MAPS 3D is a project of Istituto Nazionale di Fisica Nucleare which aims to implement an unprecedented characterisation of magnetised laboratory plasmas by means of high-resolution spectropolarimetry in the visible range. This observational technique is routinely applied to astrophysical plasmas in order to derive physical parameters of stars. Through the experimental set-up of MAPS 3D spatial resolution of the emitting plasma will be obtained, giving the possibility to localise the emitters from hot and cold plasma regions, as well as to estimate the charge state distribution. This innovative plasma characterisation, not achievable by means of the present diagnostic techniques, will allow a better comprehension

of laboratory plasma structure, heating processes and magnetic confinement, with strong implications for Accelerator Physics, Plasma Physics, Astrophysics and Nuclear Astrophysics

Giarrusso, M.

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DOI 10.1393/ncc/i2019-19227-2Communications: SIF Congress 2018IL NUOVO CIMENTO 42 C (2019) 227The INFN project MAPS 3DM. Giarrusso(∗)INFN, Laboratori Nazionali del Sud - Catania, Italyreceived 15 February 2019Summary. — MAPS 3D is a project of Istituto Nazionale di Fisica Nuclearewhich aims to implement an unprecedented characterisation of magnetised labora-tory plasmas by means of high-resolution spectropolarimetry in the visible range.This observational technique is routinely applied to astrophysical plasmas in order toderive physical parameters of stars. Through the experimental set-up of MAPS 3Dspatial resolution of the emitting plasma will be obtained, giving the possibility tolocalise the emitters from hot and cold plasma regions, as well as to estimate thecharge state distribution. This innovative plasma characterisation, not achievableby means of the present diagnostic techniques, will allow a better comprehensionof laboratory plasma structure, heating processes and magnetic confinement, withstrong implications for Accelerator Physics, Plasma Physics, Astrophysics and Nu-clear Astrophysics.1. – IntroductionMAgnetised Plasma Spectropolarimetry 3D (MAPS 3D) is a two-year project ofIstituto Nazionale di Fisica Nucleare (INFN) which aims to characterise magnetisedlaboratory plasmas in an innovative way through high-resolution spectropolarimetry,an observational technique commonly adopted in Astrophysics to determine physicalparameters of stellar atmospheres. The project started in 2018 at INFN-LaboratoriNazionali del Sud (INFN-LNS) in collaboration with researchers from Istituto Nazionaledi Astrofisica-Osservatorio Astrofisico di Catania (INAF-OACT), Università degli Studidi Catania, University of Michigan, University of Cambridge and European SouthernObservatory. It can be considered as a transversal project both from the operating pointof view, since it requires instruments and competences from different fields of Physics,and from the application point of view: a detailed plasma characterisation is mandatoryfor multiple purposes and will have strong implications for Plasma Physics, AcceleratorPhysics, Astrophysics and Nuclear Astrophysics.(∗) E-mail: marina.giarrusso@lns.infn.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 M. GIARRUSSOLaboratory plasmas can be created through the Electron Cyclotron Resonance (ECR)heating in magnetic traps, where gases injected in a chamber are ionised by microwavesand confined by magnetic fields. These plasmas represent a useful test bench for stud-ies involving different fields of Physics: from the astrophysical point of view, the ECRplasmas reproduce many stellar environments, so that the analysis of their physical con-ditions should allow to obtain advancement in observational Astronomy, improving ourknowledge about Cosmos. Also, a better comprehension of the ECR plasma ignition andstability is required to fully control the ion beam extracted from an ECR Ion Sourcefeeding particle accelerators (ECRIS [1]). Large efforts are being made for this purposeand an improvement in the knowledge of physical plasma properties is then mandatory.2. – Plasma diagnostic techniques: state of the artIn this section I summarise what widely discussed in [2], referring the reader to thisarticle for details.Present diagnostic techniques developed for laboratory plasma range from X-ray tonear infrared. Non-invasive methods are based on imaging and low-resolution (R =λ/Δλ < 40000) spectroscopy in the visible range and in the X-ray domain, and donot affect the plasma physics with respect to probes. Anyway all methods only giveinformation about the electron component, in terms of density, temperature