Radio-Frequency Generation Of An Electron Plasma In A .
UNIVERSITÀ DEGLI STUDI DI MILANOSCUOLA DI DOTTORATOFISICA, ASTROFISICA E FISICA APPLICATADIPARTIMENTO DI FISICACORSO DI DOTTORATO DI RICERCA INFISICA, ASTROFISICA E FISICA APPLICATACICLO XXVRadio-Frequency Generation of an ElectronPlasma in a Malmberg-Penning Trap and itsInteraction with a Stationary or PulsedElectron BeamSettore Scientifico disciplinare FIS/03Tesi di Dottorato di:Muhammad IkramSupervisore: Dr Massimiliano RoméCoordinatore: Prof. Marco BersanelliAnno Accademico 2013-2014
iCOMMISSION OF THE FINAL EXAMINATION1) Prof. Francesco PegoraroDepartment of Physics, University of Pisa, Italy2) Prof. Giuseppe GoriniDepartment of Physics, University of Milano Bicocca, Italy3) Dr. Marco CavenagoINFN Laboratori Nazionali di Legnaro, ItalyFinal examinationDate: March 28, 2014Università degli Studi di Milano, Dipartimento di Fisica, Milano, Italy
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iiiAcknowledgementsI am grateful to Prof. Marco Bersanelli, the director of PhD school of physics astrophysics andapplied physics university of Milan and the examination committees of the XXV cycle for admissionto the PhD courses for providing me with the opportunity to pursue a Ph.D. in plasma physicsgroup.I would like to heartiest thanks of my supervisor Dr. Massimiliano Rome, whose friendly attitude;practical advice and fruitful discussion contributed enormously to the success of this thesispossible. I acknowledge useful discussions with Prof. Roberto Pozzoli, Dr. Giancarlo Maero, DrBruno Paroli and Prof. Franca De Luca of Plasma Physics Group, Department of Physics, Universityof Milano.I would also like to thanks Dr. Marco Cavenago, INFN Laboratori Nazionali di Legnaro, Prof.Pegoraro, university of Pisa and Prof Giuseppe Gorini, university of Milano Bicocca for reading themanuscript of my Ph.D. thesis and giving me valuable comments.I offer special thanks to the Andrea Zanzani (secretary of PhD school) and Dr Bruno Paroli for theircooperation on very short notice at numerous occasions during my stay in Milano.Thanks to Francesco Cavaliere director of the mechanical workshop and members staff DanieleVigano, Attilio Gandini for the constant and fundamental contribution in experimental researchwork.I am thanking to professor Dr. Robert E. H. Clark University of Texas San Antonio (UTSA), USA forhis continuous encouragement advices time-to-time as well as fruitful scientific and daily lifediscussions.I personally obliged to chairman emerging nation science foundations (ENSF), Professor K. TahirShah and his team for their remarkable contribution to promote scientific research andsustainable technological development in emerging nations across the world. I thanks to get twotravel grant from the ENSF to attend doctoral admission examination and to return to Islamabadfrom South Korea to proceed to attend PhD School, Department of Physics at University of Milano.I particularly owe my deepest gratitude to my sweet father Ghulam Sarwar (late) who was one ofeducation loving person. I always remember his continuous meeting and discussions with myschool/college teachers in connection of my educational output, shortcoming and improvement. Iam thanking of my love-full mother, brothers professor Muhammad khan, Bashir Muhammad andsisters for their valuable teachings, love, and blessings, which always help me to achieve my goals.It will be unfair if I not thanks to my caring wife Neelofar for her affection, her support and mostimportantly her patience during our stay in via Risorgimento 161, Sesto rondo Milano. My lovelydaughter Laiba born on dated 13 January 2014 was very excitement movement for my wife andme during the tough time of writing of this thesis. I would like to dedicate this thesis to all of them.In general, during my stay and experiences here I feel pleasures to say that Italy is my secondhome country.Muhammad Ikram Safi
