Submarine Supplier Days 2023

I attended Submarine Supplier Days 2023 in Washington, D.C. March 7 and 8. It is an opportunity for companies contributing to building the latest attack and ballistic missile submarines to get together. The two big programs are for the Columbia Class fleet ballistic missile submarine and the Virginia Class attack submarines. Australia will be buying four of th e latter. I attended the meeting to introduce people to the potential of PFRC as a power plant for future submarines. The first day was a series of presentations on the latest submarines.

On the next day we visited the offices of our N.J. U.S. Representatives and Senators to gain their support for the submarine programs. Here is the inside of the Senate office building with its Calder sculpture.

It was fun to meet people building the submarines. One company in New Jersey has a sole source contract to weld components of the submarines. Each weld is signed by a welder so it can be traced back should a problem arise.

I learned that there are major problems with materials supply and with finding workers to build the submarines. Lead times on some materials can be 80 months. The issues of on-again/off-again production were also discussed. We all agreed, as did the Congressional and Senate staffers, that continuing resolutions were bad.

You can get our submarine brochure here.

New paper published on analyzing and mitigating pulse-pile-up artifacts in PFRC-2 plasma x-ray spectra

A new paper,Analysis and Mitigation of Pulse-Pile-Up Artifacts in Plasma
Pulse-Height X-ray Spectra
” by Taosif Ahsan and our team has been published open-access in MDPI Plasma. It describes the implementation of an algorithm, the two-photon trapezoidal uncorrelated-pulse model, to improve analysis of x-ray spectra emitted from PFRC-2 plasma. This model was developed to reduce artifacts in x-ray spectra caused by pulse pile-up, PPU (the phenomenon where x-ray photons are recorded nearly simultaneously so that only one x-ray photon is recorded with a combined energy), and diagnose the tail region to see if it is a pulse-pile-up artifact or if it has physical origins.

Four scenarios are shown to illustrate pulse pile-up. The top left plot has two trapezoidal pulses overlapping close enough so that the registered peak (energy) is the addition of the peaks of the individual pulses. The bottom right plot is a case where the individual peaks are detected and so pulse pile-up is not an issue. The top right and bottom left plots are in-between cases where there is enough overlap to result in a combined pulse with an intermediate energy recorded. This figure is described in the published paper.
More figures from the published paper showing the successful mitigation of pulse pile-up using the model derived in the paper.

Experiments on the Princeton Field-Reversed-Configuration (PFRC-2) device explore nearly pure, ca. 99%, partially ionized, warm hydrogen plasmas. For these, great interest lies in the tails of the X-ray spectrum. The tail region is important as an electron temperature in the PFRC can be estimated by fitting a Maxwellian distribution. Small tails of high-energy electrons in the energy distribution (EED), even comprising less than 1% of the plasma density, can have large effects on the resistivity, stability, and reaction rates of the plasma.

This paper is a step toward understanding how PPU affects the tail region of spectra for detector-formed trapezoidal pulses. Here we focus on relatively low count rate (≤0.1/deadtime) spectra where primarily only two-photon pile-up needs to be considered. Extension of this work to multi-photon pile-up will be necessary to develop an analytical tool to diagnose and mitigate pile-up effects in the tail regions of higher count-rate spectra.

Winter Mechanical Engineering Internship

During my time at Princeton Satellite Systems, I worked on a momentum unloading project for NASA’s Gateway, a component of the Artemis program. I designed a deployable parasol that is controlled by Canadarm using Solidworks.

Solidworks is a platform I am familiar with, but I was still able to learn new functions. My favorite part of working with Solidworks is the puzzle-like nature of assemblies. When trying to make dynamic parts you have to think about how to best add relations without over-constricting or under-constricting the part. Once I finalized my initial design I was able to attend a Zoom meeting and present it to another company.

When not working on my Gateway project, I fiddled with the 3D printer to print models of the PFRC fusion reactor.

