Anania, M. P.

Biagioni, Angelo

Chiadroni, E.

Cianchi, A.

Ferrario, M.

Filippi, Francesco

Giribono, Anna

Mostacci, Andrea

Palumbo, Luigi

Zigler, A.

English

FILIPPI, FRANCESCO

GIRIBONO, ANNA

MOSTACCI, Andrea

PALUMBO, Luigi

BIAGIONI, ANGELO

Archivio della ricerca- Università di Roma La Sapienza

PLASMA DENSITY PROFILE CHARACTERIZATION FOR RESONANTPLASMAWAKEFIELD ACCELERATION EXPERIMENT AT SPARC_LABF. Filippi∗, A. Giribono, A. Mostacci, L. Palumbo,SBAI, ‘Sapienza’ Università di Roma, Via A. Scarpa 14-16, 00161 Roma, Italy.INFN-Roma1, Piazzale Aldo Moro, 2 00161 Roma, ItaliaM. P. Anania, A. Biagioni, E. Chiadroni, M. Ferrario,Laboratori Nazionali di Frascati, INFN, Via E. Fermi, Frascati, ItalyA. CianchiDipartimento di Fisica, Università di Roma ‘Tor Vergata’,Via della Ricerca Scientifica 1, 00133 Roma, ItaliaA. Zigler,Hebrew University of Jerusalem, Jerusalem 91904 (Israel)AbstractNew generation of particle accelerators is based on theexcitation of large amplitude plasma waves driven by ei-ther electron or laser beams, named as Plasma Wake-field Accelerator (PWFA) and Laser Wakefield Accelera-tor (LWFA), respectively. Future experiments scheduled atthe SPARC_LAB test facility aim to demonstrate the ac-celeration of externally injected high brightness electronbeams through both schemes. In particular, in the so-calledresonant PWFA a train of more than two driver electronbunches generated with the laser comb technique resonantlyexcites wakefields into the plasma, the last bunch (witness)is injected at the proper accelerating phase gaining energyfrom the wake. The quality of the accelerated beam dependsstrongly on plasma density and its distribution along theacceleration length. The desired density can be achievedwith a correct shaping of the capillary in which plasma isformed. The measurements of plasma density, as well asother plasma characteristics, can be performed with spec-troscopic measurements of the plasma self emitted light.The measurement of density distribution for hydrogen filledcapillaries is here reported.INTRODUCTIONPlasma-based acceleration techniques have proved theability to accelerate electron beams at high energies overshort distances due to the high acceleration gradient thatplasma can sustain. Recent works [1, 2] have increasedthe interest in these techniques although the acceleratedbeams have showed a considerably high energy spread. Inorder to reduce the deterioration of the beam quality, atthe SPARC_LAB facility [3] we aim to externally inject ahigh brightness beam produced by a RF based linac into aplasma wakefield previously excited by either electron orlaser beams [4]. The amplitude of the wakefields, their spa-∗ Francesco.Filippi@roma1.infn.ittial dimensions and the consequent accelerating gradientdepends on the electron plasma density and its variationalong the longitudinal dimension of the capillary [5]. More-over, the current flow caused by the ionization discharge isan important parameter that acts on the propagation of thebeams inside the capillary. It is straightforward that a deepknowledge of the plasma density and its evolution, as wellof the current flow needed to ionize the gas, is mandatoryfor a correct control of the accelerated beam.EXPERIMENTAL SETUPFigure 1: Experimental setup for the measurements of themain characteristics of the hydrogen filled capillary plasmasource.WEPMY007 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-22554Copyright©2016CC-BY-3.0andbytherespectiveauthors03 Alternative Particle Sources and Acceleration TechniquesA22 Plasma Wakefield AccelerationThe experimental layout for the spectroscopic analysisis shown in Fig. 1. A sapphire capillary of circular crosssection of 1mm diameter is fed by two gas inlets of 0.5mmdiameter placed at 7.5mm from the respective end. For ourexperiments we decided to use hydrogen gas produced byelectrolysis in a NM Plus Hydrogen Generator (Linde). Thepressure of the gas can be varied with a pressure regulatorplaced more than 1m far from the aperture valve. For themeasurements shown in this article, the pressure was set to250mbar on the pressure regulator. A high speed solenoidvalve