Flying focus: Spatial and temporal control of intensity for laser-based applications

Abstract

Turbulent dynamos that exponentially amplify initially small, seed magnetic fields are crucial in magnetizing the Galaxy and beyond. Until now, the ideal environment for turbulent dynamos to grow has been difficult to recreate. In a new approach, we leverage the long pulse capability of the OMEGA-EP laser to recreate the highly conductive and inviscid (Re m $ 5500; Pr m տ 1) growth environment of the turbulent dynamo within the magnetized plasma jet ablated from a simple cone target of CH plastic. In 3-D FLASH simulations of our scheme, we find that the ideal dynamo environment is a typically $1 mm 3 , տ 1:5 keV hot spot where the laser beams intersect to produce maximum direct heating of the jet plasma. The dynamo environment is maintained from the onset of steady flows through the $10 ns length of the laser pulse. For a plasma vorticity of 0.3-3.0 ns-1 and a dynamo active over $5 ns, the magnetic energy increases on an exponential trajectory by more than a decade. Fourier analysis reveals that the dynamo progressively saturates up to E B =E K $ 20% from small scales k տ 30 cm À1 to large in the time it is sustained. We find robust agreement between the evolution of magnetic energy spectra extracted from the FLASH physics simulation and that derived from synthetic sheath-accelerated proton deflectometry images, thereby demonstrating that the dynamo activity can be quantified in a real experiment.

Reciprocating propagation of laser pulse intensity in free space

“…In this work, we have introduced a new type of laser pulse intensity propagation or a new type of optical control, i.e., the reciprocation FLFO. Based on the recently reported FLFO (or named sliding focus) that created by combining temporal chirp and longitudinal chromatism together [24][25][26][27], when both the Rayleigh length and the temporal chirp are dramatically increased, different from the previous result the produced FLFO would present an unprecedented motion form: flying forward-backward-forward along a straight-line in free space, showing a longitudinal reciprocating trajectory. The existence condition of a clear reciprocation FLFO with a high longitudinal spatial resolution is also analyzed and introduced.…”

“…Currently, the spatiotemporal (ST) coupling is frequently used to modulate the propagation or structure of a pulsed beam, which permits both velocity control (i.e., superluminal or subluminal, and accelerating or decelerating) and direction control (i.e., forward or backward) [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. The first example is the 3-dimensional (3-D) flying focus (FLFO) within the extended Rayleigh length independently demonstrated by Quéré, et al [24,25] (originally named "sliding focus") and Froula, et al [26,27] (originally named "flying focus"), respectively, which can propagate at an arbitrary group velocity in free space including all motion forms of superluminal or subluminal, accelerating or decelerating, and forward or backward propagations. The second example is the 2-D optical ST wave-packet demonstrated by Abouraddy, et al [28][29][30][31][32][33][34], which can also propagate at an arbitrary group velocity in free space including all above motion forms.…”

Dephasingless Laser Wakefield Acceleration

“…Spatiotemporal shaping of laser pulses provides control over the velocity of an intensity peak or ponderomotive force [17][18][19]. By stretching the region over which a laser pulse focuses and adjusting the relative timing at which those foci occur, the velocity of an intensity peak can be made to travel at any velocity, independent of the group velocity.…”

“…By stretching the region over which a laser pulse focuses and adjusting the relative timing at which those foci occur, the velocity of an intensity peak can be made to travel at any velocity, independent of the group velocity. This property has already been exploited in proof-of-principle simulations to improve Raman amplification, photon acceleration, and relativistic mirrors, and to generate Cherenkov radiation in a plasma [17][18][19][20][21][22][23] and in experiments to drive ionization waves at any velocity [24]. For LWFA, a spatiotemporally shaped laser pulse could drive a wakefield with a phase velocity equal to the speed of light in vacuum ( v p = c), eliminating dephasing and greatly reducing the accelerator length by allowing for operation at higher densities.…”

“…In addition, the stretched focal region eliminates the need for an external guiding structure or self-guiding. This was previously proposed using chromatic focusing of chirped laser pulses, but the technique inherently generated long pulses, diminishing its utility for LWFA [19]. Furthermore, the maximum extent of the focal region was limited to a single dephasing length.…”