A drop of methanol is distorted to form a cup of hot plasma. Plasma waves are driven in this cup by an intense laser, releasing hot electrons, followed by accelerated protons. Credit: The image has been created by the authors.
A high energy proton accelerator, that can be used on a table-top has been enabled by TIFR researchers using university-class lasers.
Laser Ion acceleration uses intense laser flashes to heat electrons of a solid to enormous temperatures and propel these charged particles to extreme speeds.
These have recently gained traction for applications in selectively destroying cancerous tumour cells, in processing semiconductor materials, and due to their excellent properties – for imaging and fusion relevant conditions.
Massive laser systems with several Joules of light energy are needed to irradiate solids for the purpose.
This produces a flash of ions which are accelerated to extreme speeds. Thus, emulating large million-volt accelerators is possible within the thickness of a hair strand.
Such lasers are typically limited to a few flashes per second to prevent overheating and damage to laser components.
Thus laser driven ion accelerators are limited to demonstrative applications in large experimental facilities.
This is far from real world applications that desire that the flashes of high velocity ions are available much more frequently.
Small lasers supplying several thousands of flashes are routinely present in small university laboratories, operating at a thousandth of a Joule of laser pulse energy.
Known mechanisms of laser-driven ion acceleration would predict that ion acceleration by a few kilovolts is possible in these conditions.
This is far below the MeV range ions driven by large scale lasers and this trade-off poses a fundamental challenge in developing ion sources with a high rate of repetition.
Bridging the gap
In a recent study, SV Rahul and Ratul Sabui from the Tata Institute of Fundamental Research (TIFR) Hyderabad, led by Professor M Krishnamurthy, have bridged this gap – producing Megavolt energy protons using few millijoule lasers, repeating a thousand times a second.
They leverage a well-known impediment to laser ion acceleration schemes – namely pre-pulses to their advantage.
Pre-pulses are small bursts of laser energy preceding an intense laser pulse. They originate in laser systems due to various imperfections.
The ion acceleration process relies on the premise of a single intense laser pulse heating a target. However, pre-pulses prematurely alter the surface of the solid, often even destroying the fine features present on them.
Dedicated systems are often necessary to suppress pre-pulses, adding to complexity and limiting the scalability. Instead of removing the pre-pulse, the TIFRH group demonstrated a method to harness its effects.
In their experiments, the pre-pulse sculpts a hollow cavity in a liquid microdroplet, creating a low-density plasma. This becomes a fertile ground where laser pulses are absorbed to drive a pair of gigantic waves in the plasma.
These waves tend to rapidly collapse as they travel, releasing bursts of energetic electrons. These electrons are eventually responsible to drive efficient acceleration of protons to hundreds of kilovolts.
Operating at a thousand times per second and employing millijoule energy laser pulses, approach enables efficient ion acceleration.
Without requiring extreme laser intensities or suppression of parasitic pre-pulse, this approach paves the way for high-repetition-rate laser-driven ion accelerators on university lab tabletops.
The study is published in Physical Review Research.
