Author Archives: Sangeeta Punjabi-Vinoth

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

Posted on by
Reply

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.

Research paper on “A diagnostic to measure neutral-atom density in fusion-research plasmas” has been published in the Review of Scientific Instruments

Posted on by
Reply

A research paper on neutral-atom density diagnostics on the PFRC-2, written with our colleagues and collaborators, has been published and is titled “A diagnostic to measure neutral-atom density in fusion-research plasmas” DOI: https://doi.org/10.1063/5.0101683. It is part of the “Proceedings of the 24th Topical Conference on High-Temperature Plasma Diagnostics.”

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.

Collisional Radiative Model paper Published in Review of Scientific Instruments 

Posted on by
Reply

Our new collisional radiative model paper is titled “Evaluation of a collisional radiative model for electron temperature determination in hydrogen plasma” DOI: https://doi.org/10.1063/5.0101676. It is part of the “Proceedings of the 24th Topical Conference on High-Temperature Plasma Diagnostics.”

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.

Plasma oscillations seen in seed and RMF plasma using Phantom Camera

Posted on by
Reply

The plasma rotation of the PFRC is being analyzed by the Phantom Camera in the Princeton Plasma Physics laboratory.

Video recording of RMF plasma

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.

Upgrade underway of Princeton Field Reversed Configuration-2

Image

Reply

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!

PFRC-2 has been funded by ARPA-E OPEN 2018, NASA NIAC and DOE Fusion Energy Sciences grants.