Allison emittance scanner project – Senior design project

I will elaborate on my senior design project, in which I invested hundreds of hours in during my Bachelor studies. The project was the heart of my mechanical engineering program and a prerequisite for graduating. I was advised by the head of engineering in the SARAF (Soreq Applied Research Accelerator Facility). Throughout the project, I utilized the experience and knowledge of field experts working in the department. These experts referred us to relevant scientific literature and studies that could expand the understanding of both theory and implementation of the topics.

The project’s goal was to design an RMS emittance scanner (in short, emittance is a quantity that indicates the quality of the beam). The device was meant to fit the size of 50 [mm] in length 30 [mm] in width and 30 [mm] in height, this is due to the geometry available at the critical point along the beam path where the measurement was needed. The project itself was developmental in nature, it demanded open-mindedness and different ways of thinking.

The main motivation for the project (other than graduating and so on) is divided into a few points:

  1. Diagnostic information at a critical point along the beam, this spot is between the RFQ and the LEBT, these are two components of the beam path. Their function is unimportant for the purpose of this passage, however, I do want to emphasize that it’s crucial for the crew of the accelerator to have an emittance read there.
  2. The facility itself does many different things, some of them have to do with scientific advancements in the material science branch of research. Others are simpler yet impactful. For example, one of the uses for the particle accelerator is creating radiology medication for cancer patients as a form of treatment for some types of cancer. This is a somewhat complex fabrication process, and due to the radioactivity and half life time of the substance, can not be flown into the country from a different facility.
  3. This is a somewhat indirect reason, but knowing the two-dimensional distribution of the beam (or, knowing what the emittance signature look like) can mean the difference in a few million dollars for a project. In some cases, this can make or break a project.

The end result of the project is the world-first design for a miniature Allison emittance scanner which incorporates the heat exchanger mechanism. The device dimensions are as stated above while the heat power it is designed to deal with is as high as 400 [W], depending on the beam operating properties. This results in extreme heat fluxes estimated at 1.5 [kW/cm²].
The thesis depicts the development process, calculations and different concepts throughout the project.
I produced a paper about the project which is in peer review at the moment, awaiting publication in an engineering journal.

After contacting the Nuclear research facility in Soreq I was offered three different projects to work on, the first was considered easy, the second hard, and the third one was labeled too hard (for my team and for the time frame of the bachelor design project, of course, no project is too hard). My team and I chose the second project because of its challenging yet feasible characteristics. At the time, I believed the senior design project reflects something for the future of my career and thought that a more complex and deeper project will be a stepping stone for growth later on.

We followed the guidelines and protocol of the facility, the SARAF personnel holds many years of experience and numerous projects successfully completed at the facility. I find the project protocol to be sensible and I would like to manage my future projects this way. The project’s milestones were:

  1. Particle dynamics and theory
  2. Requirements Document
  3. Literature review
  4. Design review
  5. Materials review
  6. Preliminary design review presentation
  7. Thermal study
  8. Electrical study
  9. Critical design review presentation
  10. Mechanical design
  11. Manufacturing
  12. Experiments and evaluation
  13. Project report

Particle dynamics and theory – The very first task I had was to acquire knowledge on the subject of particle dynamics. The way they behave in space when ionized and accelerated through a vacuum. Since particle dynamics is not covered in the undergraduate curriculum I had to learn about it on my own, which lead to a lot of scientific papers being spread across my desk for quite a while. This was challenging but I think it was a positive step because you can’t work on a system you don’t understand, you can’t diagnose a system you don’t know the ins and outs of. After two months of reading different Ph.D. theses and scientific papers, analyses about particle distribution and space charge I was finally a bit more knowledgeable about the system my team and I had to work on. In addition, I delved into the 4 different spatial distribution possibilities and the probability of each, as well as, how complex each of them is to work with. We relied heavily on the knowledge and advice of the head of engineering and the head of physics in the facility, they supervised and helped us with what they could in terms of guidance. After acquiring basic knowledge on how the system works, we were able to define the problem in engineering terms, while also understand the forces at play. This paved the way to the requirements document

Requirements document – The requirements document is a design guide, you can’t start a serious engineering project without a knowing what you are trying to produce, the very purpose of the project is materialized through the requirements document. The document was the guide that drove many decisions and was the reference for many questions I had during the project. It includes geometrical constraints, functionality requirements, an interface table, the thermal specifications functionality, Heat dissipation and the properties of the returning water. Together with the project leader, we came up with about 30 different requirements that were the backbone of the project and kept us from straying away from the main purpose, I always looked at the document to see what the client ordered in order to make appropriate decisions or in case there were disputes.

Literature review – At this point, I know what the project’s purpose is, and it’s time come up with a plan of how to get from A to Z. The purpose of the literature review is to research the existing technologies in the market, what devices are used, where, and what differentiates one from the other. After reviewing 4 different devices, Slit & Wire system, Tomography, Allison emittance scanner, and the pepper pot method, the conclusion was that we will develop an Allison-type emittance scanner. The Allison emittance scanner is the only system from those researched that can work under the geometrical constraints presented to me. Due to both miniature dimensions and the high heat flux, it became apparent that this will be a challenging project.

Design review – After choosing the type of scanner to work on comes the design review. The design review is similar to the literature review but instead of looking at the different types of scanners we look at the different designs for the specific scanner that was chosen. This included reading through some scientific papers published by TRIUMF in Canada, Stokli’s work and a few more. Most noticeably, the idea of a heat shield was drawn from one of TRIUMF‘s designs, however, it was still unknown to me how intricate and detailed the design becomes when scaled down in order of magnitude.