lets the gas flow inside the capillary. The valve is setto be opened for 3 ms before the discharge. This time isdictated by the constrain to keep the pressure inside the testchamber below 1 · 10−7 mbar before every shot. A voltageof 20 kV is applied between the two ends of the capillaryby a capacitor of 2 nF charged by a discharge circuit. Thecurrent of the discharge can be monitored by a Pearson cur-rent monitor placed around one of the two wires that feedsthe capillary electrodes. The light emitted by the plasmais collected by a system of lenses that produce an image ofthe capillary onto the entrance slit of an imaging spectrom-eter (SP 275 of the ARC Corporation). The spectrometerhas an aperture ratio of f/3.8, focal length of 275mm, andit is equipped with a grating with groove density of 2400g/mm. This method of diagnosis has already been used inour laboratory for the measurement of plasma density inlaser trigger ablative capillaries [6]. A fast-gated intensifiedcamera (Andor iSTAR 320) is used as the detector for thespectrometer.EXPERIMENTAL RESULTSA profile of the discharge current has been acquired afterevery shot. Its temporal profile depends on the ionizationprocess and on the discharge circuit that has been set up[7]. An example of current profile with 20 kV of voltageapplied to the discharge circuit is shown in Fig.2. The sameprofile has been observed for many shots under the sameexperimental conditions showing a very good reproducibility.The peak current of the discharge can be varied by varyingthe voltage applied to the discharge circuit, as shown in Fig.3.Both the reliability and the reproducibility of the dischargeparameters allow a good control of the ionization processand the current evolution during the discharge.The light emitted by the hydrogen is collected and sent tothe spectrometer by an imaging system composed by threelenses and two mirrors. The capillary is imaged on the en-trance slit of the imaging spectrometer, then with the gatedcamera it is possible to analyze the image of the capillaryspectrally dispersed by the spectrometer. The analysis ofthe Stark effect on the Balmer beta line can lead to plasmadensity information around the emitting atoms [8]. By trig-gering the intensified camera with different delays it waspossible to scan the temporal evolution of the plasma den-sity along the entire longitudinal dimension of the capillary.The results of this analysis are shown in Fig.4. To help thereconstruction of the spatial dimension of the spectroscopicFigure 2: Discharge current profile when a voltage of 20 kVis applied to the discharge circuit.Figure 3: Maximum current peak value measured by varyingthe voltage applied to the electrodes.Figure 4: Plasma density evolution along the longitudinalcapillary dimension. Every measurement has been acquiredwith 100 ns of time gating, varying the trigger of 100 ns afterevery shot. The dark shadows on the image are the shadowsof electrodes (thicker) and reference wires (thiner) used tospatially calibrate the spectrograms.images two wires of 0.45mm diameter were tied on the capil-lary. The shadows of these wires, together with the shadowsof the electrodes, allows to reconstruct the longitudinal di-mension of the spectroscopic image. The spectrograms withProceedings of IPAC2016, Busan, Korea WEPMY00703 Alternative Particle Sources and Acceleration TechniquesA22 Plasma Wakefield AccelerationISBN 978-3-95450-147-22555 Copyright©2016CC-BY-3.0andbytherespectiveauthorsthe plasma density informations are acquired every 100 nsafter the discharge trigger for a gate opening time of 100 ns.In every acquisition, the plasma shows the maximum densityin the middle of the capillary, then it decreases closer to theelectrodes to almost the 65% of its maximum value. After1500 ns from the discharge trigger the signal is too weak tobe analyzed and the gate time of the intensified camera mustbe modified.We have analyzed the temporal density evolution by averag-ing the density in the middle of the capillary. The averagevalues of the density between the 14th and the 16th millime-ter of the capillary are shown in Fig.5. The plasma densityreaches