In this paper, a femtosecond two-photon-absorption laser-induced-fluorescence (fs-TALIF) diagnostic was designed, installed, and operated on the Princeton-Field-Reversed Configuration-2 device to provide non-invasive measurements of the time and spatially resolved neutral-atom densities in its plasmas. We demonstrated that fs-TALIF can provide spatially, to ±2 mm, and temporally resolved, to 10 µs, measurement of the density of certain previously inaccessible atoms, e.g., atomic hydrogen (Ho).
Calibration of the Ho density was accomplished by comparison with Krypton (Kr) TALIF. Measurements on plasmas formed of either molecular hydrogen (H2) or Kr fill gases allowed examination of nominally long and short ionization mean-free-path regimes. With multi-kW plasma heating and H2 fill gas, a spatially uniform Ho density of order 1017 m−3 was measured with better than ±2 mm and 10 µs resolution. Under similar plasma conditions but with Kr fill gas, a 3-fold decrease in the in-plasma Kr density was observed.
Ho density is essential to several plasma diagnostics including time-of-flight and ion energy analyzers, and high-resolution spectroscopy, as by CPT (coherent population trapping) and DFSS (Doppler-free saturation spectroscopy). It was also used in Collisional Radiative Modelling for predicting the Electron temperature diagnostic in PFRC-2.
TALIF H-α signal (arb units) at r = 40 mm vs time for (identical) RMFo-heated discharges (Pf ∼ 60 kW). The maximum Ho density is 2 × 1017 m−3. The non-zero Ho density before and after RMFo is due to the seed plasma. RMFo power applied between 3.7 and 9.5 ms.
On September 22 Marilyn, Eric, and I visited ITER, the International Tokamak Experimental Reactor in Saint-Paul-lez-Durance, France, about 45 minutes from Aix-en-Provence. We took the TGV from Paris to Aix-en-Provence.
Our tour started with a talk by Akko Maas who gave a great presentation on fusion. He talked about building ITER. The complexity of the project and the large international team both present challenges. He also discussed the advantages of fusion in comparison to wind and solar. He noted that while a fusion reactor would have some waste, both wind and solar, when decommissioned, have waste. He talked about the next phase after ITER called DEMO. ITER is designed to produce 500 MW of fusion power from an input of 50 MW heating power. Akko had a slide listing some of the commercial fusion efforts.
Katya Rauhansalo was our tour guide. She had a couple of assistants. They were all really helpful and very knowledgeable. We discussed many fine points of Tokamak design and fusion in general. Marilyn, Eric, and I were combined with a larger group, due to Covid absences. We chatted with members of the other group about PFRC.
A Tokamak is shown below. The green coils are the center stack coils used to induce a current in the plasma. The gray coils are the poloidal coils. The purple coils are the toroidal coils. In ITER, all coils are superconducting. The green donut in the middle of the D coils is the plasma.
The following image shows the Tokamak building.
The first stop was the manufacturing facility for the poloidal coils. The following video shows a crane in operation in the assembly hall.
The top and bottom coils are small enough that they can be shipped complete. The others need to be manufactured. The following figure shows the cryostat for testing the poloidal coils.
This poster gives the details of the testing.
We then moved through the entrance to the Tokamak. We were able to enter the Tokamak building itself. Here is Eric in front of an installed toroidal superconducting coil.
First plasma was scheduled for 2025 but may be delayed. This was partly due to Covid and partly due to the inevitable technical glitches in such a complex project.
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!
This paper talks about a collisional-radiative (CR) model that extracts the electron temperature, Te, of hydrogen plasmas from Balmer-line-ratio measurements and is examined for the plasma electron density, ne, and Te ranges of 1010–1015 cm−3 and 5–500 eV, respectively. The first tests of the CR model on the Princeton Field Reversed Configuration-2 (PFRC-2) have been made, including comparisons with other diagnostics. These comparisons are informative as different diagnostics sample different parts of the electron energy distribution function.
Extracted Te(t) for the data for two values of Pc. The shaded region represents the statistical error bar.
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.
This is a really excellent article on nuclear fusion, “Small-scale fusion tackles energy, space applications,” by M. Mitchell Waldrop, written January 28, 2020, Vol 117, No. 4 for the Proceedings of the National Academy of Sciences of the United States of America (PNAS). The article quotes team Dr. Cohen and Mr. Paluszek and provides an excellent and technically accurate discussion of FRCs, heating methods, and fusion fuel physics.
My name is Pavit Hooda, and I was an intern at the Princeton Plasma Physics Laboratory during the summer of 2022. In my time there, I took on the start-up problem of the Direct Fusion Drive (DFD) and developed a compelling solution. A system to power on or re-start the DFD in space is essential for its use, especially in long-duration missions. Therefore, my work has helped us get closer to a space-faring future where the DFD is the means of propulsion for humanity’s missions to the Moon, Mars, and beyond.