Materials review – The device itself will operate in a vacuum environment and will be acted upon by a high energy ion beam. Therefore it has to be fitted for these conditions. The device has to be able to withstand the vacuum environment while also not contaminating it with oxygen and other materials, all the while, being made of metals that will not become radioactivated when interfacing with the ion beam. We sat down with the facility’s material and vacuum expert and discussed the different materials that should be used in a vacuum and why. We also had a discussion about the manipulators; how they work, how the interaction between vacuum and environment happens. This lead us to research and summarize the properties of the materials at our disposal, melting point, thermal conductivity, electrical conductivity, thermal expansion coefficients and more are all properties that play a part in choosing the materials out of which the device will be made of.
Regardless, I had to find a material that isolates electrically but conducts thermally. Apparently, there are two materials that answer this requirement, Mica, and Kapton, utilizing these materials was key to the success of the project.

Preliminary design review (PDR) presentation – There is usually one of these given per project, we, however, had two of these, one for the school and one for the facility. The purpose of this presentation is to make a small stop in the project and get input from other departments in the facility. The project members (my partner and I) should present a preliminary mechanical design and the layout of the project, in which case the more experienced personnel can voice their thoughts and we can adjust accordingly. This is also done so that people who are not involved in the project can speak their mind without being too emotionally attached to the work done, the feedback can be perceived as objective and constructive, usually.

The thermal study – Was the very heart of the project, I can write about it for days, and I have, I will try to keep it short.
This was the most challenging and the longest task we had during the project. It took more than 3 months and numerous simulations to determine the thermal design that would be adequate for the purpose of the device. The process of the thermal design started at the end of one of the meetings. Each member was to come up with concepts for tackling the heat dissipation problem, we created a qualitative study for each of them for us to determine if they were potential solutions we will be able to implement. We went through about 5 different concepts for the heat exchanger mechanism until we decided to develop the one we thought has the best possibility of success. The chosen concept was a dual planar fin array heat exchanger at the front of the assembly, the thickness of merely 8 [mm] from front to end, but in it lies the geometry to dissipate the heat generated by a deuteron ion beam, as well as the first stage of the emittance measurement. The first measurement stage is called the “entry slit”. We were able to calculate the estimated power in watts that the beam will apply on the device, and by using that piece of information we were able to perform calculations in order to design the most adequate device.

The electrical study – In terms of electrical design, there were a few challenges I had to meet. The device itself was comprised of several components; the electrodes, the front slit assembly, the back slit assembly and the Faraday Cup. The Faraday Cup has to be electrically isolated since it is the measuring component, after fractioning the beam using the slits, the measurement that’s being made is a measurement of charge. The material called Kapton we discussed before helped in this design since we could have the components fixed to each other without interfering with the measurement while still dissipating heat, even from the Faraday Cup (heat power is still applied to it, to the magnitude of 10’s of Watts). The electrodes, which play an integral part in the measurement method, had to be isolated from their surroundings as well, the isolation was done via the usage of alumina spacers between the fixture rods and the electrodes themselves.

Critical design review (CDR) presentation – At this point of the development process, I know what the whole assembly is going to look like with only a few changes left for our team to make. The CDR is another presentation stage that should happen in any project, it again lets experienced individuals who are not emotionally involved ask questions and raise issues that may have been overlooked by the people working on the project. Generally, this is a good thing when the presentation remains civilized and without the interference of ego and emotions. For me, it was an overall positive experience. For instance, one of the questions we were asked was, what is the range of temperatures and flow rates that the device can operate under without compromising the measurement.

Mechanical design – After the CDR took place and the feedback it generated was worked on, we were able to attend the finalization of the mechanical design. The small details of the heat exchanger were determined, the parts that were to be manufactured were finalized so that they are manufacturable and will fit the assembly process. The back assembly, where the Faraday Cup is fixed was made manufacturable, and feedthrough assembly was designed according to the space available along the beam path. I produced Drawings and sent them to be approved by the different experts.

Manufacturing – The manufacturing process had to be carefully considered for the assembly. The process of assembling this device includes brazing and the careful alignment of the three, 0.05 [mm] wide, 20 [mm] long slits, which is done by drilling 2 holes through the three different plates and running a rod through each to ensure the alignment of these slits.

Experiments and evaluation – These will be dealt with in the future. My professor insisted on doing experiments on the device after it has been manufactured, not only for good measure but also to ensure it works before performing a “live” test. At the point in time he suggested this, we were a bit hesitant, however, in retrospective, this is the right methodology to go about your design.

Project Report – A document much like this except far longer has been written by my partner and me,  it meticulously describes the different phases I summarized in this document. Complete with results, reasonings, and a detailed thermal study.

 

Conclusion

The project is defined as successful, the project was completed on time, it achieved its goal and was delivered within the budget limits.

A world-first Allison emittance scanner design was detailed and produced, a device able to withstand extreme heat flux while still performing its role as a diagnostic tool for the particle beam, even at a full load of 40 [MeV] particles, at 10 [mA] current.

During the project, I developed and honed my skills as an engineer, at the end of the project I have the tools to take up any project without my superior having to micro-manage me. I have dealt with a big chunk of self-learning about a subject that maybe isn’t even being taught to engineers until their 3rd degrees. And became knowledgeable enough about the subject to design a diagnostic tool for the particle beam, while also maintaining all the critical constraints.

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