and energydistribution, not being able to characterise ions.2.1. Optical emission spectroscopy . – Indeed in the visible range Optical EmissionSpectroscopy (OES) also provides relative abundances of atomic and molecular popula-tions by applying the line intensity ratio method [3]. Nevertheless, a strong limit of OESis the absence of spatial resolution: spectra are obtained by averaging the contributionsof emitting plasma volumes with very different temperatures and densities, so that it isnot possible to localise hot and cold regions inside the chamber.A further limit is the low spectral resolution which prevents line identification, asshown in fig. 1. Only lines with distance equal to, or greater than, the instrumentalprofile width Δλ = λ/R can be distinguished at wavelength λ, that is the lower thespectral resolution the worst the line identification and then the ion characterisation.Low R-values also affect the determination of emitter velocities v from Doppler shift,which cannot be obtained better than Δv = c · Δλ/λ = c/R (where c is the speedof light in vacuum). Analogously, the width of a spectral line has to be larger thanthe instrumental profile in order to appreciate broadening mechanisms (thermal andcollisional).Table I reports the values of the quantities mentioned above for different spectralresolution and at wavelength λ = 500 nm. Note that the ion temperature (and then theenergy) is expected to be of a few eV, measurable only with spectral resolution of theorder of 100000.3. – Spectral emission from magnetised plasmasWe here recall that in the presence of a magnetic field B, the atomic energy levelssplit in sublevels, so that the spectral lines also split in a series of Zeeman components.By indicating with m the magnetic quantum number, these are called π components ifrelated to transitions with Δm = 0, σb (blue-shifted) components for Δm = +1 and σr(red-shifted) components for Δm = −1. By indicating the average separation betweenTHE INFN PROJECT MAPS 3D 3Fig. 1. – Synthetic solar spectra computed in the same wavelength range with low and highspectral resolution R.the σb and σr components with Δλσ, under the weak-field approximation the intensityof B in G is given by(1) |B| = Δλσ2 · 4.67 · 10−13λ2geff,where geff is the effective Landé factor and λ is expressed in Å. Then, Zeeman splittingshave to be detected to avoid incorrect results in line identification as well as to determinethe magnetic field strength. Since the instrumental profile width corresponds to theminimum Δλσ that can be observed, the spectral resolution sets the minimum magneticfield modulus measurable (table I).Table I. – For different values of spectral resolution R chosen as order of magnitude, at afixed wavelength λ = 500nm, the instrumental profile width Δλ (corresponding to the minimumwavelength separation detectable), the minimum ion velocity vmin measurable from Doppler shifttogether with the corresponding kinetic energy Kmin for a plasma of40Ar, and the minimummagnetic field modulus |B|min are listed. The field modulus have been calculated according toeq. (1) by assuming Δλσ = Δλ and geff = 1.R Δλ [nm] vmin [km/s] Kmin [eV] |B|min [T]250 2 1200 3 · 105 855000 .10 60 750 4.310000 .05 30 190 2.1100000 .005 3 1.9 .214 M. GIARRUSSOFig. 2. – Zeeman components as observed along and perpendicular to the magnetic field direction.In addition, the absence of information about the magnetic field geometry can lead towrong estimation of plasma parameters when spectroscopically determined, since boththe strength and the shape of spectral lines from magnetised plasma depend on theangle θ between the line of sight and the field direction. As shown in fig. 2, it meansthat by observing along B only the σ components circularly polarised at their maximumintensity are present, since the π component (always linearly polarised) is parallel to thefield and disappears. As θ increases, the intensity of the π component also increases,while σ components appear to be elliptically polarised and their intensity decreases.Spectra obtained at θ = 90◦ will show the π component at its maximum intensity and σcomponents linearly polarised perpendicularly to B at their minimum intensity.Then simultaneous measurements of magnetic field intensity and geometry are neededin order to avoid wrong interpretation of spectra, i.e., of the plasma physical condition.For this purpose the only non-invasive technique is polarimetry. Polarisation can bedescribed by Stokes parameters, obtained through high-resolution spectropolarimetricobservations. As an example, fig. 3 shows synthetic Stokes profiles recorded for the caseθ = 0◦: Stokes I (representing the line intensity, i.e., the observed spectrum) shows twolines, Stokes Q/I (related to linear polarisation parallel and perpendicular to B) andStokes U/I (related to linear polarisation at 45◦ with respect to B) are null, while StokesFig. 