Although I have used 3D printers several times before, this time was more of a learning process. I was an acting 3D printer technician and wrote a guide with troubleshooting tips for future employees. Due to problematic unspooling and tangled filament the printer became jammed a few times, and I was unable to do the typical loading/unloading to set the filament free. This gave me the opportunity to take apart the 3D printer and see the internal mechanisms, which in turn allowed me to unjam the printer and solve the problem. I was thrilled to see inside the 3D printer and how the parts blend together!

Through my internship I learned about the complexity of the design process and how many things you need to consider when creating a product. Conceptualizing is one step, but bringing that concept into the real world requires much more research and planning. Overall, this internship was a great opportunity that allowed me to learn how to solve several engineering problems.

PFRC Article in the Journal of Fusion Energy

Our latest paper, The Princeton Field-Reversed Configuration for Compact Nuclear Fusion Power Plants, is available in the Journal of Fusion Energy, Volume 42, Issue 1, June 2023. This paper is the first released in “The emergence of Private Fusion Enterprises” collection. A view-only version is available for free here.

Our paper gives an overview of the Princeton Field-Reversed Configuration (PFRC) fusion reactor concept and includes the status of development, the proposed path toward a reactor, and the commercialization potential of a PFRC reactor.

The Journal of Fusion Energy features papers examining the development of thermonuclear fusion as a useful power source. It serves as a journal of record for publication of research results in the field. This journal provides a forum for discussion of broader policy and planning issues that play a crucial role in energy fusion programs.

Bright plasma pulses achieved for various gases at a new frequency in the PFRC-2

We have been operating the Princeton Field-Reversed Configuration-2 (PFRC-2) at the new, lower frequency of 1.8 MHz since mid-November. We have blogged about some developments towards operating at this new frequency including installation of the new capacitors as well as power supplies and holders for the belt coils.

The frequency we describe is that of the rotating magnetic field (RMF) which is generated by four radio-frequency antenna loops surrounding the machine. The RMF is responsible for creating a higher density field-reversed configuration plasma out of an initial lower density seed plasma and for heating the ions and electrons in the plasma.

The video below shows bright plasma pulses of increased density driven by RMF, now in various gases (argon, helium, and hydrogen):

Achieving bright plasma pulses is an important first step in operating at the new RMF frequency. This frequency will be within the range at which we expect ion heating to occur once we finish installation of the belt coils to increase magnetic field. We first observed bright plasma pulses at the new frequency of 1.8 MHz in argon gas due to its lower ionization potential in comparison to that of molecular hydrogen. In the experiment runs following the run with argon, we tuned parameters such as magnetic field, pressure, and seed plasma power until we began to see bright flashes in helium and hydrogen (where there is still a small percentage of argon). We are continuing work on optimizing the bright flashes for these gases.

In the next few months we plan to increase the magnetic field and measure the plasma with the ion energy analyzer built by the Princeton Plasma Physics Laboratory (PPPL) for this experiment with support from ARPA-E. This will show us how much plasma heating is happening!

In other good news, our two INFUSE awards with PPPL, which were announced this summer, have finally received all necessary approvals from DOE and are kicking off. Sangeeta and I (Chris) are at the lab helping to run the PFRC-2 experiment every week and will soon be running software simulations for the INFUSE projects. We will be studying plasma stabilization techniques and new antenna configurations, all to maximize plasma heating efficiency!

Stay tuned as we continue to update on our progress with the PFRC-2!

NIF: Net (Scientific) Gain Achieved in Inertial Fusion! What is the impact on PFRC?

The internet was abuzz last week with the news that the National Ignition Facility had achieved that elusive goal: a fusion experiment that achieved net (scientific) energy gain. This facility, which uses 192 lasers to compress a peppercorn-sized pellet of deuterium and tritium, released 3 MJ of energy from 2 MJ of input heat.

We have to use the caveat that this is “scientific” gain because it does not account for the total amount of energy needed to make the laser pulse. As a matter of fact, the lasers require 400 MJ to make those 2 MJ that reach the plasma. If we account for this energy, we can call it the “wall plug” gain or “engineering” gain since it includes all the components needed. This gain for laser-induced fusion is still less than 1%, because the lasers are very inefficient.

