a single printed circuit board for
ease of implementation onto the particle accelerator. We also recommend that the team make a
new pulse generator with several connection points for attenuators, specifically with an option to
add an attenuator before the stretcher to allow for directly varying the pulse before having it
stretched out. This would allow for a better testing environment before the BPM is actually
implemented onto the particle accelerator.
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References
1. G. Dattoli and A. Torre. (1989, March 1) Free Electron Laser Theory. Retrieved from
http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/20/052/20052737.pdf
2. lightsources.org.(2013) Figure retrieved from http://www.lightsources.org/what-free-electronlaser
3. Max VanKeuren and Stephen Watras. (2014) Beam Position Monitor for CSU Accelerator
Laboratory. Retrieved from http://projects-web.engr.colostate.edu/ece-sr- design/AY13/
beam/Documents/FinalReport.pdf
4. analog.com. Figure retrieved from http://www.analog.com/media/en/technical-documentation/
data-sheets/AD585.pdf
5. ti.com. Figure retrieved from http://www.ti.com/lit/ds/symlink/ads1115.pdf
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Appendix A - Abbreviations
A/D - Analog to Digital
BPM - Beam Position Monitor
BJT - Bipolar Junction Transistor
DC-DC - Direct Current converts voltage level to another
FEL - Free Electron Laser
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
Op-Amp - Operational Amplifier
PCB - Printed Circuit Board
V/V - Volts per Volts
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Appendix B – Budget
Purchased Items
Description
Cost ($)
Quantity
Part Number
Sample and Hold Op- 39.57
Amp
1
AD585AQ
Adafruit ADC
14.95
1
ADS1115
Arduino
19.04
1
Leonardo
Total Cost = $ 73.56
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Appendix C – Project Plan Evolution
Below are images of our Gant Chart detailing the progression of our work throughout the past
school year.
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