Princeton Plasma Physics Laboratory Research Articles

Operational Experience with the DATS Sampler during the Canadian Tritium Modeling Experiment

An international tritium model validation experiment was held at Chalk River, Canada, during June 1987. The Princeton Plasma Physics Laboratory (PPPL) Differential Atmospheric Tritium Sampler (DATS) was one of the many types of tritium samplers used for this experiment. Besides the modeling data that were produced from this experiment, the authors learned how well our tritium samplers performed when a known tritium quantity was released. The DATS were set up at 50, 100, 200, and 400 meters downwind from the release point. Data were collected during the release period and for the next 24 hours. While the units worked very well in the field, valued operational experience was gained in the recovery of the tritium from the silica gel. Because of delays in the analysis of the collected samples, it became difficult to recover the HTO fraction quantitatively. Indications are that molecular sieves are more suitable for samples which are not going to be processed immediately. This paper reports on the field set up, the measurement results, and operational experience.

LOCUS database system

LOCUS is a database system for creating, manipulating, and interactively investigating databases. LOCUS was written in response to the need to study the data from the experimental devices at the Princeton Plasma Physics Laboratory and is now also used extensively at other fusion laboratories. An example of its use is the determination of scaling laws for tokamak plasmas. To do this, LOCUS allows the user to enter and modify data, define subsidiary variables as arbitrary functions of original data, plot and fit such variables, and perform multiple linear regression analysis. Through a flexible interactive interface the program is easy to learn and largely self-documenting. There are two implementations of LOCUS: one that manipulates data in VAX/VMS ISAM files and another that manipulates data in the commercial relational database management system INGRES.

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CIT project.

CIT (Compact Ignition Tokamak) project is planned to be the major next project in US magnetic fusion program. The mission of the CIT will be to realize, study and optimize fully ignited plasma discharges. This review introduces the conceptual design of CIT, especially on key technologies and physical bases which was presented at technical review committee meeting held in January, 1988, at Princeton Plasma Physics Laboratory.

Fusion's $372-Million Mothball

Lawrence Livermore National Laboratory's Mirror Fusion Test Facility (MFTF-B) was built to prove that a design using magnetic mirrors could compete in an international race to produce a commercial fusion reactor. The MFTF-B was to be used to propel the magnetic mirror program into a real contest against Livermore's arch rival: the Princeton Plasma Physics Laboratory and its tokamak machine. On 21 February 1986, after 9 years of construction and $372 million, MTFT-B was officially dedicated. However, due to budget restriction DOE demanded the facility be put on standby the very next day.

Measurements of the Neutron and Gamma-Ray Fluences in the TFTR Test Cell Due to a Point Source Simulating D-T Fusion Plasma Neutron Production

Spectral distributions of high-energy neutrons (0.9 less than or equal to E/sub n/ less than or equal to 14.5 MeV) and of high-energy gamma rays (0.4 less than or equal to E/sub ..gamma../ less than or equal to 9.4 MeV) due to a deuterium-tritium (D-T) neutron point source simulating the extended fusion plasma neutron source in the Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory are reported. A D-T neutron generator was positioned inside the vacuum vessel at ten different locations around the torus. Neutrons and gamma rays were detected by a liquid-scintillator-based detector (4.65-cm diam X 4.22 cm high) with electronic pulse-shape discrimination to differentiate between events in the detector due to incident neutrons and those due to incident gamma rays. The detector was placed on the median plane of the reactor at 8.85 m from the geometric center of the TFTR. Two spectral distributions, one for neutrons and the other for gamma rays, were obtained for each of 18 measurements. The neutron data exhibit a high-energy peak dominated by uncollided primary-energy neutrons and a low-energy contribution from the scattered neutrons. The gamma-ray data exhibit a high-energy contribution due to neutron capture gamma rays and amore » low-energy contribution due to gamma rays following neutron inelastic scattering reactions.« less

Hydrogen isotope retention and release from copper

The Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory is considering the use of tritium neutral beams during the deuterium–tritium phase of operation in 1990. A concern is the tritium inventory that will develop in neutral beam components such as the copper calorimeter. In this paper we report on measurements and calculations of hydrogen isotope retention and release for copper under ion bombardment. Retention of deuterium trapped in the near-surface region is measured as a function of temperature using the D(3He, p) α nuclear reaction technique with an 800-keV 3He+ analysis beam. The rate of molecular recombination (Kr) at the copper–vacuum interface is determined from plasma driven permeation studies in our Tritium Plasma Experiment facility. The measurements indicate that Kr=6.7±0.1×10−18 cm4/s over the temperature range 575–825 K, in agreement with theory. These data are used as inputs to the diffuse computer code to provide a model for tritium–copper interactions. The model is tested against measured pressure rises during discharges in the TFTR neutral beam line, and is then used to predict tritium inventory in the copper ion beam dumps during future TFTR tritium operation.

