Sub-100 psec x-ray gating cameras for ICF imaging applications

Abstract

The glass laser Inertial Confinement Program in the USA has made significant achievements in laser compression of capsules filled with thermonuclear fuel. To achieve a high degree of implosion, a symmetric irradiation is essential and is achieved by a large number of smoothed laser beams directly impinging on a target or by indirect drive(1 '2)• The key diagnostic used to monitor arid adjust the symmetry of indirect drive at LLNL and to some extent direct drive at LLE is time resolved two dimensional x-ray imaging, which allows the experimentalists to observe in emission the symmetry of the imploding shell during the implosion phase, and the size and shape of the hot fuel in the stagnation phase. An example of a sequence of 14 images graphically illustrating a direct drive implosion with a well balanced direct drive laser system is shown in Fig. 1. The need for this capability has focussed the high speed photography effort at LLNL and LLE. In the last several years, demonstrated that 80 Ps x-ray gating with 30 mm detector plane resolution is routine, and 35 ps x-ray gating has been achieved and will soon be in routine use soon. Electro optic tubes can achieve x-ray gating but the limited spatial and temporal resolution(3' 4), caused us to focus our effort on the gating of microchannelpiate (MCP) proximity focussed x-ray detectors. The only significant disadvantage of the gating MCP's over electro-optical tubes is that many images on exactly the same line of sight cannot be achieved. However, multiple images with gated MCP's can be obtained along the lines of sight which are insignificantly different from each other, with spatial and temporal resolution that are more than adequate. Most importantly, gated MCP detectors are sufficiently practical that several of them are in routine use on large laser systems such as the Nova glass laser at LLNL and the Omega glass laser at LLE. In this paper we report on several of the technical advances made at LLNL in the gating of MCP x-ray detectors over the past two years, and show typical results obtained from implosions. The essential features of a gated MCP detector are shown in Fig. 2. A pulsed voltage (typically 1 kV, 100 psec) is applied across a MCP. The voltage is applied by gold conducting layers on the MCP, which form a microstrip line with the glass of the MCP being the dielectric(5). While the voltage is applied the photo electrons resulting from xray photons incident on the coated surface of the MCP are amplified with typically i03 gain. Because the gain is non-linear with the applied voltage, (subject to consideration on the electron transit line in the MCP, Section IV) there can be narrowing of the 'optical' gate with respect to the electrical gate. This relaxes the pulse voltage requirement for the MCP, to FWHM -100 psec, and voltages of typically 1 kV into typically 25W(6). In section II we discuss the novel approach we use for the generation of the electrical pulses required for the gated MCP cameras. The spatial resolution and the factors affecting it are described in Section III. We discuss the temporal resolution of the MCP detectors in Section IV, and in Section V we discuss the configuration of microstrip coating on MCP detectors we have used and some typical results from laser driven implosions. Off line tests showing ultra font gating are described in section VI.