Using a low amplitude, chirped-frequency localized potential drive(W. Bertsche, J. Fajans and L. Friedland, Direct Excitation of High-Amplitude Chirped Bucket-BGK Modes, Phys. Rev. Lett., 91: 265003, 2003. ), we excited undamped large amplitude electrostatic plasma waves in a relatively hot plasma. We believe these waves to be BGK waves, stationary, non-linear kinetic waves which are untouched by classical Landau damping. Even though BGK modes underpin much of kinetic wave theory, direct experimental evidence of undamped BGK waves has proven elusive. Large-amplitude responses have been observed in the past, however such structures have generally been unstable and short-lived. Other excitations generated during continuous driving have resulted in stable but low-amplitude waves. Our technique generates a tailored distribution function along with a self-consistent field, yielding large oscillations long after the drive has been removed. A theory for this excitation has been developed, which agrees with many features observed experimentally(L. Friedland, F. Peinetti, W. Bertsche, J. Fajans, and J. Wurtele, Driven Phase Space Holes and Synchronized Bernstein, Green, and Kruskal (BGK) Modes , Phys. Plasmas (accepted, 2004).). Restricted two-dimensional PIC simulations of an electron plasma column with a localized chirped drive are in close agreement with experimental data. This technique may lend laboratory insight to physical phenomena observed in other fields such as laser plasma interactions.
[CI1B.002] Diocotron Echoes: Direct Visualization of Phase Space Mixing and Unmixing
Jonathan Yu (University of California, San Diego)
We have observed diocotron wave echoes in magnetized pure
electron plasma columns, demonstrating the reversible nature
of spatial Landau damping, with reversibility limited by
weak collisional velocity scattering. The waves are k_z=0,
E \times B drift surface waves, and also
represent Kelvin waves on an ideal fluid with sheared
rotation ømega_E(r). We excite an initial diocotron wave
with azimuthal mode number m_i. This wave rapidly Landau
damps due to phase mixing, leaving spiral filaments of
perturbed density. At time t= \tau, a second wave is
excited with mode number m_s, and this wave also damps.
The echo then spontaneously appears at time t_e = \tau \,
m_s/( m_s - m_i), with mode number m_e = m_s - m_i.
Experiments directly image the wave damping, showing the
spiral wind-up of density filaments, and showing the
unwinding that results in the echo. The basic echo
characteristics, including mode number, appearance time, and
nonlinear ``saturation" effects, agree with a simple
nonlinear 2D ballistic theory. The plasma dynamics is
complicated, however, by 3D ``end" effects that make
ømega_E dependent on an electron's z-velocity,
i.e.~ømega_E(r,v_z). Surprisingly, the echo is not
destroyed by v_z-dependent particle drifts in the end
containment fields; in essence, separate v_z-classes of
particles execute separate wind-up and unwinding, resulting
in the same echo. The echo is however, destroyed at late
times by collisional intermixing of these separately
evolving v_z-classes, and the maximal observed echo times
agree with a second-order kinetic theory. In addition, we
find that excessively large second wave excitations degrade
the echo even sooner than collisions; this may be due to
excitation-induced velocity mixing as particles move
radially, or due to excitation-induced \theta-drifts.
[CI1B.003] Nonlinear physics of laser-irradiated micro-clusters
Boris Breizman (University of Texas)
A nonlinear theory has been developed to describe electron
response and ion acceleration in dense clusters that are
smaller in size than the laser wavelength. This work is
motivated by high-intensity laser-cluster interaction
experiments. The theory reveals that the breakdown of
quasi-neutrality affects the cluster dynamics in a dramatic
way. The laser creates an ion shell that expands quickly due
to its own space charge whereas the central part of the
cluster expands at a much slower rate under the thermal
electron pressure. The developed theory also shows a trend
for the electron population to have a two-component
distribution function: a cold core that responds to the
laser field coherently and a halo that undergoes stochastic
heating. As the ion shell expands, the potential well for
the electron core becomes shallower, producing a leak of the
core electrons. The response of the cold electron core to
the laser field is similar to that of a driven nonlinear
pendulum. As a result, the third harmonic signal from an
isolated cluster exhibits resonant enhancement when the
laser frequency is close to 1/3 of the core eigenfrequency.
It is essential that the ion background has to be
non-uniform to produce a nonlinear electron response. The
theory has been extended to describe coherent response of
multiple clusters. This extension addresses recent fs
pump-probe experiments that exhibit a narrow peak in
third-harmonic emission from argon clusters under a probe
pulse at time delay of 300 fs following heating by a short
pump pulse, in contrast with a much broader resonance in
linear absorption. The third-harmonic feature is sensitive
to focus and interaction geometry, which indicates that
phase matching in the evolving cluster plasma is primarily
responsible for the third harmonic peak.
[CI1B.004] An Alfven Wave Maser in the Laboratory
James E. Maggs (Physics and Astronomy Department, UCLA)
A frequency selective Alfvén wave resonator results from the application of a locally non-uniform magnetic field to a plasma source region between the cathode and anode in a large laboratory device. When a threshold in the plasma discharge current is exceeded, selective amplification produces a highly coherent, large amplitude shear Alfvén wave that propagates out of the resonator, through a semi-transparent mesh anode, into the adjacent plasma column where the magnetic field is uniform. This phenomenon is similar to that encountered in the operation of masers/lasers at microwave and optical frequencies. The current threshold for maser action is found to depend upon the confinement magnetic field strength, B_0. Its scaling is consistent with the condition for matching the drift speed of the bulk plasma electrons with the phase velocity of the mode in the resonator. The largest spontaneously amplified signals are obtained at low B_0 and large plasma currents. The magnetic fluctuations, \delta B, associated with the Alfvén maser can be as large as \delta B/B_0 = 1.5 % and are observed to affect the plasma current. Steady-state behavior leading to coherent signals lasting as long as 3 msec can be achieved when the growth conditions are well-above threshold. The maser is observed to evolve in time from an initial m = 0 mode to an m = 1 mode structure in the transition to the late steady-state. The laboratory phenomenon reported is analogous to the Alfvén wave maser proposed to exist in naturally occurring, near-earth plasmas.