The National Ignition Facility (NIF) for laser driven nuclear fusion at the Lawrence Livermore National Laboratory (USA) (Moses, Reference Moses2008) finished the research campaign budgeted for the end of September 2012 without reaching the final goal (Clery, Reference Clery2012). Further steps are needed to achieve the world's very first break-even for generating more nuclear power than applied for the ignition. The generated fusion reactions need more than 1000 times higher gains. The project for the crucial option of future energy production developed most outstanding technology for building the world's largest laser and is one of the highest global achievements in science and engineering.
For the next steps, we present an example on how a comparably little change of working conditions can result in a most enormous increase of nuclear fusion gains rising by a factor of more than 1000. This is an experience from the same field of lasers which happened as an example of the new nonlinear physics, opening a whole new dimension of exploration for knowledge and over-whelming qualities of living (Hora, Reference Hora2000).
The highly sophisticated work with the most outstanding results of NIF was summarized (Glenzer et al., Reference Glenzer and Moses2011) by an extremely experienced large team. The crucial importance of this work into the next phase of exploration may be illustrated by statements from the leading conference in this field, September 2011, the “7th Inertial Fusion Science and Applications” in Bordeaux/France. A keynote lecture was presented by the Under-Secretary of Energy of the US-Department of Energy, Steven E. Koonin. He mentioned that his department was then publishing a report about energy. In the discussion, the leader of NIF, Ed Moses, asked why the report did not mention any word about nuclear fusion.
Koonin replied that this will change when NIF achieves ignition. One year later this goal still has to be reached (Clery, Reference Clery2012) where the difficulties may very probably be due to the fact that all was exclusively based on the scheme of indirect drive spark ignition (Lindl, Reference Lindl, Hora and Miley2005) where a gain G of nearly N = 1015 fusion neutrons was produced by laser pulses of E = 1.8 MJ energy. This low gain may not be a surprise because the warning was given by a key expert in a summary at an earlier IAEA-Fusion conference about laser-driven fusion (Meyer-ter-Vehn, Reference Meyer-Ter-Vehn1996), underlining “the most difficult aspects” for spark ignition which is extremely complicate. In contrast to this scheme of ignition, it was known before that the highest gains of neutrons N were 2 × 1014 with 35 kJ laser pulses (Soures et al., Reference Soures, Mccrory, Vernon, Babushki, Bahr, Boehli, Boni, Bradlay, Brown, Craxton, Delettrez, Donaldson, Epstein, Jaanimagi, Jacobs., Kearney, Keck, Kelly, Kessler, Kremes, Knauaer, Kumpan, Letzring, Lonobile, Loucks, Lund, Marshall, Mckenty, Meyerhofer, Morse, Okishev, Papernov, Pien, Seka, Short, Shoup Iii, Skeldon, Skoupski, Schmid, Smith, Swmales, Wittman and Yaakobi1996), however, only by using the alternative scheme of direct drive volume ignition (Hora et al., Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998). This kind of ignition uses a much less complicate scheme (Hora et al., Reference Hora and Ray1978; Hora, Reference Hora1981; Reference Hora1987; Reference Hora1991; Hoffman et al., Reference Hoffmann, Blasevic, Ni, Rosmej, Roth, Tahir, Tauschwitz, Udera, Vanentsov, Weyrich and Maron2005) as outlined by Lackner et al. (Reference Lackner, Colgate, Johnson, Kirkpatrick, Menikoff, Petschek and Miley1994). The contrast of this volume ignition against spark ignition is known. Spark ignition usually had 1000 times the lower gains under comparable conditions. Experiments with conditions of volume ignition arrived approximately at gains of N neutrons
$$N = 10^{12} E^{2}/4 \quad \lpar E \, {\rm in \, kJ}\rpar \comma$$by direct drive of micro-balloons filled with high pressure deuterium-tritium (DT) gas up to E = 35 kJ (Soures et al., Reference Soures, Mccrory, Vernon, Babushki, Bahr, Boehli, Boni, Bradlay, Brown, Craxton, Delettrez, Donaldson, Epstein, Jaanimagi, Jacobs., Kearney, Keck, Kelly, Kessler, Kremes, Knauaer, Kumpan, Letzring, Lonobile, Loucks, Lund, Marshall, Mckenty, Meyerhofer, Morse, Okishev, Papernov, Pien, Seka, Short, Shoup Iii, Skeldon, Skoupski, Schmid, Smith, Swmales, Wittman and Yaakobi1996, see Hora et al., Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998). Extending this relation should lead to about 1018 neutrons for breakeven with E = 2 MJ, see the diagram of Nakai (Reference Nakai2008) (Fig. 1). One has to be aware after the critical views (Clery et al., Reference Clery2012) that a fusion gain above breakeven up to 10, produced by incident laser energy of 2 MJ needs a neutron number up to 1019.
Fig. 1. (Color online) Measured and projected neutron gains at DT fusion using ignition by spherical laser compression depending on the energy of the laser pulse following Nakai (Reference Nakai2008).
If the earlier results of 1000 times lower neutrons with spark ignition could be applied, the result of NIF with 1015 neutrons would well be the best they could measure, as achieved but this is 1000 times too low for breakeven. It was reported (Banks, Reference Banks2013) that a review was requested by the US-Congress “from the National Nuclear Security Administration (NNSA) which funds NIF … submitted in December … it adds that the neutron yield remains a factor 3–10 less than that required to ignite “alpha heating,” in which energetic alpha particles heat the fuel — a key step toward ignition.” How should this jump of the gain happen by a factor of more than 1000 when the present gain is increased by a factor 3 only?