Nonetheless, this is great news for all fusion researchers. Since we often get asked: Has anyone achieved net (scientific) gain yet? Now we can say: Yes! It is physically possible to release net energy from a fusing plasma, to get more energy output than direct energy input. This advance has been achieved through various new technology: machine learning to select the best fuel pellets, wringing more energy from the lasers, more exact control over the laser focusing. Modern technology, especially computing for predicting plasma behavior, explains why progress in fusion energy development is now accelerating.

Tokamaks have also come close to net gain, and in fact the JT-60 tokamak achieved conditions that could have produced net gain, if it had used tritium [1].

The reason JT-60 did not use tritium in those shots is very relevant to our fusion approach, the PFRC. Tritium is radioactive, rare, expensive to handle, and releases damaging neutrons during fusion. Tritium is also part of the easiest fusion reaction to achieve in terms of plasma temperature, the deuterium-tritium reaction. It makes sense for fusion experiments to use such a reaction, but this reaction presents many difficulties to a future working power reactor.

The PFRC is being designed to burn deuterium with helium-3, rather than with tritium, precisely to make the engineering of a reactor easier. The deuterium-helium-3 reaction releases no neutrons directly. Some deuterium will fuse with other deuterium to produce neutrons and tritium, but the PFRC is small enough easily expel tritium ash. This results in orders of magnitude less neutrons per square meter reaching the walls. Once we have scientific gain, like the NIF has now demonstrated for laser fusion, we have an easier path to engineering gain — that is, net electricity.

So while the laser fusion milestone doesn’t directly impact our work on the PFRC, it is important to the field. We will continue to follow the progress of all our peers as we work to achieve higher plasma temperatures in our own experiments!

[1] T. Fujita, et al. “High performance experiments in JT-60U reversed shear discharges,” Nuclear Fusion 39 1627 (1999). DOI: 10.1088/0029-5515/39/11Y/302

APS Division of Plasma Physics 2022 Meeting in Spokane, Washington

Last week, I attended the American Physical Society Division of Plasma Physics (APS DPP) 2022 Meeting. As the name entails, it was a meeting full of plasma physics with applications ranging from astrophysics to nuclear fusion energy. There were many great talks and posters on plasma physics research by companies, national labs, and universities, and one could sense an overall feeling of excitement around fusion shared by many attendees.

I had a pleasant time in Spokane, WA. Pictures from outside of the conference center (with many conference attendees standing nearby), including the nice view from the conference center, are shown below.

I presented a talk on the Princeton Field-Reversed Configuration (PFRC) fusion reactor concept, and how we can leverage public-private partnerships for its development. The talk discussed technical details of the PFRC, including the past modeling and experiments, current investigation, and future research & development plans. The talk also described the markets and commercialization opportunities for this reactor concept, including disaster relief and asteroid deflection. Here I am at the podium speaking.

I also presented a poster on our recent investigations of x-ray diagnostics on the PFRC-2 experiment for electron temperature and density measurements, which was mounted on a poster board in the conference center. Many people came by to ask about my poster as well as about general PFRC questions, which kept me talking for the majority of the 3-hour poster block session! It was great to discuss ideas and results with many scientists and students at the conference.

Dr. Sangeeta Vinoth also had a poster at this conference on collisional-radiative model developments to extract electron temperature measurements from spectroscopy, which she presented virtually. APS DPP 2022 was an exciting conference to attend, and I’m looking forward to seeing updates from presenters at this conference. That also includes us, as we have more research and investigation to do — stay tuned!

New Paper Published: Implementation of Mylar filter in x-ray diagnostics of PFRC-2

We have recently published a research paper titled: “Use of a Mylar filter to eliminate vacuum ultraviolet pulse pileup in low-energy x-ray measurements”[1]. This paper is published as part of the “Proceedings of the 24th Topical Conference on High-Temperature Plasma Diagnostics”, along with the papers mentioned in earlier PSS blog posts on the collisional-radiative model and the neutral atom density diagnostic.