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vAbstractExperiments and numerical investigations on trapped electron plasmas and traveling electronbunches are discussed.A Thomson backscattering diagnostics set up was installed in the ELTRAP (Electron TRAP) device, aPenning-Malmberg trap operating at the Department of Physics of the University of Milano since2001. Here, an infrared (IR) laser pulse collides with nanosecond electron bunches with an energyof 1-20 keV traveling through a longitudinal magnetic field in a dynamical regime where spacecharge effects play a significant role. The backscattered radiation is optically filtered and detectedby means of a photomultiplier tube. The minimum sensitivity of the backscattering diagnostics hasbeen estimated for the present set-up configuration. Constraints on the number of photons andthus on the information one can obtain with the Thomson backscattering technique aredetermined by the relatively low density of the electron beam as well as by noise issues. Solutionsto increase the signal level and to reduce the noise are briefly discussed.The generation of an electron plasma by stochastic heating was realized in ELTRAP under ultrahigh vacuum conditions by means of the application of low power RF (1-20 MHz) drives on one ofthe azimuthally sectored electrodes of the trap. The relevant experimental results are reviewed.The electron heating mechanism has been studied by means of a two-dimensional (2D) particle-incell (PIC) code, starting with a very low electron density, and applying RF drives of variousamplitudes in the range 1-15 MHz on different electrodes. The axial kinetic energy of the electronsis in general increasing for all considered cases. Of course, higher temperature increments areobtained by increasing the amplitude of the RF excitation. The simulation results indicate inparticular that the heating is initially higher close to the cylindrical wall of the device. These resultson the electron heating point in the same direction of the experimental findings, where theplasma formation due to the ionization of the residual gas is found to be localized close to the trapwall. The simulations indicate also major heating effects when the RF drive is applied close to oneend of the trap. Similar results are obtained for an electron plasma at higher densities, simulatinga situation in which the RF is applied to an already formed plasma.With the aim to extend these RF studies to the microwave range, a bench test analysis has beenperformed of the transmission efficiency of a microwave injection system up to a few GHz. The
vitest was based on the use of a prototype circular waveguide with the same diameter and length ofthe ELTRAP electrode stack and of a coupled rectangular waveguide with dimensions suitable for afuture installation in the device.Electromagnetic PIC simulations have also been performed of the electron heating effect, againboth at very low and relatively high electron densities, applying a microwave drive with afrequency of approximately 3 GHz close to the center and close to one end of the trap.Both the bench test of the injection system and the numerical simulations indicate that the newmicrowave heating system will allow the extension of the previous RF studies to the GHz range. Inparticular, the electron cyclotron resonance heating of the electrons will be aimed to increasingthe electron temperature, and possibly its density as a consequence of a higher ionization rate ofthe residual gas. The installation of the new RF system will open up the possibility to study, e.g.,the interaction between the confined plasma and traveling electron bunches.
