Princeton Plasma Physics Laboratory (PPPL)

The Advanced Concept Torus (ACT-1) was a steady state toroidal device which was operated at PPPL. Its vacuum chamber was made of aluminum with a water cooling system and 26 toroidal field coils capable of produce a 0.56 T magnetic field on the axis. ACT-1 had a toroidal plasma with a minor radius of 10 cm and a major radius of 59 cm. It was used for the study of radiofrequency heating, specifically in lower hybrid waves and ion cyclotron waves. In 1985, ACT-1 was converted into the Current Drive Experiment (CDX)

The Current Drive Experiment (CDX) was a device located at PPPL. It was an upgrade of ACT-1 made to explore current drive and current profile control methods. CDX was the first tokamak to use the DC helicity injection concept (a method to continuously inject electrons to maintain the plasma current) to initiate and maintain the plasma discharge. In 1989 CDX was upgraded into CDX-U to explore the helicity injection in a small aspect ratio device.

The Current Drive Experiment-Upgrade (CDX-U) was an upgraded version of CDX located at PPPL. CDX-U was the first spherical tokamak in the United States and was originally made to continue the research from CDX but in a spherical tokamak configuration. In 2000, CDX-U began operations with a liquid lithium limiter in contact with the plasma. This addition made a major improvement in the energy confinement time since the lithium absorbs impurities which drive energy losses. These results led to the transformation of the device into LTX in 2005.

The Facility for Laboratory Reconnection Experiments (FLARE) is a joint project of five universities and two DoE national laboratories: Princeton University, University of California - Berkeley, University of California - Los Angeles, University of Maryland, University of Wisconsin - Madison, PPPL and LANL. It started operations in 2018 at Princeton University and since 2019 it is located at PPPL. FLARE's design is based in MRX but at a larger scale. As MRX, its main objective is to study the magnetic reconnection process for astrophysical and fusion research. FLARE has a 3m diameter and is 3.6m long and can produce magnetic fields up to 0.5T.

The Floating Multipole Machine (FM-1) was a spherator built and designed at PPPL. It was a bigger version of the LSP device. This device had an internal superconductive ring of 0.75 m major radius with a current capability of 350 kA. This ring levitated due to its own magnetic field interacting with 5 sets of stabilization coils and was able to remain afloat for an 8 hour experimental shift in a room temperature environment. FM-1 reached electron temperatures around 100 eV and ion temperatures below 5 eV . Its typical magnetic field was around 0.3 T and reached a confinement time up to 3 seconds. FM-1 is considered one of the earliest poloidal divertor experiments.

The Levitated Spherator (LSP) was the first spherator built with a levitated internal ring at PPPL. LSP was similar to SP-3 in geometry but without the internal ring supports. The idea of levitating the ring instead of using mechanical supports was to increase the confinement time by eliminating the particle losses due to asymmetries caused by the supports in previous spherators.

It had a superconductive internal ring with a radius of 0,46 m and capable of transporting a 85 kA current, that was levitated with magnetic fields. To keep the superconductivity, the ring was filled with liquid helium to keep it cold. It used ECRH heating with 10ms pulses of 1kW 2.45 GHz microwaves and steady heating of 60 W 3.5 GHz microwaves, which produced temperatures up to 10 eV. LSP was also heated ohmically, reaching temperatures up to 100 eV. This device had an average magnetic field of 0.1 T. FM-1 was a larger version of this device.

The Lithium Tokamak Experiment (LTX) was a tokamak located at PPPL that was constructed as an upgrade of CDX-U. It was the first device to have a full liquid lithium wall and was used to study lithium as a plasma facing material. LTX had a toroidal field of 0.17 T and reached a plasma current around the 100 kA. It had a major radius of 40 cm and an aspect ratio of 1.6. In 2016, LTX ended operations to be upgraded to LTX-β.

The Lithium Tokamak Experiment β (LTX-β) is a spherical tokamak based at PPPL. LTX-β is the upgraded version of LTX. Although LTX-β maintains LTX's geometry, the toroidal field, the plasma current and the heating were improved. Among the upgrades, two neutral beam injection systems were installed, both provided by TAE Technologies. The main objective of LTX-β is to test the confinement of hotter plasmas than those achieved in previous devices.

Understanding magnetic reconnection is important for both fusion physics and astrophysics communities. Due to its importance in its study, the MRX receives funding from various agencies, including DOE and NASA. MRX inspired the design of FLARE, a bigger device made to continue reconnection studies.

Model A's main goal was to demonstrate that the magnetic confinement idea works. Once this was achieved, Model A went out of operation and research continued with the construction of Model B.

Model B-1 was the first version of a series of devices built at PPPL under the name "Model-B". Like Model A, this stellarator was made from 5 cm diameter Pyrex tubes in a figure-eight configuration but with a much stronger magnetic field (around 3T). It utilized ultra-high vacuum and the plasma reached 100 eV temperatures using ohmic heating. The magnetic confinement produced with this device was excellent for single particles but it failed to maintain a confined plasma for greater than 10 ms, mainly because of collective plasma behavior and transport.

Model B-2 was a bigger version of Model B-1. It still used 5 cm cross-section diameter tubes but also included a 50 cm long section with an expanded diameter. It used ohmic heating, which produced similar results to the ones obtained in B-1, including short confinement times. Model B-2 was built to study the magnetic pumping process of heating. Although this process made it possible to reach higher temperatures, the presence of impurities and the short confinement times precluded the achievement of the expected temperature of 1 keV. This stellarator was exhibited at the United Nations' Second International Conference on the Peaceful Uses of Atomic Energy in Geneva, Switzerland.

The results of the Model B-3's ion-cyclotron heating and the general data gained about stellarator operating conditions led to the construction of Model B-66.

