Ravindra Kumar‘s first-floor lab isn’t particularly conducive to leisure. The windows are tightly shut and covered with curtains day and night. The room is a maze of tubes and wires, some taped to the floor, with little space left to move around. It also houses an extraordinary science project.
On two adjacent tables, Kumar has assembled a complicated set of devices that generate narrow beams of very powerful lasers. These are among the most powerful beams of light human beings have invented.
On a microscopic space, the lasers can generate temperatures of 3-4 million degree Celsius. In comparison, the temperature at the core of the Sun’s is 15 million degree Celsius. His equipment is among perhaps the 40-odd in the world – and the only such in India – to have created stars on a table. Kumar assembled the equipment, at a cost of around Rs 15 crore, to study matter at extreme conditions. The learnings from such research could find applications in better cancer treatment, magnetic storage systems and security scanners. And indeed in nuclear fusion- the holy grail of particle physics that promises clean, cheap and abundant energy for the planet.
On the scale of the universe, human perception lies somewhere in the middle – seconds, metres, grammes, and degrees centigrade are what we can know and feel. Nature extends to a large distance on either side of the human spectrum. Things happen in nature at tiny scales and lightning speeds. Energies of mind-boggling intensities are created in the stars.
Physicists have created equipment to probe matter at tiny scales on the earth. But astronomers cannot do experiments on a star. They have to rely on close observation to understand what happens inside them.
Powerful lasers now give scientists a tool to tame nature’s violence by creating tabletop stars. To be precise, lasers give them a tool to create extreme violence at such tiny scales as to not worry about their destructive potential. Per square centimetre, Kumar’s laser packs 100 million trillion watts of power, a billion times more powerful than lasers used in surgery.
The most powerful lasers in the world are 10 times more powerful, but it is not just the power that matters here. All the power of the laser is packed into short pulses lasting an extremely tiny amount of time. If you divide a second by a trillion and then divide each part by a thousand, you get what is called a femtosecond. It’s a shorter duration of time than the mind can fathom. The laser pulses in Kumar’s lab last 25 femtoseconds. This combination of great power and short duration does strange things to matter. Studying matter under extreme conditions can offer clues to what happens in the stars. It helps us to understand the earth as well.
Ravindra Kumar joined TIFR in 1991 after a PhD from IIT Kanpur in nonlinear optics – the study of how intense light interacts with matter. When a light pulse is shined on matter, the response of the substance scales in the same way as the intensity of light. When the power of light exceeds a certain stage, matter starts to change very rapidly. Kumar studied nonlinear optics in gases till 1997.
By then, lasers had become very powerful, due to an invention in the 1980s called chirped pulsed amplification (CPA). The inventors of CPA, Gerard Morou and Donna Strickland of the University of Rochester, won the Nobel Prize last year. Strickland was, in fact, only the third woman to win a physics Nobel.
After 1997, Kumar started building more and more powerful lasers and investigating solids. If you hit a piece of solid with a powerful femtosecond laser, matter is converted into a hot soup called plasma. That is the state in which matter exists in stars. Astrophysicists struggle to model plasma, and table-top laser experiments now provide a way to study them in a laboratory setting.
The plasma at such high temperatures is a swirling mass of violent particles. Called turbulence, it is a phenomenon not well understood by physicists. Turbulence is everywhere in nature. Kumar observed in his experiments that the turbulence in his table-top star resembles those observed around the sun, the solar wind and the earth. It was a clear indication of the connection between real stellar phenomena and the tiny artificial star.
Turbulent plasma creates high magnetic fields that is of great interest to scientists. Femtosecond lasers help produce tiny but intense magnetic fields, as intense as the powerful magnetic fields found in neutron stars, the densest objects in the universe. Magnetic fields determine the behaviour of many stars. Changes in solar magnetic fields can cause storms that destroy communication equipment on earth. Studying table-top magnetic fields can help scientists understand the weather in space.
Modelling nature in the extremes has many spin-off benefits. Hitting matter with a high-power laser can produce radiation, some of which can be of benefit to industry. A part of the radiation is in terahertz frequencies, useful for making body scanners. High-power lasers also produce protons that can be used to kill cancerous cells deep inside tissues, without damaging what¡¦s on the surface. Magnetic fields created by the laser-created plasma can help align electron spins, which in turn raises possibilities for magnetic storage devices in the future. Scientists believe that one day, femtosecond lasers can facilitate nuclear fusion, and thus create an abundant supply of inexpensive energy. There are benefits to taming violence.