its maximum around 400 ns after the discharge trig-ger, some tens of nanoseconds after the maximum of thecurrent profile. Before its maximum, the density increasesharply, then it decrease. We fit the decreasing data with anexponential curve starting when the discharge has reachedone third of its peak amplitude. The fit gave a time constantof the density decay of 479.95 ns.CONCLUSIONWe have shown the measurements of the plasma densityand current profile of an hydrogen filled capillary for res-onant PWFA experiment. The analysis of the dischargecurrent flow has shown a good shot to shot reproducibilityof the discharge current profile, and the possibility to controlits maximum peak value by varying the applied voltage withexcellent reliability. The density measurements performedwith our setup allow to detect the plasma density variationalong the longitudinal dimension of the capillary and itstemporal evolution with steps of 100 ns and with a gate timeof 100 ns. The maximum density inside the cylindrical cap-illary is in the center and the density decreases to the 65%of its value close to the electrodes. Analyzing the temporalevolution of the plasma density it has been possible to mea-sure the maximum around 400 ns after the discharge trigger,some tens of nanoseconds after the peak of the current dis-charge.Further analysis has been planned to better characterize theplasma density online during the experiment.ACKNOWLEDGEMENTAuthors would like to acknowledge V. Lollo and S. Binifor technical support.This work has been supported by the EU Commissionin the Seventh Framework Program, Grant Agreement312453-EuCARD-2 and the Italian Minister of Research inthe framework of FIRB – Fondo per gli Investimenti dellaRicerca di Base, Project n. RBFR12NK5K.Figure 5: Temporal density evolution averaged between 14and 16mm inside the capillary in logarithmic scale. Theplasma density reaches its maximum around 400 ns afterthe trigger discharge. The decrease of density starting from600 ns after the trigger has been fitted with an exponentialcurve.REFERENCES[1] M. Litos et al., “High-efficiency acceleration of an electronbeam in a plasma wakefield accelerator”, Nature, vol. 515, no.7525, p. 92-95, Nov. 2014.[2] W.P. Leemans et al., “GeV electron beams from a centimetre-scale accelerator”, Nature Physics, vol. 2, pp. 696–699, 2006.[3] M. Ferrario et al., “SPARC_LAB present and future” NuclearInstruments and Methods in Physics Research Section B: BeamInteractions with Materials and Atoms, vol. 309, pp. 183–188,2013.[4] R. Pompili et al., ”Beam manipulation with velocity bunch-ing for PWFA applications” Nucl. Instruments Methods Phys.Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip.,doi:10.1016/j.nima.2016.01.061, 2016.[5] E. Esarey, C. Schroeder and W. P. Leemans, “Physics of laser-driven plasma-based electron accelerators,” Rev. Mod. Phys,vol. 81, no. 3, pp. 1229–1285, Aug. 2009.[6] F. Filippi, et al., “Plasma density characterization atSPARC_LAB through Stark broadening of Hydrogen spec-tral lines,” Nucl. Instruments Methods Phys. Res. Sect. A Accel.Spectrometers, Detect. Assoc. Equip., 2016, http://dx.doi.org/10.1016/j.nima.2016.02.071[7] M. P. Anania, et al., “Plasma production for electron accel-eration by resonant plasma wave,” Nucl. Instruments Meth-ods Phys. Res. Sect. A Accel. Spectrometers, Detect. As-soc. Equip., 2016, http://dx.doi.org/10.1016/j.nima.2016.02.029i[8] M. A. Gigosos and A. Manuel, “Computer simulated Balmer-alpha, -beta and -gamma Stark line profiles for non-equilibriumplasmas diagnostics,” Spectrochim. Acta - Part B At. Spectrosc.,vol. 58, pp. 1489–1504, 2003.WEPMY007 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-22556Copyright©2016CC-BY-3.0andbytherespectiveauthors03 Alternative Particle Sources and Acceleration TechniquesA22 Plasma Wakefield Acceleration

Plasma density profile characterization for resonant plasmawakefield acceleration experiment at SPARC-LAB

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Plasma density profile characterization for resonant plasmawakefield acceleration experiment at SPARC-LAB

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