Artist’s Rendering of the DFD on a Mission to Mars
The problem at hand was to create an auxiliary power unit that can generate a sufficient amount of power with the use of the Deuterium fuel and liquid Oxygen oxidizer that were on board. The Deuterium is one of the fuels of the fusion within the DFD, and the Oxygen can be recycled from the cabin of the crew. After the power is generated, the objective is to eventually split the deuterium-oxide product back into its constituents for use in their respective areas of the spacecraft. This electrolysis can be done after the fusion core is started and there is a sufficient amount of surplus energy from the DFDs.
The design of the heat engine first begins with the electric pumps that feed the fuel and the oxidizer into the combustion chamber. A turbopump-based feeding system was decided against due to the low mass flow rates that are required to power the DFD. Additionally, the accurate throttle control granted by the use of electric pumps, and the ability to use the batteries on board to spin the pumps, make electric pumps the more viable option. Before the deuterium fuel is fed into the coaxial swirl injector, it is ran across cooling channels surrounding the combustion chamber. This regenerative cooling is performed to heat the deuterium to increase its reactivity and lengthen the lifespan of the combustion chamber by minimizing the effect of the high temperature it is operating at. Additionally, the cooling system provides a healthy temperature gradient for the thermoelectric generation layer that is also wrapped around the combustion chamber. The oxidizer is directly injected into the combustion from its propellant tank.
After passing through the injector and combusting in a successful ignition, the deuterium-oxide steam exhaust is directed towards a turbine system. The turbine system and the combustion chamber are attached with a flange. The turbine system consists of two sets of blades that are separated by a disk that acts like a stator in a steam turbine. The exhaust is first directed towards a doughnut-shaped casing that allows for the heavy water steam to hit the blades in a direction that is parallel to the blade disk’s central normal axis. The two turbine disks are attached to a common axis that extends outside the turbine system’s casing. The rotation of this axle is then used to generate power with an electric generator. Finally, the steam then exits through a large exhaust manifold tube that directs it to a temporary storage container. This design of a heat engine would result in producing 3 MJ, the sufficient amount of power to start up a PFRC, in about 10 minutes. An illustration of the entire design of this system can be seen below.
CAD model of the heat engine
In the pursuit to study the feasibility of this engine, various parts were selected. A 600 W electric generator that matches both the power and mass specifications of the heat engine was found and is shown below.
600 Watt Power Generator
Additionally, the turbine casing in the heat engine matches the geometry and function of a turbocharger that is found as a component in some car engines. The part is displayed below.
Turbocharger component
A significant amount of extensive work still needs to be put into the creation of this heat engine. However, I truly believe that this work presents itself as a good first step in the right direction towards this engine’s small but significant role in humanity’s journey to the Moon, Mars, and beyond.
The above media was taken when we were running with the Rotating Magnetic Field (RMF) Heating System. The video from 10 to 23 seconds shows the plasma rotations are more pronounced and then stabilized later on. The stabilization of plasma is due to the gas puff introduced.
While the PFRC is under upgrade to lower the RF frequency, we have been running the seed plasma, which is PFRC plasma operation without RMF. We also observe rotation in the PFRC seed plasma.
Video recording of seed plasma
This video is taken using Phantom Camera with 1000 frames per second.
Seed plasma seen using Phantom Camera at 20k frames per second.
Our prelaunch campaign is now live on the Spaced Ventures crowdfunding portal! We will be raising money for our new DOE INFUSE awards, to support PFRC-2 experimental operations with new diagnostics, and to design a superconducting PFRC-3!
Potential investors can go to the site, create an account and indicate interest in our raise. This is called “testing the waters!” Those who sign up now will be the first to know when our raise goes live.
Direct Fusion Drive Schematic
Thank you to the Out of This World Design graphics team and the Spaced Ventures team for their support in putting together the pitch! The beautiful new spacecraft render is now on our homepage. The team also made really cool line drawings that show how DFD works!
The Princeton Field Reversed Configuration-2 (PFRC-2) upgrade in field and frequency is underway. We are currently installing new coils around the experiment to increase the magnetic fields and new capacitors to help lower the RF operating frequency – all to reach our target milestones of measuring ion heating! This is an essential next step in our development of Direct Fusion Drive.
The power supplies are stacked in their rack, ready to supply power to the belt coils. The supplies must be programmed to energize for each pulse as they are not cooled and the coils would otherwise overheat. The belt coil holder component on the right was 3D printed at PPPL.
The new 2 nF capacitors, shown above (left image), must be enclosed in a custom copper box that will be part of the tank circuit of PFRC-2. Each component must be carefully designed, including the lengths of the connecting cables, for us to get the right frequency without exceeding voltage limits of the materials.
The above image is of the cable that will connect the tank circuit and the PFRC-2. These cables are very robust, and stiff so that the layout must be carefully planned. We will continue to post updates as we work towards that 2 MHz frequency milestone!
