Going Into the Pit
Katrina Howard, PhD student at University of Chicago
I pursue physics because the search for the most fundamental descriptions of the universe helps me understand myself. I strive toward this goal of understanding myself in all facets of my life, within and outside of my academic experience. I study accelerator physics in order to contribute to the next generation of particle accelerators, in the hope that my work will allow for the further understanding of fundamental particles and enable accelerators to be used in new avenues of scientific progress.
I arrived in the field of superconducting radio-frequency (SRF) cavities when I was first looking for an undergraduate research opportunity related to particle accelerators. My alma mater is a hot-spot for accelerator physics, so I had sent emails to several professors. When I received a response suggesting a meeting with the principal investigator of the SRF group, I went in expecting an interview. I scoured their webpage, learning about the group’s work and coming up with talking points of studies I found interesting. Instead, upon arrival, I was immediately asked how many credits of independent study I wanted to register for in the upcoming semester and then given an impromptu tour of the lab space. I couldn’t believe that expressing interest was enough to be welcomed into SRF.
Over the next two years, I fell in love with the field. I’ll tell you about the premise of SRF, briefly, to bring you along with me. An SRF cavity is the component of the particle accelerator that actually does the accelerating of the particles. One of the foundations of electromagnetism is that a charged particle is accelerated in an electric field. Similar to how a woodwind instrument is a resonator for sound, a cavity is a resonator for electromagnetic waves. We like this because we can achieve a very large electric field and maintain this field very efficiently, which we can’t do with a static field. These two parameters, the accelerating gradient and the quality factor, are the core of a fascinating optimization problem when building an accelerator. Having a larger electric field over a smaller distance means that you need less cavities to accelerate your particles to the same energy. Having a higher quality factor means that the cavities are losing less power and that you need less cooling power to dissipate that heat. Both of these contribute greatly to reducing the cost of your accelerator.
The original RF cavities were made out of copper, because its good thermal conductivity could dissipate large amounts of power-turned-heat. But another optimization problem is at play once you consider using a superconductor, a material whose resistance goes to zero below a critical temperature. Now this zero resistance figure isn’t true in an oscillating field, but we are still talking about resistance a million times lower than with the copper. At this point, even with all of the associated operating costs to immerse the cavities in liquid helium (expensive!), it becomes more efficient to use superconducting RF cavities in your accelerator. Niobium emerged as the material of choice, with the highest critical temperature of any element, at 9.2 K. The SRF community has shown that introducing certain impurities into high-purity niobium can improve performance by altering the mean free path at the surface, where the magnetic field penetrates into the material. As we approach the theoretical limits of niobium for SRF cavities, the last decade has brought immense improvements in quality factor and accelerating gradients through intentionally added impurities into the niobium surface, such as nitrogen and oxygen. These days, most work is focused on optimizing these recipes on niobium or on exploring alternative materials that would allow cavity operation at higher temperatures.
I decided I wanted to continue my work in SRF during grad school. When I began applying to physics graduate programs in 2020, I learned about how different research groups at universities and national laboratories receive funding for their work. I saw groups funded by companies, the NSF, the DoD, and the DOE, among others, and I observed how the funding source often dictated the “mission” of the research institution and gave insight to what policies and political agenda may be at play. I was wary of these motivations, as I already felt some guilt about choosing a career path that does not directly contribute towards helping people. Though I know participating in this fundamental research & development work is meaningful, it is hard to see any good coming from that in the present. I try to alleviate some of this guilt by taking a strong stance against work that causes harm. This turns out to be surprisingly difficult within physics; it is not enough to just refuse to work on weapons. While science itself does not have a morality, many of our scientific advances have been used for harm. As a result, I was unwilling to work in a lab that accepts defense funding.
Another component critical to my search was the culture of the lab. My efforts involve surrounding myself with people who respect me as a person and a budding physicist. I have found my academic path to be quite lacking in women, especially queer women, so I have learned how to build a community where it does not yet exist or has not been welcome in the past. But fundamentally I do require respect and safety to be myself. To learn about the lab environments during my remote search, I attended virtual open houses and extensively interviewed potential advisors and current grad students at the institutions to which I had been accepted. I chose my current lab for my thesis research because their SRF facilities were state-of-the-art and the DOE funding seemed plentiful for the kind of fundamental accelerator R&D work I was interested in. Most importantly, I received personal assurance from my future advisor that it was safe, and I believed that.