This research builds on the investigation of measuring electron density and temperature by collecting plasma-emitted x rays using a diagnostic called the Silicon Drift Detector (SDD). The x rays emitted via Bremsstrahlung (German word for “breaking radiation”), can be mapped to a distribution that gives electron temperature and density. We observed changes to the x-ray spectra when changing the size of the aperture during experiments with the Rotating Magnetic Field (RMF), which was found to be connected to a phenomenon called “pulse pileup”. Essentially, pulse pileup means that too many x rays coming in at once can combine in energy and so skew the distribution that is measured — this would be misleading for temperature measurements, since they are connected to the slope of the distribution! To solve this issue, we decided to investigate the use of a Mylar filter, see below, because of its favorable filtering properties relevant to our experiment:

Picture of Mylar filter, diameter ~ 1 centimeter, thickness ~ 1 micron (1/10000 cm). Image from [1].

We performed calibration with an x-ray target tube and tested the filter with various plasma conditions for the PFRC-2. When running in a high-ultraviolet-flux mode of the PFRC-2 (with RMF) we found that the Mylar filter substantially reduced the low energy signal, which supports our hypothesis that the pulse pileup was causing x rays to be measured at higher energies. See the figure below for a striking comparison between no-Mylar and Mylar cases. The Mylar filter helps us eliminate pulse pileup effects and uncover the true x-ray distribution reaching the SDD for accurately measuring electron number density and temperature in the PFRC.

Comparison of x-ray spectra for high-UV-flux condition: without Mylar (blue) and with Mylar (red). A substantial reduction of the pulse pileup helps us uncover the true x-ray spectrum for measurement. Image from [1].
[1] Galea, Swanson, Cohen, and Thomas, Review of Scientific Instruments 93, 093531 (2022);

Annie Price Presents, “Nuclear Fusion Powered Titan Aircraft” at IAC 2022 in Paris France

Annie Price, who was an intern at Princeton Satellite Systems during the summer of 2021, presented our paper, “Nuclear Fusion Powered Titan Aircraft,” at session C4,10-3.5 which was the Joint Session on Advanced and Nuclear Power and Propulsion Systems.

There were many interesting papers. One was on generating electric power in the magnetic nozzle of a pulsed fusion engine. Another was on the reliability of nuclear thermal engines. The lead-off paper was on a centrifugal nuclear thermal engine with liquid fission fuel.

Annie’s paper covered the design of a Titan aircraft that can both do hypersonic entry and operate at subsonic speeds. Her design uses a 1 MWe nuclear fusion power plant based on PFRC and six electric propeller engines.

She discussed the aerodynamic design, why Titan is so interesting and how the available power would enable new scientific studies of Titan. Annie also described how a PFRC rocket engine or power plant operates. She included a slide on our latest results.

The paper was well received. She had a couple of good questions after her talk and engaged in interesting discussions after the session. Great job Annie!

Installation of the new capacitors in the Princeton Field-Reversed Configuration-2

Further upgrades of the Princeton Field Reversed Configuration-2 (PFRC-2) are underway with the goal of achieving the milestone of ion heating. The PFRC-2 is predicted to have substantial ion heating once the RF antenna frequency is lowered and the magnetic field is increased. To lower the RF frequency, we have installed additional capacitors in the tank circuit of PFRC-2. The picture below shows three capacitors, each with capacitance of 2 nanoFarads (2 nF), installed in a custom-built copper box.

The copper box is also shown in the bottom part of the image below, where it will be connected with a robust cable to the top box, which is called the tuning box. The tuning box is an aluminum box with one fixed capacitor and two tunable capacitors which can be adjusted to change the resonance frequency of the circuit.

Changes have also been made to the inside of the tuning box in order to prevent electrical arcing, which is a common issue when working with high-power and high-voltage circuits. To help prevent arcing, conical structures of brass have been fabricated and installed. The brass structure is shown alone in the first image below and is shown enveloping the cable connection in the second image below. The shape of these structures allows a better spread of the charge in the tuning box so as to lower the chances of electrical breakdown. Taking these preventative design decisions is key to ensuring reliable operation once the upgraded system is running.