1. Optical diagnostics applied on PROTO-SPHERA plasmas
V. Lazic, G. Galatola, P. Buratti, F. Alladio, P. Micozzi, F. Andreoli, M. Nuvoli, D. Giulietti, V. Piergotti, , B. Tilia, A. Grosso
ENEA, FSN, Frascati, Italy;
ABSTRACT
In order to study temporal evolution of plasma parameters and the species (atoms, ions and electrons) in the PS reactor, we applied Optical Emission Spectroscopy (OES) in wide spectral interval, from UV to NIR. Thanks to an adjustable focusing system the optical signal was collected at different radial distances from the reactor’s center. Here we report the results of the measurements performed on PROTO-SPHERA reactor filled with hydrogen.
INTRODUCTION
PROTO-SPHERA (PS) is an innovative plasma confinement concept based on the formation of a toroidal plasma around an axial screw pinch which provides the toroidal magnetic field [1]. Central part of the plasma, i.e. Plasma Centerpost (PC) is an arc discharge driven by DC voltage between two annular electrodes. Presently, the PS is capable to routinely operate with a 10 kA stable longitudinal current in the PC and 7 kA in the Torus for 0.5 seconds.
PS is equipped with three color camera for monitoring the plasma torch, the cathode and the anode area. However, diagnostics of the plasma produced by PS has started only recently, by applying OES and interferometric measurements of the electron density. The results of spatially resolved OES detection will be exploited in future an input for the interferometric measurements, which presently consider an uniform electron density and thus supply only the average value across the observation path.
The plasma is out of Local Thermal Equilibrium (LTE) and the excitation temperatures (T) calculated from H I lines through the Boltzmann plot are T< 1500 K, the values too low and incompatible with the presence of ionic species (Cu II, W II). However, ratio of two H I lines is an indicator of relative T changes (Fig. 5-top) and from here, after the plasma formation the T keeps stable except some increase before the end of the discharge (>840 ms). Ratio I(Cu II)/I(Cu I) has the minimum value in the interval ms (Fig. 5-top). Basing on the Saha equation and assuming the partial LTE (p-LTE), the electron density Ne is maximum for the interval ms, and it decreases later.
EXPERIMENTAL
Spectral acquisition: nm by three compact spectrometers (Avantes), resolution 0.09 nm (UV) nm (IR), measuring interval 5 ms.
Optical system (Fig. 1): a fiber bundle containing three 0.6 mm quartz fibers, two lenses (f1= +50 mm, f2= -200 mm) with adjustable distance in order to collect the plasma emission from the reactor’s center (Z= 0) up to 30 cm towards the lenses (Z= -30 cm). The area imaged onto the optical fibers is reduced from diameter of 0.58 mm to 0.30 mm when passing from Z= 0 to Z= -30 cm, as simulated by the ZEMAX software. Note: the measurements at Z positions were performed for different runs.
HeNe beam: dia. 3 mm, optically attenuated, used to monitor changes in the plasma transparency and/or the refractive index.
Plasma (Fig. 2): the ignition at delay of 750 ms from the reference trigger, duration 250 ms.
Figure 5:Time behavior at Z=0 of: Top - ratio I( Hδ)/I(Hγ) and I(Cu II)/I(Cu I); Bottom – electron density
Figure 6:Time behavior at different positions Z of: Top - Hγ intensity normalized on the detection area; Bottom – electron density.
The central part of the reactor has the lowest H emission (Fig. 6-top) due to a high ionization, confirmed by calculating the Ne from Inglis-Teller formula [2-3]. After the plasma formation, at the radius of 20 cm from the center the Ne is almost an order of magnitude lower than in the first 10 cm (Fig. 6-bottom) but it increases shortly after at about 810 ms when the core (Z =0) starts to expand.
In the interferometric measurements it was assumed that Ne profile was constant over radius of 30 cm, producing the average value of 1.5*1014 cm-3. However, the initial plasma is limited to the radius of about 20 cm (Fig. 5-top) and it has much higher density at the center. Recalculating the interferometric data for the plasma core of 10 cm radius, the resulting Ne is 1.4*1015 cm-3, analogue to the values obtained by OES .
Beside oscillations due to mechanical vibrations, from the plasma ignition the intensity of HeNe line starts to grow. Increasing the plasma density, its refractive index N is reduced below 1 (Tab. 1). The extended plasma behaves as a weak negative lens and shifts forward the focal point of the HeNe beam, bringing it closer to that of the system which collects the plasma emission. In this way, a larger portion (diameter) of the HeNe beam enters into the optical fiber i.e. spectrometer.
Figure 1: Experimental lay-out for OES measurements
Figure 2: Photo of the plasma
RESULTS
The spectra (Fig. 3) from the plasma show a presence of H, O, C, Cu and W, the last two elements present in the electrodes.
Table 1: Refractive index of plasma at nm for different electron densities
Ne (1015 cm-3) N
1.5
0.74
0.4
0.23
Figure 3: Spectrum acquired from the central part of discharge at delay of 765 ms. Beside C I (247.8 nm), the lines below 300 nm mainly come from W I, W II and Cu II species.
Figure 7: Time behavior of the detected HeNe laser peak
In the plasma core (Z=0) the peak emissions from Cu I and Cu II occur at delay 765 ms, followed by peaks of H I (Fig. 4). This might indicate that Cu, with lower ionization energy than H, mainly contributes to the plasma formation and successive heating through acceleration of free electrons.
CONCLUSIONS
The central plasma column in the PROTOSFERA reactor is highly ionized, reaching the electron density Ne of about 1.4*1015 cm-3, here estimated roughly from Inglis-Teller formula.
The electron density is strongly inhomogeneous along the radial direction, where the difference covers almost one order of magnitude inside the radius of only 20 cm.
Initially, the plasma has a radius if about 20 cm and later expands to c.a. 30 cm.
The plasma behaves as a negative lens for the incoming collimated HeNe beam, increasing its collection efficiency, particularly during the maximum plasma expansion.
By combining the spectroscopic data relative to the species distribution in the reactor and the interferometric measurements that take into account such distribution, it will be possible to obtain affordable, time resolved plasma characterization.
Figure 4:Time behavior of Hδ, Cu I ( nm) and Cu II ( nm) lines at the center (Z=0)
[1] F. Alladio, et al. -“Design of the PROTO-SPHERA experiment and of its first step (MULTI-PINCH)” - Nucl. Fusion 46 (2006) S613–S624.
[3] M. Ivkovic, et. al., V. Mitrofanov, “Low electron density diagnostics: development of optical emission
spectroscopic techniques and some applications to microwave induced plasmas”, Spectrochim. Acta Part B 59 (2004) 591–605.
[2] A. V. Mitrofanov, “On a revised version of the Inglis-Teller Formula”, Sov. Astronom. J. 16(5) (1973)
iCFDT5 2018, Frascati (Italy)