The way may be by an appropriate combination of the spark ignition with volume ignition. For volume ignition, such jumps were the result of detailed computations and had led to the discovery of the volume ignition in 1978 (Hora et al., Reference Hora and Ray1978; Reference Hora1987; Lackner et al., Reference Lackner, Colgate, Johnson, Kirkpatrick, Menikoff, Petschek and Miley1994; Atzeni, Reference Atzeni1995). It was surprising that all experiments with the highest gains by direct drive were achieved with this scheme (Hora et al., Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998). Indeed this was related to the reheat based on Denis Gabors's (Reference Gabor1952) collective stopping power used in the computations for volume ignition (Hora, Reference Hora1991) since 1978. This collective stopping was used also in the alternative picosecond nonlinear force driven block ignition scheme with ultrahigh plasma acceleration (Hora, Reference Hora2009; Hora et al., Reference Hora, Miley, Ghoranneviss, Malkynia, Azizi and He2012; Lalousis et al., Reference Lalousis, Hora, Eliezer, Martinez-Val, Moustaizis, Miley and Mourou2013). The predominance of the Gabor collective effect at very high plasma densities was measured also in other in another connection (Hoffmann et al., Reference Hoffmann, Weyrich, Wahl, Gardes, Bimbot and Fleurier1990).
The giant jump in the fusion gain can be seen from the initial result which led to the discovery of volume ignition (Hora et al., Reference Hora and Ray1978) in Figure 2. Increasing the input laser energy E L only by 50% resulted in an increase of the fusion gain by a factor 2714. What was reported then and repeated in numerous computations (Stening et al., Reference Stening, Khoda-Bakhsh, Pieruschka, Kasotakis, Kuhn, Miley, Hora, Miley and Hora1992) is shown in Figure 3 with comparison of experiments (Hora et al., Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998). At adiabatic isochoric compression of spherical uniform DT fusion fuel of solid density n s and volume V 0s, fusion gains G are calculated depending on the input energy E 0 for the laser driven adiabatic compression. The reached maximum density is given by the parameter on the right-hand upper side fully drawn optimum lines. For low compression and low energy, each case produces a gain given in the kind of parabolic curve “Standard isochore” with measured examples of the points A, B, C, D. If the temperature at highest density is too low, less reactions are produced, if too high, the expansion is very fast with adiabatic expansion and the gain is lower again. Without re-heat and nearly no re-absorption of bremsstrahlung, the parabolas result in an optimum combining at the fully drawn lines. What happens above G = 8 for DT, is that the dashed parabolas jump up to nearly vertical lines. These represent the giant jump of the gain. From the printing output of the time dependence of the reaction, the results of the kind of Figure 2 showed the extremely strong increase of the gain G by a little bit increasing E 0, see the dashed deformation into a nearly vertical part of the parabolas in the upper right area of Figure 3.
Fig. 2. Time dependence of the average energy of the particles in the expanding reacting DT sphere with low gains G of 0.77 at input laser energy of 20 MJ, while 30 MJ result in a very high jump of the gain of 1900 due to volume ignition by alpha particle re-heat and bremsstrahlung re-absorption (Hora et al., Reference Hora and Ray1978).
Fig. 3. Optimum fusion gain G (fully drawn lines) of DT depending on the input energy E 0 into the reaction sphere of initial uncompressed volume V 0s with fusion burn for G of less than eight and of ignition for higher gains. The standard isochore parabola (dashed) is the deformed with a very steep increase showing a jump by volume ignition (Hora et al., Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998).
The possibility for combining the present spark ignition at NIF with the results of volume ignition to occur in the outer mantle of low temperature high density fuel may well lead to the discontinuity-like jump, how an increase of the present gains by a factor 3 to 10 can well lead to an increase of the gain by a factor of more than 1000 (!). Analyzing one case of Storm et al. (Reference Storm, Lindl, Campbell, Bernat, Coleman, Emmett, Hogan, Horst, Krupke and Lowdermilk1988), case C in Fig. 3, in Hora et al. (Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998) resulted in volume ignition of the core of the spark ignition. Conditions may well be found now, how the same volume ignition will happen in the high density low temperature mantle of the initially intended spark ignition NIF experiments.
Other very positive next steps for NIF have been mentioned e.g., in favor of direct drive by Christopher Deeney (NNSA) (Clery, Reference Clery2012) who mentioned that experiments with direct drive are being scheduled for 2013 for conditions of direct drive volume ignition as in the cases of Figure 1 in contrast to spark ignition. This should lead to the expected 1018 or more neutrons with the 2 MJ laser pulses of NIF of ns duration according to Figure 1 and Eq. (1). Much change of the laser irradiation from polar to spherical geometry may definitely not be needed for first checks. It is well known from the LLNL (Storm, Reference Storm1986) that the comparably very high gains (see experimental point C in Fig. 3) with direct drive (Hora et al., Reference Hora, Azechi, Kitagawa, Mima, Murakami, Nakai, Nishihara, Takabe, Yamanaka, Yamanaka and Yamanaka1998) were performed with non-spherical irradiation. When this will be successful, the re-design of NIF to spherical irradiation may be interesting if not an idealized optical irradiation front cannot be found by numerical techniques for polar irradiation.
