Transcript Diapositiva 1 - Dipartimento di Fisica

Congresso del Dipartimento di Fisica
Highlights in Physics 2005
11–14 October 2005, Dipartimento di Fisica, Università di Milano
Coherent structures and diocotron instability in electron plasmas
G. Bettega*†, F. Cavaliere*, M. Cavenago+, F. De Luca*†, D.Ghezzi*°, A. Illiberi*†, R. Pozzoli*†, and M. Romé*†
* Dipartimento
† INFN, sezione di Milano
di Fisica, Università di Milano
+ INFN, Laboratori Nazionali di Legnaro
°Politecnico di Milano
THE MALMBERG-PENNING TRAP: EXPERIMENTAL PARAMETERS
Schematic of the devide
Injection Hold Dump Cycle
The plasma is axially trapped within a potential well
created by two negative voltages; the radial
confinement is obtained by the application of an axial
magnetic field which keeps the plasma in rotation
around its axis: the JxB electrodynamical force
which generates is inward directed and balances the
electrostatic repulsion bteween the particles
FIELDS OF STUDY: 2D FLUID DYNAMICS, FORMATION AND
EVOLUTION OF COHERENT STRUCTURES , PLASMA MODES
A trapped particle axially oscillates at a frequency much larger than the ExB motion one: on this
“slow” time scale it is equivalent to a rigid charged rod undergoing a pure ExB (therefore
incompressible) motion (“point vortex”). The whole plasma behaves 2D (cold) fluid whose dynamics is
well modeled by the Euler equations for an ideal (not compressible, inviscid) classic fluid
FORMATION OF 2D VORTICES
A small initial density perturbation,
present as a noise on a ring shaped plasma,
propagates azimuthally on its surface,
exponentially growing in time and driving
the dynamics in a strongly non linear
regime. This kind of instability, known as
Diocotron Instability, is analogous to the
well known Kelvin-Helmoholtz instability of
the fluid with shear flow. The initial phase
of a trapped plasma evolution (~100 ms) is
characterised by the formation, the
interaction and the dissipation of small 2D
vortices as long as a smooth radial
equilibrium is reached.
EXPERIMENTAL PARAMETERS
COHERENT STRUCTURES IN TRAPPED PLASMAS. FREE RELAXATION OF 2DTURBULENCE: VORTEX CRYSTALS FORMATIONS
The plasma is generated by a thermionic spirally
shaped cathode, so the initial density distribution
reflects the shape of the source. Such an initial
density diatribution is strongly unstable and becomes
turbulent: N>>1 vortices (electron columns) form,
which non linerarly interact through merger events
and originate filaments of low electron density. The
number of vortices decreases in time towards a single
vortex state according to a power law
For some source parameters the free relaxation of the initial turbulence
is halted and a VORTEX CRYSTALS state forms, made of a stable
rotating arrays of vortices. The measured number of vortices versus
time abandones the power law and forms a plateau.
The self organisation properties of the 2D turbulence are investigated
within the context of variational methods (maximisation of entrophy,
minimisation of enstrophy, RMFE – Regional Maximum Fluid Entrophy -).
Similar vortices structures are also observed in many natural occuring
flows (hurricanes, the Great Sopt on Jupiter) which can be modeled as
two dimensional; the similarity between 2D vortices interactions and
galaxies merger is striking
Once formed the vortex crystal states last for thousands of bulk plasma
rotation and are finally dissipated by the non zero viscosity. The
relationship between the initial conditions (source and injection
parameters) and the formation of a vortex crystal is not well understood
l=1 DIOCOTRON MODE: DIAGNOSTIC, INSTABILITY AND ACTIVE CONTROL
The diocotron modes can be also diagnosed by passive, non desctructive methods, collecting the induced charge signals which forms on
the electrodes of two aximuthally sectored cylinders. The small voltage signals are amplified and conditioned, then acquired and
sofware processed by a LabVIEW virtual instrument: their spectra give informations about the frequencies and the amplitudes of the
waves. The fundamental wave (l=1 diocotron mode) - which is equivalent to an off-axis plasma rotation is predicted - stable by the
theory, but is found UNSTABLE in the experiments: the plasma is driven towards the wall (curve -a-, free evolution of the instability)
PERSPECTIVES... future work ...
Spectrum of the signal collected on a -probe
EXPERIMENTS ON 2D COHERENT STRUCTURES IN TRAPPED
PLASMAS AND IN LOW ENERGY ELECTRON BEAMS
Extended experimental scanning of the initial conditions
originating monotonicaly decreasing sequences of vortices and
vortex crystal states
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Application of the super-resolution techique (wavelet
transform) to the acquired images in order smaller 2D coherent
structures
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2D Turbulence spectra (fluctuations, cross correlations of
relevant quantities, energy spectrum, vortices cascades)
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Plasma off-axis rotation. CamV Apparatus @ San
Diego University (http://sdphca.ucsd.edu/mpegs)
Ions removing fields
The role of the ionisation processes in determining the l=1 diocotron mode instability
has been recently evidenced: the application of proper “ions removing field” has been
succesful in controlling the increase the instabilty: the use of this techique allows to
double the lifetime of the plasma.
When the field are applied the ions are
ejected out of the plasma confinement
volume and the instability is removed.
Compare the evolutions in the graph ...
(a) free evolution of the instability
(b) ions removal from 1s to 2s
(c) ions removal from 2s to 3s
MANIPULATION OF PLASMA MODES WITH EXTERNAL
CIRCUITS
Active control of the l=1 diocotron mode with time varying ions
removing fields
➢
Active control of the l=1 instabilities with phase shifter and
negative resistance circuits
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Active control of the l=1 diocotron using external resonant (LC)
networks
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HIGH FREQUENCY ELECTROSTATIC DIAGNOSTIC SET UP
Set up of the electrostatic diagnostic for the plasma high
frequency modes of oscillation (amplification, filtering,
interfacing ...)
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