Comparisons of Calculated and Measured Spectral Distributions of Neutrons from a 14-MeV Neutron Source inside the Tokamak Fusion Test Reactor

A recent paper presented neutron spectral distributions (energy greater than or equal to 0.91 MeV) measured at various locations around the Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory. The neutron source for the series of measurements was a small deuterium-tritium generator placed at various positions in the TFTR vacuum chamber. The results of neutron transport calculations are presented and compared with these experimental data. Applications to breeding blankets are discussed.

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Princeton Tokamak Reaches Record Plasma Ion Temperature

The large Tokamak Fusion Test Reactor at the Princeton Plasma Physics Laboratory has entered a new regime of “enhanced confinement” operation. The first spectacular consequence of this new regime is the attainment of a record ion temperature of 20 keV—more than 200 million kelvin. For the first time, a tokamak is creating plasmas with ion temperatures hot enough for ignition and fusion‐reactor operation.

Tritium Assay of Li2O in the LBM/LOTUS Experiments

The Lithium Blanket Module (LBM) is an assembly of over 20,000 cylindrical lithium oxide pellets in an array representative of a limited-coverage breeding zone for a toroidal fusion device. A principal objective of the LBM program is to test the ability of advanced neutronics coding to model the tritium breeding characteristics of a fusion device blanket. The LBM has been irradiated at the Ecole Polytechnique Federale de Lausanne (EPFL) LOTUS facility with a 14 MeV point-neutron source. Princeton Plasma Physics Laboratory (PPPL) and EPFL assayed the tritium bred in lithium oxide diagnostic samples placed at various positions in the LBM. PPPL employed a thermal extraction technique while EPFL used a dissolution method. The results form the assay are reported and compared to MCNP Monte Carlo neutronics calculations for the LBM/LOTUS system.

Neutron diagnostics on TFTR utilizing the Campbelling technique

Modified commercial equipment installed on the tokamak fusion test reactor (TFTR) at Princeton Plasma Physics Laboratory (PPPL) utilizes Campbell’s mean square voltage theorem to monitor the neutron source strength at neutron count rates orders of magnitude above the capability of the count rate mode. Campbelling uses the large amplitude fluctuations from neutron fission events in the detectors to discriminate against small amplitude γ ray and other noise events. Source strengths yielding equivalent count rates a factor of 5 greater than possible in the conventional count rate mode have been obtained to date. The concept of Campbelling is discussed and the particular application to TFTR is illustrated. Fundamental advantages are the extended useful range of the detectors by a factor of ∼104 and gamma rejection by a factor of ∼103. Some results are shown and the neutron source strengths obtained are compared to those from conventional counting circuits and from other detectors whose outputs have not yet suffered counting losses.

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Verification of the Taylor (minimum energy) state in a spheromak

Experimental measurements of the equilibrium in the S-1 spheromak [M. Yamada, J. Sinnis, H. P. Furth, M. Okabayashi, G. Sheffield, T. H. Stix, and A. M. M. Todd, in Proceedings of the US-Japan Symposium on Compact Toruses and Energetic Particle Injection (Princeton Plasma Physics Laboratory, Princeton, NJ, 1979), p. 171] by use of magnetic probes inside the plasma show that the final magnetic equilibrium is one that has relaxed close to the Taylor (minimum-energy) state, even though the plasma is far from that state during formation. The comparison is made by calculating the two-dimensional μ profile of the plasma from the probe data, where μ is defined as μ0 j∥/B. Measurements using a triple Langmuir probe proved evidence to support the conclusion that the pressure gradients in the relaxed state are confined to the edge region of the plasma.

