Ignition research continues at the giant laser facility, but the end of the calendar year looks like a more significant date than that set by Congress.

Staff at the National Ignition Facility (NIF) say that work to generate inertial confinement fusion with energy gain will continue as planned, despite the end of the official “ignition campaign” last week.

September 30 saw the expiration of an arbitrary “deadline” for achieving ignition that was set by US Congress, prompting speculation about the future of the laboratory, whose primary function is to simulate the physics of nuclear weapons, but for which fusion energy has become another long-term development target.

Just before that date, which marked the end of the US government’s financial year rather than anything of more scientific significance, a New York Times article suggested that the failure to meet the ignition goal could have “serious repercussions” for not only the giant Lawrence Livermore National Laboratory project (estimated cost so far: $5 billion), but federal financing of “big science” in general. A follow-up editorial in the same newspaper added that Congress would need to look hard at whether either of the “stockpile stewardship” or long-term energy goals could be pursued on a smaller budget.

NIF officials have long expressed their confidence that the system will eventually succeed in “bringing star power to Earth”, as a giant banner at the facility puts it, and told optics.org earlier this year that it was “tantalizingly close” to that goal. But others, including those working on the rival magnetic confinement approach to fusion, are more skeptical, doubting that the laser technique will ever work on a scale that makes for a practical energy source.

A memorandum to the Department of Energy dated July 19 added fuel to that skepticism, even though advisor and memo author David Crandall wrote that the functionality of the laser, its diagnostics, optics and targets – as well as the laser operations performed by the NIF team - were all “outstanding”.

The problem was that the same memo also noted that “considerable hurdles” must still be overcome to reach the ignition goal, or to observe unequivocally the phenomenon of alpha heating – a key element of the fusion process. Given those issues, Crandall and his fellow reviewers said that the probability of demonstrating ignition before the end of this year was now “extremely low”, and that even the less ambitious goal of showing unambiguous alpha heating would be “challenging”.

“While no reviewer thought ignition likely before December 31, 2012, some thought the intermediate goal of measurable alpha heating (increasing the neutron yield) might be achieved within that time, and several expressed optimism about achieving ignition at NIF within a few years,” concluded the memo.

Model problems
According to the same document, the reason has nothing to do with NIF not working to its specifications – on the contrary, the 192-beam system is actually outperforming expectations in many areas. The key problem seems to be that the “hohlraum” ignition target and its interaction with the laser is not behaving in the way that physical models had predicted.

“The coupling of the laser through the radiation inside the hohlraum to the capsule is less efficient than expected and the physical ablation process is somewhat different than expected - resulting in a lower implosion velocity than is predicted to be required for ignition,” the review panel wrote.

NIF told optics.org that it was working to resolve what it called the “remaining few issues” towards achieving ignition in its current campaign. According to officials, that campaign has so far been able to demonstrate the fundamental conditions required to achieve ignition – though, crucially, not all at the same time.

“Achieving ignition conditions requires four things,” the lab explained. “An implosion velocity of 370 km/second, creating a symmetrical hot spot at the center of the target, proper plasma mix and uniform compression.”

On the question of alpha heating, NIF says that alpha particles have been produced from fusion reactions, and have compressed fuel to a sufficient density to re-deposit energy.

However, the lab concedes that its experiments have not yet produced the kind of results that had been predicted in its models, and said that it was continually refining these as more experimental data was collected. It is also working to produce even higher laser energy pulses (of 2 MJ, compared with the 1.8 MJ thought to be sufficient) as one way to overcome the less efficient coupling between the laser and hohlraum than was initially expected.

In demand

“September 30 marked the end of the National Ignition Campaign, but does not mark the end of ignition research or an expiration of the value of the facility,” said officials. Highlighting the scientific value of the system, they added: “As a measure of its success, there are now requests from its user communities for more than 500 experiment days in 2013, about twice the NIF capacity. Requests for use of NIF extend for many years into the future.”

As things stand, NIF will be able to continue operations as planned through fiscal year 2013, though its funding beyond that remains to be determined by future government budget cycles.

Responding to the reaction to the expiration of the Congress “deadline” in some quarters, the lab stressed the unpredictable nature of the work, saying: “Ignition experiments on NIF are continuing steps in a well-managed and deliberate scientific program, not the ‘pass/fail’ event that it has become - and one that should be tied to the process of discovery science and the expansion of knowledge, not fiscal year boundaries.”

In fact, it looks like the end of this calendar year could turn out to be a much more significant date than September 30, in terms of NIF’s future direction and the ambitious goal of harnessing “star power” on Earth. Crandall’s memo raises the prospect that NIF will take on a quite different role if experimental results and computer models for ignition continue to contradict each other.

The memo states that if alpha heating and “further substantial progress” towards ignition is not demonstrated before the end of December, the ignition program should be redirected to a “broader and more balanced research program” – suggesting that the pursuit of fusion power will take a back seat.

Mike Hatcher

Editor in Chief of optics.org

For more information visit: www.optics.org