Top-level schematic of the experiment. Sixty OMEGA beams drive a spherical implosion, which is backlight by the EP laser-generated protons and imaged on a radiochromic film detector.
A coronal plasma forms around an imploding capsule due to ablation blow off from the OMEGA drive. The coronal plasma can flow around the backlighter foil to reach where the short-pulse beam propagates. This impedes the short-pulse propagation to the focal point, leading to reduced proton maximum energy and yield.
Sixty OMEGA beams drive the capsule implosion. X rays from the capsule can preheat the backlighter foil, which will reduce the backlighter performance.
If there is a pathway for fast electrons to form a return current to the backlighter foil within the backlighter pulse, then the sheath field can be neutralized. This reduces TNSA production. Return current can be mitigated by ensuring that the target scale lengths are large enough that the current cannot flow during the pulse duration.
Backlighter design used in these experiments. Shown is a cross section, where the design has cylindrical symmetry around the central axis (except for the target-positioned stalk).
Images of fielded backlighters. From top to left: (a) and (b) two isometric views of a backlighter, (c) side-on view of backlighter, (d) view from TCC of backlighter, and (e) and (f) shadowgraphs of pre-shot backlighter and capsule in OMEGA target chamber.
The radiography time-of-flight, magnification, and EP backlighter performance depend on the backlighter-object distance (d o ) and object-film distance (d i ). Optimized values, as used in these experiments, are d o = 1.2 cm and d i ∼ 30 cm.
Radiochromic (RC) film pack design for detection of proton radiographs. The pack consists of interleaved filters (Al or Ta) and films.
Sensitivity versus proton energy for film 5, chosen as an example of typical behavior. (a) Full range of proton energies typically produced. (b) Zoomed in view of the peak structure.
Time-of-flight curves for d o = 0.6 cm (dotted line), = 0.9 cm (dashed line), and = 1.2 cm (solid line). The points mark specific film energies (see Table II).
RC film sensitivity, as energy deposited per proton, folded with an assumed exponential proton distribution and plotted versus initial energy. Ten of 11 films are shown, from film 1 to 10 from left to right (Film 11 is of-scale to the right). The dashed line represents the assumed exponential normalized source distribution.25
RC film sensitivity, as cumulative energy deposited for protons with energy ⩽E vs E (i.e., a running integral of Fig. 11. Each film is normalized to the total sensitivity. Ten of 11 films are shown, from film 1 to 10 from left to right (Film 11 is off scale to the right).
(a)–(d) A series of radiographs of the filamentary field structure around an imploded capsule, OMEGA shot 61 250. For film energies and timing, see Table II. (e) Timing for OMEGA shot 61 250, corresponding to radiography presented in (a)–(d). The black curve represents the average OMEGA implosion drive intensity (pulse shape FIS3601P) as measured on shot. The vertical dashed line shows when the EP short pulse beam was fired. The gray box represents the sample times for Films 3 through 9, which recorded useful data on this implosion (see also Table II).
Comparison of 60-beam OMEGA radiographs using backlighters without (left) and with (right) a preplasma shield, as discussed in Sec. III. The implosion is at the center of each image; (a) is dominated by large-scale diffuse structure and (b) is dominated by filamentary field structures.
Film 1, d i = 69cm, OMEGA shot 61 247. The image is of an undriven implosion. In this case, d i is too large so only the lowest proton energy film recorded useful data. On a nominal performance backlighter shot, with d i optimized, Film 1 saturates.
Film 1, OMEGA shot 63 031. (a) Normal film scan and (b) enhanced contrast image. The EP backlighter was fired at reduced energy (40 J) for timing purposes without an implosion target in place, which also gives a uniformity measure. Here we can see some low-amplitude large-scale spatial non-uniformities, but clearly distinct from implosion effects in Fig. 13.
Film pack filter materials and thicknesses.
Film pack proton energy of maximum sensitivity, ε, and time-of-flight (TOF) to the subject implosion d o /v p for d o = 1.2 cm.
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