Model B-640 was the Model B-64 reassembled in a race-track geometry (represented in the 0) for an experimental campaign in 1956. In this new geometry, the rotational transform was zero and a strong ohmic heating was used. Even though it was not realized at that moment, that characteristic made the device a tokamak and not a stellarator. For this reason the B-640 is considered the first tokamak in the United States.

Model B-65 was a race-track shaped stellarator built at Princeton University. It was the first race-track device with helical windings to provide rotational transform and magnetic shear. It also included Model B64's divertor which improved the heating, which led to reported temperatures above 100 eV. Model B-65 was also used for the first high power ion-cyclotron heating experiments using deuterium.

Finally, in 1969, Model C stopped operations and was converted into a tokamak geometry, changing its name to Symmetric Tokamak (ST).

The National Spherical Torus Experiment (NSTX) was a spherical tokamak built at PPPL. It began operations in February of 1999. The NSTX device was built with the objective of studying the behavior of the spherical tokamak configuration in the mega ampere regime. It was designed to work with a confinement magnetic field of 0.55 T and to reach a central temperature between 1 to 3 keV. In 2012, NSTX ended operations to undergo a mayor upgrade. Once this upgrade was finished, the device became NSTX-U.

The National Spherical Torus Experiment Upgrade (NSTX-U) is the name which NSTX, located at PPPL, is known after a major upgrade that began in 2012. The main upgrades made were a larger centerstack to improve the magnetic confinement, an additional neutral beam injector to improve heating and structural enhancements to adapt the coils and vacuum vessel to the new working conditions. NSTX-U was operational for 10 weeks in 2016 but a failure in one poloidal coil forced the device to shut down and undergo a major repair process. It is expected that NSTX-U will resume operations in 2021.

The Princeton Beta Experiment (PBX) was an upgrade made to PDX at PPPL. Its objective was to produce plasma with higher β values than the obtained in conventional tokamaks by reducing some instabilities presented in other devices. This reduction was achieved by shaping its plasma cross section like a kidney bean, which also increased the plasma current capacity of the plasma. PBX was able to achieve β values of 5.3%, a record at its time. In 1986, PBX was shut down to be transformed into PBX-M.

Princeton Beta Experiment Modified (PBX-M) was an upgrade of PBX. It was an advanced concept tokamak built with the objective to show the practicality of controlling the plasma current and pressure distributions to achieve stable and advanced operating regimes and to avoid plasma disruptions. As part of the modifications, the plasma major radius was increased from 1.45 m to 1.65 m, which made it possible to include a divertor and new coils inside the vacuum vessel. Other upgrades were the improvement of the plasma shaping control system, the installation of an Ion Bernstein wave antenna and the installation of new diagnostics. With this modifications, the volume-averaged toroidal β values and ion temperatures reached higher values than in PBX, reaching a β of 6.8% by 1988.

PBX-M final operation occurred in November 1993, but it remained unused at PPPL until 2003, when it was finally disassembled.

The Poloidal Divertor Experiment (PDX) was a tokamak built in PPPL in the late 70's that was designed with a poloidal divertor. Its main goals were to study the effectiveness of the poloidal divertor in the control of impurities and instabilities, as well as the study of different plasma cross section shapes in the device. In 1982, PDX was modified into a closed divertor geometry and the outer divertor coils were removed. In 1983, PDX was shut down and was converted into PBX.

Proto S-1A (sometimes known as Proto S-1A/B) was a prototype of S-1 built at PPPL in 1980. Its parts were a scaled down version of the S-1 device, 1/6 of the size of the main device. It was made to demonstrate the viability of the spheromak concept. Proto S-1A had a 15cm major radius and 3cm minor radius flux core which contained a 3-turn poloidal flux coil and a 40-turn toroidal flux coil. The core was covered with a 3mm thick stainless steel enclosure that functioned as a vacuum vessel and helped to symmetrize the induced fields. Discharges were made using hydrogen, helium and argon with pressures around 20 to 50 mTorr. Proto S1-A was able to maintain its fully formed configuration for 12 μs. After Proto S-1A, another prototype was built: Proto S-1C

Proto S-1C was a prototype of S-1 built at PPPL. It was a bigger version of Proto S-1A and 1/3 of the size of S-1. It was used for impurity control studies.

The S-1 was a spheromak located at PPPL. Its main objective was to study confinement and MHD stability of the spheromak concept. It began operations in September 1983, after two operational prototypes were tested: Proto S-1A and Proto S-1C. During its operation, the toroidal plasma currents reached approximately 600 kA and maximum plasma electron temperatures up to 500 eV. The vacuum vessel was made of two stainless steel domes with a large viton seal between them. Its base pressure was around 10^-8 torr.

In 1987 S-1 was shut down for budgetary reasons. The studies of the effects of spheromak plasma compression could not be completed before the project ended.

Symmetric Tokamak (ST) was the first tokamak in the US, designed and built at PPPL. The origin of the design of ST was the Model C stellarator. After results in T-3 revealed that heating was more successful in tokamaks than in early stellarators, the team at PPPL decided to change the stellarator configuration to a tokamak configuration. The transformation required changes in the vacuum vessel geometry, changing from racetrack to a circular configuration. The magnetic coils were designed to produce magnetic fields between 3.5 and 4.5 T. After 5 months of extensive transformation, the ST finally started operations on May 1st, 1970. The ST quickly achieved similar results to T-3, which led to the end of the stellarator era at that time. ST achieved electron temperatures of 2.2 keV.

In December 1993, the team at PPPL started an experimental campaign with the TFTR of 50/50 D-T discharges. During this campaign, TFTR established a world record of 6.3 MW fusion energy production, a milestone previously held by JET. In May 1994, TFTR set another fusion energy production record of 9.2 MW.