Petawatt proton beams at Lawrence Livermore

Nearly 20 years ago, researchers conducting experiments on Lawrence Livermore National Laboratory’s (LLNL) Nova Petawatt laser system — the world’s first quadrillion-watt laser — discovered that when the system’s intense short-pulse laser beams struck a thin foil target, an unexpected torrent of high-energy electrons and protons streamed off the back of the target.

Earlier this month, an international team of researchers used the Nova Petawatt’s successor, the National Ignition Facility’s (NIF) petawatt-class Advanced Radiographic Capability (ARC), to begin developing an experimental platform that promises to turn Nova’s surprise discovery into a powerful new source of protons to study the extreme conditions deep inside the planets and the stars, enhance targeted tumor therapy and advance the frontiers of high energy density (HED) science.

In two NIF Discovery Science experiments, the researchers fired four ARC beamlets at a 33-micron-thick titanium foil, setting up a strong electrostatic sheath field called a Target Normal Sheath Accelerating (TNSA) field perpendicular to the target (normal is a geometric term for perpendicular). As the field blew away from the back of the target, it accelerated high-energy protons and ions from the contamination layer of proton-rich hydrocarbons and water coating the target’s surface, all moving rapidly in the same direction.

In the experiments, two of NIF’s 192 beamlines were split to form the four short-pulse ARC beamlets. The beamlets were fired simultaneously for 10 or one picoseconds (trillionths of a second), generating up to 200 terawatts (trillion watts) of power per beamlet. The total of about 700 terawatts in the second experiment was the highest peak power yet generated on NIF.

ARC’s high peak power is made possible by a process called chirped-pulse amplification, in which a short, broadband pulse generated by an oscillator is stretched in time to reduce its peak intensity, then amplified at intensities below the damage threshold in the laser amplifiers, and finally compressed to a short pulse and highest peak power in large compressor vessels.

The new Discovery Science platform, supported by LLNL’s Laboratory Directed Research and Development (LDRD) program, is designed to study the physics of particle-beam generation at previously unexplored ultra-high short-pulse laser energies and long pulse durations. Coupled to NIF’s 1.8 million joules of ultraviolet energy, the capability will enable myriad HED applications and allow the creation and study of extreme states of matter.

After amplification in the NIF laser, the ARC beamlets are compressed in the Target Bay and focused to Target Chamber Center.

NIF is the world’s only facility capable of achieving conditions like those in the interiors of stars and giant planets. Using ARC short-pulse generated proton beams for ultrafast heating of matter to extreme states will enable opacity and equation-of-state measurements at unprecedented energy-density states.

In addition, “protons deposit their energy very specifically,” noted LLNL postdoc Derek Mariscal, lead experimentalist for the project. “That’s why protons are promising for applications such as tumor therapy. You can send a beam of protons toward a tumor and get it to deposit all of its energy exactly where you want it to without damaging other areas of the body.

“Likewise with a solid material,” he said. “(The proton beam) deposits its energy where you want it to very quickly, so you can heat up a material really fast before it has time to hydrodynamically expand — your material stays dense, and that’s the name of the game — high energy, high density.”

Once the proton-acceleration platform has been demonstrated and understood, Mariscal said, the next step in the project will be to fire the ARC beams at a deuterated carbon (CD) foil to generate a beam of deuterons. “You could impact those onto a second foil, like lithium fluoride or beryllium, and then you get a beam of neutrons — a real, laser-like neutron source, only using two beams of NIF instead of all 192.”

Goat Guy objects to tumor therapy application from Livermore press release

But, as with most things, I [Goat Guy] ended up reading the article which goes at length to identify uses for the thus-generated proton pulses … to irradiate tumors very specifically and similar. And I was struck by a “nominal tautology” that just won’t go away.

Namely, that if short pulse proton beams are useful for irradiating tumors, then giant laser facilities are not the efficient way to generate them. It also brings into question whether short duration is in fact important at all. For example, the passage of an ion beam thru living tissue like it or not does damage as it ploughs thru the layers of cells between source and target. Ideally, the radiologist tunes the energy of the beam(s) to be high enough to make it JUST to the tumor site, but low enough not to overshoot the tumor by very much. To ‘contain’ energy deposition at the tumor as one might expect.

But even with that idea, the reality is that all the tissue between the exterior of the Body and the site of the tumor must also be exposed to the high energy radiation beam. This reducible condition is dealt with by rotating either the patient’s orientation relative to the machine, or the beam’s entry-angle relative to the body. Or both. The used-to-be-hard (but now all automated in software) problem was calculating how to adjust the energy of the beam in real-time as it is rotated past thicker and thinner sections of the body. And turning it off when passing over critically vital parts of one’s anatomy (think ”the heart” or ”the esophagus” as functional body-parts that one simply cannot survive not having for more than a handful of minutes at a go.)

When I think of that, I find that there is no correlation between (beam-intensity × dwell fraction) and damage done .

Oh sure, if one is to extend the idea of a “pulse” to days, the total energy of the beam is spread out so long that ongoing self-repair mitigates the total summed damage. But on timescales below seconds, all damage is the same damage. Especially as a beam’s pulses extend deep below the microsecond-to-nanosecond timeframe.

All of the above is cited to point out the underlying disingenuousness of the article’s basis. The National Ignition Facility super-incredible lasers are tools looking for plausible future work to do. Solutions waiting for problems to solve. A humanitarian narrative on an otherwise fantastically expensive machine whose real purpose is closely related to nuclear weapons simulation research.

And finally… the bit that just got me to write this comment to start with, was the suggestion at the end of accelerating deuterons, or proton-neutron atomic nuclei as something wickedly important to the medical-irradiation side of LBL/LNL’s charter. This is unadulterated horsehurl. Going back to the 1930s, physicists have been successfully accelerating quite intense proton, deuteron, triton and helium (alpha) nuclei … and for that matter virtually ALL light nuclei isotopes … at will. In – yes fairly large – industrial machines, but still not NIF sized money-pits.

I probably am misguided for being so angry at this article’s point(s), because primary research is definitely important. But trying to co-opt a plausible medical objective with a ten billion dollar laser’s lack of practical purpose just feels smarmy. Intellectually smarmy.

There you have it. GoatGuy

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