3. – Synthetic Stokes profiles as observed when the line of sight and the magnetic fielddirection are coincident (i.e., θ = 0◦).THE INFN PROJECT MAPS 3D 5V/I (related to circular polarisation) presents clockwise and counter-clockwise profiles,i.e., the two σ components.In conclusion, a deeper knowledge of plasma properties, mainly in terms of ChargeState Distribution (CSD) and ion temperatures, needs new diagnostic tools based onhigh-resolution spectroscopy spatially resolved. Polarimetric observations are also re-quired for determining the magnetic field geometry, in order to properly analyse spectrafrom magnetised plasmas.4. – MAgnetised Plasma Spectropolarimetry 3DMAPS 3D was born from the idea to improve our knowledge about ECR plasma prop-erties by means of instrumentations and methodologies imported from Astrophysics. Theproject aims to obtain an innovative characterisation of magnetised laboratory plasmasthrough high-resolution spectropolarimetry in the visible range. As stated before, this isa well-consolidated observational technique routinely applied to astrophysical plasmas inorder to correctly estimate physical parameters of stellar atmospheres, such as effectivetemperature, surface gravity, chemical composition, and magnetic field intensity and ge-ometry. MAPS 3D is a fundamental step for the INFN project PANDORA (Plasmas forAstrophysics, Nuclear Decays Observation and Radiation for Archaeometry [4]).MAPS 3D takes place at INFN-LNS, which hosts the necessary equipment. In par-ticular, the Flexible Plasma Trap (FPT [5]) where the plasma is generated and lon-gitudinally confined through a simple mirror magnetic configuration. It consists of acopper cylindrical chamber of length 260 mm and diameter 82 mm, surrounded by threecoils for a maximum magnetic field intensity |B|max = 0.5T, and equipped with threeguides, orthogonal to each other, for microwave radio frequency launching parallel andperpendicular to the B-direction.In an agreement of collaboration between INFN-LNS and INAF-OACT, a Memoran-dum of Understanding was signed in August 2017 and the Spettrografo Alta RisoluzioneGalileo (SARG [6]) was moved from Telescopio Nazionale Galileo (TNG, La Palma,Canary Island) to LNS. The white-pupil échelle spectrograph SARG allows to obtainspectra between 370 and 1000 nm in a single shot with constant resolution R = 164000,e.g., lines separated up to 0.003 nm at 500 nm can be distinguished. This high-resolutionspectrograph represented a valuable tool in Astrophysics, since it has been widely usedfor stellar spectroscopic observations (e.g., [7-11]). When mounted at TNG, SARG wasequipped with a polarimetric unit implemented by OACT researchers [12]. Magneticfields weaker than 100 G have been measured on the surface of stars (see [13,14]).Two are the main goals of MAPS 3D: the realisation of three polarimeters for the tridi-mensional spatially resolved spectropolarimetric analysis of plasma, and the implemen-tation of a novel on-line diagnostic tool for magnetised plasma based on high-resolutionspectropolarimetry, as described in the following.4.1. Instrumental tools. – The spectropolarimetric analysis aimed by MAPS 3D re-quires three polarimetric units (left panel of fig. 4) coupled to FPT 90◦ from each other,i.e., located at the microwave launching entrances (right panel of fig. 4), and fibre-linkedto SARG. By varying the polarimeters focus it will be possible to point-by-point mapthe plasma. This experimental set-up allows, for the first time, a tridimensional char-acterisation of plasma and magnetic field inside the chamber, with spatial resolution upto 1mm3.6 M. GIARRUSSOFig. 4. – Left: mechanical drawing of a polarimetric unit of MAPS 3D. Right: the experimentalset-up: the three polarimeters will be coupled to the Flexible Plasma Trap and fibre-linkedto the high-resolution spectrograph SARG, in order to point-by-point map the plasma up to1 mm3.Following the successful configuration of the most sensitive spectropolarimeters, e.g.,SARG at TNG, CAOS (Catania Astrophysical Observatory Spectrograph) at OACT [15]and PEPSI (Potsdam Echelle Polarimetric and Spectroscopic Instrument) at LBT (LargeBinocular Telescope) [16], to build up each polarimeter, the dual beam swapping tech-nique configuration was adopted. Data reduction and Stokes parameters are obtained asin [15].Figure 5 shows one polarimeter