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Magnetic flux trapping during field reversal in the formation of a field-reversed configuration

The flow of plasma and magnetic flux toward a wall is examined in a slab geometry where the magnetic field is parallel to the wall. Magnetohydrodynamic (MHD) flow with a quasisteady approximation is assumed that reduces the problem to three coupled ordinary differential equations. The calculated behavior shows that a thin current sheath is established at the wall in which a variety of phenomena appear, including significant resistive heating and rapid deceleration of the plasma flow. The sheath physics determines the speed at which flux and plasma flow toward the wall. The model has been applied to the field-reversal phase of a field-reversed theta pinch, during which the reduced magnetic field near the wall drives an outward flow of plasma and magnetic flux. The analysis leads to approximate expressions for the instantaneous flow speed, the loss of magnetic flux during the field reversal phase, the integrated heat flow to the wall, and the highest possible magnetic flux retained after reversal. Predictions from this model are compared with previous time-dependent MHD calculations and with experimental results from the TRX-1 [Proceedings of the 4th Symposium on the Physics and Technology of Compact Toroids, 27–29 October 1981 (Lawrence Livermore National Laboratory, Livermore, CA, 1982), p. 61] and TRX-2 [Proceedings of the 6th U.S. Symposium on Compact Toroid Research, 20–23 February, 1984 (Princeton Plasma Physics Laboratory, Princeton, NJ, 1984), p. 154] experiments.

The role of magnetic reconnection and differential rotation in spheromak formation

The physical processes involved in the formation of a spheromak are studied. Using one-fluid axisymmetric resistive magnetohydrodynamic equations together with a temperature equation, investigations of the essential physics aspects of the formation specifically for the new Maryland Spheromak (MS) [J. Antoniades, C. Chin-Fatt, A. DeSilva, G. Goldenbaum, R. Hess, and R. Shaw, in Proceedings of the Sixth U. S. Symposium on Compact Torus Research, Princeton, 1984 (Princeton Plasma Physics Laboratory, Princeton, NJ, 1985), p. 65] have been performed. These studies elucidate the role of differential rotation in accelerating the penetration of toroidal field Bφ and in forcing I≡ rBφ to be a flux function. Another important aspect of the formation is the magnetic reconnection that occurs at the X point between the two reversal coils. A strong toroidal current is generated by the reconnection, and the X point is converted into an O point that finally becomes the magnetic axis of the spheromak. In this latter phase of reconnection, the plasma is heated very strongly. Studies have also been done on the effect of the difference in phase and time scales for the reversal and Iz banks for the purpose of optimization. The effect of radiation from carbon and oxygen is examined and found to have very little effect on the proposed formation scheme. The results are also relatively insensitive to changes in the anomalous electron thermal conduction models used.

A New Tritium Monitor for the Tokamak Fusion Test Reactor

At DT-fueled fusion reactors, there will be a need for tritium monitors that can simultaneously measure in real-time the concentrations of HTO, HT and the activated air produced by fusion neutrons. Such a monitor has been developed, tested and delivered to the Princeton Plasma Physics Laboratory for use at the Tokamak Fusion Test Reactor (TFTR). It uses semipermeable membranes to achieve the removal of HTO from the sampled air for monitoring and a catalyst to convert the HT to HTO, also for removal and monitoring. The remaining air, devoid of tritium, is routed to a third detector for monitoring the activated air. The sensitivities are those that would be expected from tritium instruments employing conventional flow-through ionization chambers: 1-3 ..mu..Ci/m/sup 3/. Its discriminating ability is approximately 10/sup -3/ for any of the three components (HTO, HT and activated air) in any of the other two channels. For instance, the concentration of HT in the HTO channel is 10/sup -3/ times its original concentration in the sampled air. This will meet the needs of TFTR.

The FELIX Experiments: Measurements of Electromagnetic Effects

Three major series of experiments have been conducted with the FELIX facility at ANL. Experiments on the coupling between eddy currents and angular displacements of loops and plates in crossed constant and changing magnetic fields demonstrated that the coupling effects reduced the peak currents and deflections. These experiments were carried out by a team of investigators from Princeton Plasma Physics Laboratory (PPPL) and ANL and are reported elsewhere. Experiments measuring eddy current effects in flat plates provided data to validate 2-D and 3-D eddy current computer codes. Experiments measuring eddy current effects in hollow cylinders with different ratios of thickness to diameter explored the limitations of using 2-D codes to model 3-D geometries. The plate and cylinder experiments are described in detail.