Observations of Electromagnetic Fields and Plasma Flow in Hohlraums with Proton Radiography

Abstract

We report on the first proton radiography of laser-irradiated hohlraums. This experiment, with vacuum gold (Au) hohlraums, resulted in observations of self-generated magnetic fields with peak values $10 6 G. Time-gated radiographs of monoenergetic protons with discrete energies (15.0 and 3.3 MeV) reveal dynamic pictures of field structures and plasma flow. Near the end of the 1-ns laser drive, a stagnating Au plasma ($10 mg cm À3 ) forms at the center of the hohlraum. This is a consequence of supersonic, radially directed Au jets ($1000 m ns À1 , $Mach 4) that arise from the interaction of laser-driven plasma bubbles expanding into one another. A high-Z enclosure, i.e., hohlraum, creates an environment filled with a nearly blackbody (Planckian) radiation field when irradiated by high-power lasers or energetic ions [1,2]. The cavity generates intense thermal x rays at a radiation temperature (T r ) of hundreds of eV. Hohlraums have been extensively used as radiation sources or platforms for a wide range of basic and applied physics experiments. In studies of laboratory astrophysics and highenergy-density (HED) physics [3,4], for example, hohlraums are used for creating and simulating various extreme HED conditions, including those of stellar and planetary interiors. The hohlraum radiation field is used to compress spherical capsules, through capsule ablation, to high temperature and density in indirect-drive inertial confinement fusion [1,2].The use of hohlraums requires an understanding of physics details, such as coupling efficiency, plasma conditions, instabilities, radiation uniformity [1,2,5,6], and cavity shape [1,2,7,8]. Electric (E) and magnetic (B) fields generated by several processes may have important effects on hohlraum physics and overall performance [9]. B fields inside a hohlraum can reduce heat flow, since cross field thermal conductivity is modified by a factor of ð1 þ ! 2 ce 2 Þ À1 , where ! ce is the electron gyrofrequency and is the collision time [10,11]. E fields may modify the plasma conditions and, if sufficiently large, could enhance thick-target bremsstrahlung at x-ray energies well above the Planckian background.For low-intensity laser drive, such as used in most hohlraum experiments [1][2][3][4][5][6][7][8][9], the dominant source for B-field generation is expected to be nonparallel electron density (n e ) and temperature (T e ) gradients (rn e  rT e ) [10,11]. The E field is expected to result from electron pressure gradients (rP e ) [10,11]. Despite such expectations, prior to this work, no direct experimental measurement and characterization of such hohlraum fields have been made.The first observations of E and B fields and their evolution in hohlraums, made with time-gated monoenergetic proton radiography [12], are presented in this Letter. Coupled plasma flow dynamics were also observed. Simultaneous imaging with two discrete proton energies breaks any inherent degeneracy between E and B.In the radiography setup (Fig. 1) the backlighter is a D 3 He-filled, thin-glass-shell targe...

Measurements of hohlraum-produced fast ions

“…Since SRS and TPD only occur in underdense plasmas, ≤ n c /4 and n c /4, respectively, where n c is the critical density, these mechanisms would require a large-volume ablated low-density gold plasma. Since previous proton radiography experiments showed that the timescale for hohlraum wall motion 19 is longer than the E field generation time 18 , SRS and TPD in the ablated wall plasma cannot explain the data.…”

“…8) we conclude that these fast protons cannot be produced by any LPI mechanism at the LEH window. This production mechanism is therefore associated with the general hohlraum charging, as discovered using charged-particle radiography 18 , which creates E fields of order 10 9 V/m. Such strong fields can create runaway ions.…”

“…The NIC design uses gas-filled hohlraums to impede plasma flow at the hohlraum wall [18][19][20] . This requires a thin window at the laser entrance hole (LEH) to contain the initial gas fill.…”

“…Alternatively, PDI requires a volume of plasma near n c , which means that it will be more efficient later in time when the gold plasma gradient scales are longer. Since the previously published data 18,19 demonstrate that the hohlraum potential is built up early in time, we associate the hohlraum-charging fast ions with RA.…”

The effect of shock dynamics on compressibility of ignition-scale National Ignition Facility implosions

“…Only data from the equator (DIM 90-78) is shown. This is because the polar yield data is affected by transverse electromagnetic field structures at the LEH, which can cause deflections and thus a reduction in the apparent yield observed by the WRFs on the pole [34][35][36] . A significant shot-to-shot yield variation is observed, i.e.…”