that I have assembled together with the OACT re-searchers involved in the project. It was tested at OACT by acquiring spectra of a Th-Nelamp, both without and with a uniform magnetic field |B| ∼ 100G directed along theline of sight. The polarimeter was fibre-linked to the spectrograph FRESCO (Fiber-opticReosc Echelle Spectrograph of Catania Observatory, see [17] for a description and [18] fora spectropolarimetric use) with R = 20000. Figure 6 shows the observed Stokes profiles:Stokes I (left panel) has the same shape for both cases (with and without B), since thespectral resolution of FRESCO does not allow to detect the Zeeman splitting. Stokes V(not normalised with respect to I) is shown in the right panel without (dashed line) andin the presence of (solid line) magnetic field. In the latter case, the profile reveals thetwo σ components, helping us to correctly interpret the spectrum.Ordinary Beam FibreExtra-ordinary Beam Fibre/2 Retarder /4 Retarder CollimatorFocal PlaneFig. 5. – Polarimetric unit of MAPS 3D as assembled at OACT. The Wollaston prism is notvisible in the picture.THE INFN PROJECT MAPS 3D 7Fig. 6. – Observed Stokes I and V profiles of a Th-Ne lamp spectral line, obtained at OACTwith the spectrograph FRESCO (R = 20000) and the polarimeter shown in fig. 5, both without(dashed line) and with (solid line) a uniform magnetic field of about 100 G along the line ofsight. The Stokes I one is the same with and without the field, since the spectral resolution doesnot allow to detect the wavelength separation of Zeeman components.4.2. Diagnostic tools. – Contrarily to stellar atmospheres, for which the Local Ther-modynamic Equilibrium (LTE) approximation is assumed, ECR plasmas are describedby the Collisional Radiative model. Then a proper interpretation of emission spectrafrom FPT plasmas needs a novel numerical code taking into account the non-LTE pop-ulation and the Zeeman splitting of lines. This code will be developed on the basis ofCHIANTI [19, 20], the nowadays most advanced database and code for calculating theemissivity of solar optically thin plasma. In addition, a pipeline for on-line reductionand analysis of spectropolarimetric data is planned in order to: 1) measure the ion tem-perature versus single/gas mixing operations, and/or in presence of kinetic/cyclotronturbulences; 2) provide spatially resolved measurements of the plasma distribution ofions at different charge states in order to investigate the mechanism of ion confinementand the build-up of the multiply charged ions; 3) measure the plasma magnetic field, withparticular interest in the local effects of magnetic reconnection and turbulence on-set dueto the magnetic curvatures. Indeed, these measurements are routinely obtained in thecase of solar corona, stellar magnetism and planetary nebulae.The implementation of such a diagnostic tools is now ongoing, according to the mile-stones timetable of the project.∗ ∗ ∗The author gratefully acknowledges the 5th Nat. Comm. of INFN supporting thegrant MAPS 3D, as well as all the colleagues involved in the project. Special thanks go toDr. Matteo Munari and Dr. Ricardo Zanmar Sánchez for their efforts in the realisationof polarimeters.8 M. GIARRUSSOREFERENCES[1] Geller R., Electron Cyclotron Resonance Ion Sources and ECR Plasmas (IOPPublishing) 1996.[2] Giarrusso M. et al., JINST, 13 (2018) C11020.[3] Fantz U. et al., Nucl. Fusion, 46 (2006) S297.[4] Mascali D. et al., Eur. Phys. J. A, 53 (2017) 145.[5] Gammino S. et al., JINST, 12 (2017) P07027.[6] Gratton R. et al., Exp. Astron., 12 (2001) 107.[7] Catanzaro G., Leone F. and Dall T. H., Astron. Astrophys., 425 (2004) 641.[8] Ryabchikova T. et al., Astron. Astrophys., 438 (2005) 975.[9] Ryabchikova T. et al., Astron. Astrophys., 462 (2007) 1103.[10] Stift M. J., Leone F. and Landi Degl’Innocenti E., Mon. Not. R. Astron. Soc., 385(2008) 1813.[11] Sachkov M. et al., Mon. Not. R. Astron. Soc., 389 (2008) 903.[12] Leone F. et al., Proc. SPIE, 4843 (2003) 465.[13] Leone F. and Catanzaro G., Astron. Astrophys., 425 (2004) 271.[14] Leone F. et al., Mon. Not. R. Astron. Soc., 401 (2010) 2739.[15] Leone F. et al., Astron. J., 151 (2016) 116.[16] Strassmeier K. G. et al., Astron. Nachr., 336 (2015) 324.[17] Frasca A. and Catalano S., Astron. Astrophys., 284 (1994) 883.[18] Leone F., Catanzaro G. and Catalano S., Astron. Astrophys., 355 (2000) 315.[19] Del Zanna G. et al., Astron. Astrophys., 582 (2015) A56.[20] Young P. R. et al., J. Phys. B, 49 (2016) 074009.

The INFN project MAPS 3D

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The INFN project MAPS 3D

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