Session ZI03: Invited: Post Deadline

First physics results from the Wisconsin HTS Axisymmetric Mirror (WHAM)

WHAM aims to confine and stabilize an axisymmetric mirror plasma using 17 T, 55 mm diameter bore HTS mirror coils and with its recent first plasmas set a new world record

for steady field applied to a magnetic confinement fusion experiment.  The WHAM device is now operating with an array of high power heating and control systems, including a 500 kW, 110 GHz gyrotron, a 1 MW, 25 kV neutral beam injection system, and multi-megawatt radially controlled plasma biasing actuator for stirring the plasma to provide vortex stabilization of MHD interchange, as well as a substantial base-line diagnostic set that will be described. One major accomplishment of the first campaign was to (1) the commissioning of the CFS supplied HTS magnets to full field operation, and (2) to demonstrate robust and reliable operation of the heating systems and diagnostic set in the presence of the high magnetic field. In the first experimental campaign the vacuum field was entirely due to the two HTS magnets corresponding to a mirror ratio of 70. The application of up to 400 kW of ECH, limited to 10 ms pulse lengths, demonstrated robust target plasma formation achieving average line densities ~ 3 and in separate plasmas a midplane averaged beta of > 5%. These ECH plasmas show clear evidence of both a cold high density plasma and a low density, long-lived hot electron plasma (with hard x-rays of energy > 100 keV observed) that appear to modify MHD stability (both positively and negatively). Preliminary data analysis indicates high m-number flute modes with real frequencies in the 40-60 kHz range suggestive of hot electron interchange in low ion temperature plasmas. Biasing of the tungsten limiter modifies MHD activity and by optimizing the bias has led to an increased plasma stored energy by 50% (and can also be used to drive MHD and make confinement worse). Neutral beam injection has successfully been used to fuel the plasma by injecting into ECH target plasmas, but at present the lack of wall conditioning

in this first campaign indicates that charge exchange of the fast ions on residual and recycled neutral particles is limiting the build-up of fast ion pressure and controlling the overall confinement.   

Reliable operation of a laser plasma accelerator driven free electron laser

Laser plasma accelerators (LPAs) have emerged as a viable alternative to traditional accelerators for various applications, thanks to their capability to generate high-brightness

beams and much higher accelerating gradients. This enables more compact designs for future light sources, such as free electron lasers (FELs). FEL technology leveraging LPA sources is

progressing swiftly, with several key milestones achieved in recent years. However, significant work remains to be done to move from proof-of-concept experiments to the dependable

operation of LPA-driven FELs. Recent initiatives at the BELLA center's Hundred Terawatt Undulator beamline, which includes an electron beam transport section leading to a 4-meter-

long, strong focusing undulator, have successfully demonstrated the consistent operation of a high-gain FEL in the SASE regime. SASE gain is detectable on 90% of shots with measured

SASE gain in excess of 1000.   

Universal non-Maxwellian equilibria in near-collisionless plasmas

A fundamental tenet of thermodynamics is that chaotic systems will relax to maximum-entropy states. In plasmas, the chaos is conventionally provided by

interparticle collisions and the universal maximum-entropy equilibrium is a Maxwellian distribution. However, in collisionless plasmas, the chaotic state is due to collective

turbulent dynamics. We will argue theoretically, and show numerically, that such plasmas still relax towards universal equilibria that are non-Maxwellian, featuring a

power-law distribution of particle energies. This is achieved via an entropy-maximization procedure that accounts for the short-time conservation of certain collisionless

invariants. The conservation of these collisionless invariants endows the system with a partial ‘memory' of its prior conditions, but is imperfect on long time scales due to the

development of a turbulent cascade to small scales, which breaks the precise conservation of phase volume, making this memory imprecise. The equilibria are still

determined by the short-time collisionless invariants, but the invariants themselves are driven to a universal form by the nature of the turbulence. This is numerically confirmed

for the case of beam instabilities in one-dimensional electrostatic plasmas (see Ewart et al. 2024, E-print arXiv:2409.01742), where sufficiently strong turbulence appears to

cause the distribution function of particle energies to develop a universal power-law tail, with exponent -2 (as predicted in Ewart et al. 2023, J. Plasma Phys. 89. 905890516).   