viiTable of ContentsChapter ONEIntroduction1.1Overview of non-neutral plasmas1.2Physics of a single-component plasma in Penning Trap1.3Confinement theory and limitation1.4Motivation1.5Layout of the thesisChapter TWOExperimental system and Thomson backscattering diagnostics2.1 The Malmberg-Penning trap ELTRAP2.2 Electrode assembly2.3 Vacuum system2.4 Magnetic field2.5 Electron source and 2D beam scanner2.6 Thomson backscattering diagnostics and limitations2.6.1 Estimate of the minimum sensitivity2.7 Improvements of the S/N ratio2.8 Conclusions and OutlookChapter THREEWave-particle interactions in non-neutral plasmas3.1 Introduction3.2 RF electron plasma generation in a Penning–Malmberg trap3.2.1 Stochastic heating3.2.2 Electron heating by a Fermi acceleration mechanism
viiiChapter FOURWaveguide design and microwave injection bench test4.1Waveguides4.2Waveguide design with the geometrical constraints of ELTRAP4.3Waveguide modes transform, attenuation and excitation technique4.4RF power transmission in a waveguide4.5Power flow measurement devices4.5.1 Directional coupler4.5.2 Power detector calibration4.6Power flow measurements in a prototype circular waveguideChapter FIVENumerical simulations of the RF heating of a non-neutral plasmain a Penning-Malmberg trap5.1Introduction5.2OOPIC simulation set-up5.3Electrostatic vs electromagnetic simulations5.3.1 Time step and cell size5.4Electrostatic PIC simulations5.5Simulations of RF heating at low electron plasma density5.6Simulations of RF heating at high electron plasma density5.7RF heating with a background gas5.8Simulations of the ECRH in ELTRAPChapter SIXSummary
ixList of FiguresFigure 2.1 Picture and schematic of the internal OFHC electrodes assembly aligned and mountedon an aluminum bar, with the circuits used to measure the induced charge signals induced (e.g.,on electrodes S2 and C4). Bottom: indicative scheme of the electrode potentials, with confinementbetween electrodes C1 and C8 biased at a negative voltage V and RF drive of amplitude ‘A’ on C7.Figure 2.2 Picture of an azimuthally four sectored electrode, built of four insulated parts.Figure 2.3 Schematic of the cylindrical electrodes assembly within the vacuum chamber with thereentrant flange, the pumping system and the solenoid with the iron yoke of ELTRAP. The ironstructures used to generate the magnetic field and some shims positioned at the coil endsconcentrate the field’s lines to the axis. The coil is made of three windings connected in series.Figure 2.4 Schematic of the digitally controlled 2D beam scanner system for the electron bunchalignment at focal point of the infrared laser beam on the geometrical axis of the trap.Figure 2.5 Sketch of the Thomson backscattering diagnostic set-up. A pulsed electron bunch isextracted by an UV laser beam impinging on a photocathode set at a potential of 1-20 kV. Thebunch is focused by the axial magnetic field B 0.2 T of the trap. The trap electrode S4 can beused to detect the bunch crossing via induced current. A 2D beam scanner, combined with thecharge readout from a Faraday cup, sets the current flowing in two orthogonal sets of dipole coilsand deflects the bunch transversally until the bunch transverse position reaches the desiredinteraction point. The radiation of an IR laser is filtered and focused onto the same interactionpoint by a suitable optical system. The backscattered radiation is optically filtered and detected bya photomultiplier (PMT).Figure 2.6 Residual background noise measured by PMT after subtracting the coherent noise foraveraging times of 0.1 s (gray), 1 s (red) and 5 s (blue), respectively. After t 30 ns the noise levelincreases due to the stray light produced by the laser hitting the internal structures of the vacuumchamber.
xFigure 2.7 Expected scattered photons signal for a bunch of density 3.6 1011 cm 3 and radiusrb 0.3mm in the present set-up configuration. We assume a Gaussian signal profile with a FWHMof 5 ns. Part of the signal is overlapped with the stray light noise (see also Fig. 2.6).Figure 3.1 Sketch (a) of the ELTRAP set-up as used for RF discharge and plasma confinementexperiments, with a CCD camera for optical diagnostics. Sketch (b): indicative scheme of theelectrode potentials, with confinement between electrodes C1 and C8 biased at a negative voltageV and RF drive of amplitude ‘A’ on electrode C6.Figure 3.2. Optical measurement of the transverse density profile during the discharge. Theplasma can be observed after 300 ms. The generation takes place initially mostly in the peripheryand successively extends to the whole space.Figure 3.3: Axially-integrated density profiles during plasma formation, for times between 300 and420 ms. On the left, profiles along the vertical axis y, normalized to the maximum measured value.On the right, azimuthally-integrated profiles, normalized to the total charge. The profile evolutionis not continuous but follows three successive shape groups: (a) for 300 320 ms, (b) for 330 350ms, (c) for 360 420 ms.Figure 3.4. Total charge confined after 4.5 s. The plasma is formed and confined between C1 andC8 (top, long trap) or between S2 and C8 (bottom, short trap). The legend specifies the electrodeused as antenna for the RF excitation.Figure 3.5: Ionization rate measured in terms of density growth at the trap wall, normalized to themaximum measured value. The data are grouped according to figure 3.3 (circles correspond togroup (a), squares to (b) and triangles to (c)) and are fitted with exponential laws of inverse timeconstants 81.3, 25.2 and 9.9 s 1, respectively.Figure 3.6: Limit energy distributions f(E) of a trapped electron, after 107 interactions with anoscillating barrier. Geometrical parameters of the ELTRAP device have been used: confinementbetween electrodes C1 and C8, RF drive on electrode C7. In the left panel, the amplitude of RFdrive is varied while keeping the frequency at 1 MHz. In the right panel, the frequency is varied ata constant amplitude of 3.8 V.