My research focuses on the nature of impurities and defects in niobium and how they affect the performance of SRF cavities. I question why some impurities improve cavity performance while others hinder it. Low purity niobium has been investigated in the past for the purpose of cost reduction and possible high quality factor. I am not interested in using this material in accelerators, to be clear. This is a fundamental R&D investigation into the behavior of intrinsic impurities so that the mechanism of recent impurity-based improvements can be better understood and improved upon.
In this study, I combine RF testing with material science characterization to understand the impact of impurities/defects in the superconducting niobium lattice. RF testing involves submerging the cavity in a liquid helium bath inside a dewar (large pit in the ground), somewhat resembling the image associated with this essay. We couple RF power into the cavity to generate the resonant electromagnetic waves. At a given input power, we know the accelerating gradient from simulations with the cavity geometry, and we can calculate the quality factor by measuring the reflected and transmitted power. The materials study involves samples cut from the same niobium sheet as the cavity. I have learned many material science techniques to explore the microscopic properties of the crystal structure. The goal is to gain insight how to develop new surface treatments involving the impurities and defects intrinsic to niobium.
Many folks think a PhD is difficult because of rigor of the science. The science is the fun part; my work is what sustains my spirit and motivates me to press on. It is difficult because of the people. Despite the care I take in creating a support system of my peers and advisors, my experiences with my colleagues and with departmental and legal bureaucracy have been with significant adversity. I must constantly advocate for myself just to survive and to overcompensate for some perceived stain on my reputation. Having to stand up for myself has made me stronger, but this should not be an extra requirement for my success.
I was on vacation when I started receiving an influx of messages from colleagues asking if I had seen a paper posted online about preparing the lab to carry out the P5 plan. I was familiar with the recommendation for continued support of general accelerator R&D; this document was not about that. Many of my colleagues were aware I had experienced an incident of similar genre as those described in the paper. They asked about my reaction, they asked if I knew about those incidents, they asked if I knew the people affected, they asked if I was involved in writing the paper. The contents of the paper were news to me, but unfortunately not surprising, because I have personal experience with the reporting and investigation mechanisms discussed. I also have personal experience with those mentioned in the paper for academic nepotism: the same one who denied that my safety had been compromised in order to protect an illusion. The illusion of safety, I think, had already been shattered. An updated edition of the paper described further details. Through all official channels, my issue is considered to be resolved. My claim was substantiated, disciplinary actions were carried out, and accommodations have been put in place. So why do I still feel like garbage? I am still haunted by the influence of those who hurt me, to the point I have been prescribed benzodiazepines for when I encounter them in order to not lose my composure at work. Often, I wonder why I continue in this field in the face of so much pain. I am staying for the same reason I began this work: for the love of the science and with the support from the communities I have built.
As I progress further into my career, I have become increasingly aware of the impact of science policy on students’ access to graduate research opportunities. Over the years, and especially since this January, I have witnessed a dramatic shift at the lab. My group’s funding has been tightened, which has limited my work to less expensive options like materials studies and minimized expensive cavity tests. These limitations have caused significant delays to many of my projects, which will delay the completion of my degree. The laboratory culture has been diminished, where we have to “follow the law” in order to evade federal scrutiny and additional defunding. Fearing the restriction of my communities and maintaining hypervigilance of the shifting funding situation makes it very difficult to focus on and carry out my research. We are expected to behave business as usual, as if the core structures of our government are not being bulldozed in front of our eyes. In the meantime, I will continue working on my thesis research and advocating for myself and my communities, while I plan my escape to conduct my postdoc experience abroad. I look forward to continuing research on SRF R&D elsewhere after completing my PhD, free of these hostile systems.
Acknowledgements: This essay was written for the Science Policy & Advocacy for Research Competition (SPARC) at the Universities Research Association.