Direct observation of ion cyclotron damping of turbulence in Earth's magnetosheath plasma

The turbulence in space plasmas undergoes dissipation at the kinetic scales, where the turbulent fluctuations in the electromagnetic fields are on the order of ion-cyclotron motions and

smaller. An open question in space plasma turbulence is what dissipation processes are removing energy from the turbulent fluctuations and converting it to ion and electron energies. The rapid in-situ measurements of magnetic fields, electric fields, and ion and electron phase space densities by NASA’s Magnetetospheric Multiscale (MMS) mission enable us to observe which

dissipation processes are present and to which particles the energy is being transferred. In this work we have identified ion cyclotron waves embedded in a turbulent cascade when the MMS spacecraft were located in the Earth’s magnetosheath, and the subsequent dissipation of turbulent energy through ion cyclotron damping. Combined with a previous analysis of the same interval of data which showed the presence of electron Landau damping, we identify the dissipation of a large fraction of the turbulent energy into the ions and electrons. The turbulent cascade rate at a scale larger than that of the ion cyclotron motion is quantified and directly compared to the dissipation of this energy into the ions at the ion kinetic scale. Team acknowledgement: We would like to thank the MMS instrument teams for the quality and care in calibration of their data.   

Experimental determination of the electrical conductivity of compressed matter using ultrafast terahertz radiation

While the electrical conductivity is a vital parameter for describing HED matter, the direct measurement of conductivities in laser-driven compression experiments is

challenging. Modelling processes such as planetary dynamos and the stability of inertial confinement fusion implosion requires detailed knowledge of the DC-conductivity and

the equation of state of these exotic states of matter. It has recently been demonstrated that terahertz pulses are suitable to measure the electrical conductivity of warm dense

matter and dense plasmas, as THz fields are sufficiently slowly varying that they behave like DC fields on the timescale of electron-electron and electron-ion interactions, and

hence probe DC-like responses. Here, we present recent experimental results combining long-pulse and short pulse laser systems to measure the THz reflectivity of

shock compressed matter. By taking advantage of the different Hugoniots of polystyrene and polyethylene terpthalate, our measurements span different regions of phase space

including above the carbon melting line, regions where diamonds are expected to form, and regions where hydrogen is predicted to undergo an insulator-to-metal phase

transition. From our data we extract optical constants of the compressed plastics including the refractive index and provide estimates of the electrical conductivity.   

Tailoring high amplitude plasma waves via autoresonant beat-wave excitation

Autoresonant excitation of plasma waves can be realized by using a chirped beat frequency of two driving lasers [R.R. Lindberg et al 2004 PRL 93, 055001]. The resulting robust and stable plasma waves can be controlled through the chirp, reaching amplitudes above the Rosenbluth limit [M.N. Rosenbluth and C.S. Liu 1972 PRL 29, 701] and allowing an optimization of the resulting electron acceleration. Going beyond previous studies using a cold electron fluid model, we revisit this scheme with particle-in-cell (PIC) simulations [M. Luo et al. 2024 Phys. Rev. Res. 6, 013338] to clearly identify its validity range. We push the description to a regime where previously overlooked fluid non-linearities, self-injection and kinetic effects become important. We show that frequency chirp allows efficient control of the wave amplitude and self-injection even in this new regime. We also explore multiple-dimensional effects impacting the transverse structure of the wakefield when strong electron acceleration is achieved. The analysis, supported by large scale 2D PIC simulations, shows that in spite of the reduction of spatial coherence, the acceleration of self-injected electrons remains at 70% to 80% of that observed in one dimension.