xiFigure 4.1 Sketch of a rectangular waveguide with dimensions a 7.18 cm, b 3.71 cm and length7 cm, and of a circular waveguide with a 9 cm diameter and 102 cm length.Figure 4.2 Schematic diagram of the waveguide design and coupling with the electrode stack ofELTRAPFigure 4.3 The cut-off frequency of rectangular waveguide of dimension a 7.18 cm and b 3.71 cmFigure 4.4 The cut-off frequency of circular waveguide of diameter 9 cm.Figure 4.5 Schematic diagram of field transition from rectangular (TE10 mode) to circularwaveguide (TE11 and TM01 modes) via T-junction.Figure 4.6 Field distribution inside of rectangular waveguide for TE10 and TE20 modesFigure 4.7 Field distribution inside of circular waveguide for TE11 and TM01 modesFigure 4.8 Attenuation for TEmn and TMmn modes in a circular waveguide of radius a 1.5 cm.Figure 4.9 Attenuation for TEmn and TMmn modes in a circular waveguide of radius a 3 cm.Figure 4.10 Schematic diagram of directional couplerFigure 4.11 Schematic diagram of the detector calibrationFigure 4.12 Schematic diagram of experimental set up and transmitted power from RF generatorto output of directional couplerFigure 4.13 Return loss measurements of rectangular waveguide onlyFigure 4.14 Arrangement of power flow measurements with a 31 cm rectangular waveguide
xiiFigure 4.15 Return loss measurements of the rectangular waveguide coupled with the circularwaveguide.Figure 4.16 Return loss measurements of the rectangular waveguide coupled with the circularwaveguide and an additional cylinderFigure 4.17 Arrangement of power flow measurements compatible with geometrical constraints ofELTRAP.Figure 4.18 Transmitted power of rectangular wave-guide compatible with geometrical constraintsof ELTRAP coupled with prototype circular waveguide.Figure 4.19 Reflected power of rectangular wave-guide compatible with geometrical constraints ofELTRAP coupled with prototype circular waveguide.Figure 4.20 Return loss measurements of rectangular waveguide compatible with geometricalconstraints of ELTRAP coupled with prototype circular waveguide.Figure 4.21 Transmitted power of rectangular wave-guide compatible with geometrical constraintsof ELTRAP coupled with the prototype circular waveguide and an additional cylinder.Figure 4.22 Reflected power of rectangular wave-guide compatible with geometrical constraints ofELTRAP coupled with the circular waveguide and an additional cylinder.Figure 4.23 Return loss measurements of rectangular waveguide compatible with geometricalconstraints of ELTRAP coupled with the prototype circular waveguide and an additional cylinder.Figure 4.24 Return loss measurements of rectangular waveguide compatible with geometricalconstraints of ELTRAP coupled with 1). Prototype circular waveguide, 2). Prototype circularwaveguide and additional cylinder (combine plot of figure 4.20 and 4.23).Figure 5.1 Simulation setup and input file structure of the PIC code.
