Tunable OMEGA P9 (TOP9) Beam Project at LLE

May 2019

The TOP9 beam transported from OMEGA EP
to the OMEGA Target Bay

In both indirect- and direct-drive inertial confinement fusion (ICF) experiments, energy can be lost or misdirected when the laser beams cross with each other on their way to the target. This phenomenon, known as cross-beam energy transfer (CBET), scatters light from one beam to another, mediated by a plasma grating, and leads to beams giving up part of their energy to other beams. Splitting the OMEGA beams into three different groups, each with a separate laser wavelength (color), has been proposed because simulations modeling wavelength detuning have shown to limit the energy transferred between beams. In fact, wavelength detuning is predicted to recover much of the drive power currently lost to CBET on OMEGA. Consequently, validating CBET numerical models is an important component of simulating ICF implosions. This quest for validation stimulated the development of a CBET platform at LLE where the Tunable OMEGA P9 (TOP9), a wavelength-tunable laser, was built to study CBET into well-characterized stationary plasmas. The TOP9 system will enable robust tests of integrated CBET hydro models and demonstrate CBET mitigation using wavelength shifting.

The TOP9 beam uses OMEGA EP's short-pulse Beamline 1, for which the front end was enhanced to provide a narrowband wavelength-tunable option. The tunable optical parametric amplifier (OPA) source utilizes the OMEGA EP short-pulse optical parametric chirped-pulse–amplifier (OPCPA) system with a narrowband tunable seed laser injected into the preamplifier (bypassing the stretcher). Reconfiguration between the standard OPCPA and the tunable OPA, including timing and diagnostic changes, can be accomplished in less than 8 h.

The TOP9 development team

The TOP9 system consists of several beam transport and structural components. The periscope intercepts the OMEGA EP beam for transport to the P9 OMEGA Target Chamber port. An addition was built onto the North End Mirror Structure in the OMEGA Target Bay to support the image relay tube and downstream optics.

The beamline can deliver up to 0.5 TW in pulses as long as 1 ns in duration or 0.1 TW for pulse widths up to 2.5 ns with a wavelength that is tunable from 350.2 to 353.4 nm. Potentially harmful nonlinear optical effects were simulated and found to pose minimal threat within the system operating envelope.

A recent experiment conducted at LLE using the TOP9 beam employed a 2-mm-outer-diam supersonic gas-jet nozzle that emits a mixture of 45% nitrogen and 55% hydrogen gas. For the first 500 ps, nine large-diameter [850-µm full width at half maximum (FWHM)] plasma-forming beams were overlapped at target chamber center in a quasi-symmetric fashion, each with ≈180 J of energy. Over the next 500 ps, a single pump beam (with an angle of incidence 21.41° away from co-propagating with TOP9, 70 J of energy, and 127 GW of power) was turned on along with TOP9 at a lower incident power of 10 GW. The wavelength of TOP9 was varied between shots in order to trace out the ion-acoustic wave (IAW) resonance. To concentrate the interaction in the uniform region and increase the single-pump CBET gain, smaller (163-µm-diam FWHM) phase plates were used for the TOP9 and pump beams.

Additional heater laser beams are aimed nearly counter-relative to TOP9 and are designed to overfill the interaction volume. These can be activated to enhance the Langdon plasma effect in the experiment. There is very little interaction of these beams with the pump and TOP9 because of their wavelength and angles of incidence. All beams other than TOP9 used polarization smoothing.

For the final 500 ps, the pump was turned off but TOP9 remained on, providing a baseline plasma transmission measurement. The TOP9 beam terminated on a thin diffuser sheet inside the target chamber and was measured in transmission by a charge-coupled–device camera and a fiber-coupled streaked spectrometer (examples of the data from each diagnostic are shown here).

All laser drive fusion schemes require many overlapping laser beams to propagate through plasma in order to deposit energy at desired locations. The overlapping beams, however, can exchange energy, which subsequently impacts the distribution of laser intensity on the target. An incomplete understanding of this process can introduce errors in capsule implosion symmetry and laser drive. Four decades ago, a scientist at Lawrence Livermore National Laboratory, A. B. Langdon, predicted that the presence of the lasers in the plasma modifies the underlying plasma conditions in a way that is not taken into account in our computer modeling. It was later predicted that, if true, that modification also changes the way in which crossed lasers exchange energy. However, there was little direct evidence in fusion-relevant conditions supporting either of those theories.

Recent LLE experiments have provided data that support Langdon's theories. By creating a highly uniform plasma, heating it with multiple laser beams in a manner similar to fusion experiments, and then using a Thomson-scattering diagnostic to probe the resulting plasma conditions, we have observed the deviation from equilibrium plasma conditions that was predicted by Langdon. Using the TOP9 wavelength tunable laser, we then conducted a CBET experiment in the resulting plasma conditions and validated the expected impact on that process.