xiiiFigure 5.2 Cell distributions of the electrode stack (10 cylinders) in the simulation domain of thePIC code.Figure 5.3 PIC scheme for electrostatic and electromagnetic simulationsFigure 5.4 Schematic diagram of electrostatic PIC simulations of an electron plasma formed andconfined between C1 and C10 biased at a negative voltage -100 V and RF drive of amplitude A 5 Vand 10 V applied on 1) C5 and C7 at 5 x 107 m-3 2) and C5 at 1012 m-3.Figure 5.5 Axial temperature vs time for an electron plasma density 5 x 107 m-3 and an RF drive ofamplitude A 5 V applied on Cylinder-5 (First simulation scheme mentioned in figure 4.4).Figure 5.6 Axial temperature vs time for an electron plasma density 5 x 107 m-3 and an RF drive ofamplitude A 10 V applied on Cylinder-5 (First simulation scheme mentioned in figure 4.4).Figure 5.7 Axial temperature vs time for an electron plasma density 5 x 107 m-3 and an RF drive ofamplitude A 5 V applied on C7 (2nd simulation scheme mentioned in figure 4.4).Figure 5.8 Axial temperature vs time for an electron plasma density 5 x 107 m-3 and an RF drive ofamplitude A 10 V applied on C7 (2nd simulation scheme mentioned in figure 4.4).Figure 5.9 The electrostatic PIC simulation of axial kinetic energy (eV) (electrons velocities in theaxial (z) direction) of electron plasma 5 x 107 m-3 as a function of radial position. The confinedelectrons is between C1 and C8 biased at a negative voltage -100 V and RF (MHz) drive ofamplitude A 5 V applied on Cylinder-5 (1st simulation scheme mentioned in figure 4.4).Figure 5.10 Axial kinetic energy (eV) as a function of the radius. The electron density is5 107 m 3 and the RF applied on C5 has an amplitude of A 10 V (1st simulation schemementioned in figure 4.4).Figure 5.11 Axial kinetic energy (eV) as a function of the radius. The electron density is5 107 m 3 and the RF applied on C7 has an amplitude of A 5 V (2nd simulation schemementioned in figure 4.4).
xivFigure 5.12 Axial kinetic energy (eV) as a function of the radius. The electron density is5 107 m 3 and the RF applied on C7 has an amplitude of A 10 V (2nd simulation schemementioned in figure 4.4).Figure 5.13 Axial temperature vs time for an electron plasma density 5 10 7 m 3 and an RF drive ofamplitude A 5 V applied on C5 (3rd simulation scheme mentioned in figure 4.4).Figure 5.14 Axial temperature vs time for an electron plasma density 5 10 7 m 3 and an RF drive ofamplitude A 10 V applied on C5 (3rd simulation scheme mentioned in figure 4.4).Figure 5.15 Axial kinetic energy (eV) as a function of the radius. The electron density is1012 m 3 and the RF applied on C5 has an amplitude of A 5 V.Figure 5.16 Axial kinetic energy (eV) as a function of the radius. The electron density is1012 m 3 and the RF applied on C5 has an amplitude of A 10 V.Figure 5.17 (a) Electron axial temperature (eV) vs time for a 5 MHz excitation frequency ofamplitude A 5V and electron density 5 10 7 m 3 where hydrogen used as background gas atpressures of 10 6torr ,10 7 torr and 10 8 torr (b) electron neutral collision time vs. simulation time(s) at a pressure of 10 6 torr .Figure 5.18 For the same data as in figure 5.17, electron neutral collision time vs. simulation time(s) at a pressure of (c) 10 7 torr and (d) 10 8 torr .Figure 5.19 (a) Electron axial temperature (eV) vs time for a 5 MHz excitation frequency ofamplitude A 5V and electron density 1012 m-3 where hydrogen used as background gas atpressures of 10 6torr ,10 7 torr and 10 8 tor
microwave heating system will allow the extension of the previous RF studies to the GHz range. In particular, the electron cyclotron resonance heating of the electrons will be aimed to increasing the electron temperature, and possibly its
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