I am doing a presentation on cooling techniques for low temperature physics. I know laser cooling and evaporative cooling are major ones. What other methods are used to cool particles to the lowest temperatures?
The method of adiabatic demagnetization: you apply a field to an insulated collection of paramagnetic spins or magnetic moments (eg., a rare-earth salt), then remove the field. The system of spins re-organize into chaos (i.e. wiping out the order that the field induced), absorbing energy from the surroundings...and cooling is achieved. It's the preferred method of cooling to the very lowest temperature ever achieved, in the MICROkelvin range of temperature.
Laser Choices-zero in on your Laser Specifications Before Buying
It will save a lot of time and effort if you determine the type of laser you need for your application before shopping around. This article will lead you through the “mysteries” of lasers to your final selection. We will not go through the technical theories of how a laser works, but what lasers are used for what applications. We will determine the proper choices for Marking, Engraving, Cutting, Welding, and Scribing.
There are many types of lasers, each having different characteristics and differing interactions with various materials. We need to know the lasers output wavelength, average power, peak power, pulse rate, beam quality, and beam size. It will also help us to understand the conversion efficiency and consumable requirements of a particular laser to evaluate the operating cost.
The following are the various types of lasers we will be considering for the applications I mentioned above:
The CO2 Laser, with a wavelength of 10,600 nm (nanometers), reacts best with organic materials, wood, plastic, paper, glass and fabrics but can be used for metal applications at the higher power levels. With output power levels from 10-Watts to 25-Kilowatts, these lasers can be used for marking, engraving, cutting, welding and scribing. Metals are very reflective to the wavelength of CO2 Lasers and they do not work well for marking metals due to the lower power levels required for marking. CO2 Lasers can operate in the continuous wave (CW) mode or a pulsed mode. However, the peak power in the pulse mode generally never exceeds twice the CW power.
The latest CO2 lasers, 10-watts to 500-watts are generally RF excited diffusion cooled and sealed units. 10-100-watt CO2 lasers are air cooled and water cooled at power levels above 100-watts. With the water cooling requirement, a refrigerated water chiller is generally necessary. A CO2 laser is approximately 10% efficient, so 90% of the input power is dissipated in heat that needs to be removed from the laser by either air or water cooling, thus further decreasing the wall plug efficiency. At power levels over 500-watts, CO2 lasers need to be provided with a laser make-up gas (the lasing medium) to maintain the output power level of the laser. This make-up gas is a consumable and adds to the operating cost. The sealed lower power CO2 lasers generally can last 3 to 5 years before needing to be recharged with gas.
The “Q” Switched Nd:YAG or Vanadate Laser with a wavelength of 1,060 nm is best used for marking and scribing applications. These lasers react well with metals, ceramics, and plastics for marking applications. The average output power levels of these lasers generally range from 5-watts to 100-watts, and the newer units are diode pumped (excited) rather than flash lamp pumped. The unique feature of this Nd:YAG laser is the “Q” Switch which turns the laser beam on and off at frequencies from 1 kHz to 50 kHz.
On the off cycle, the diodes continue to pump energy into the laser crystal so that when the beam is turned on again it releases a very high peak power pulse in the multi-kilowatt range. This high peak power pulse quickly breaks down the surface of the material being marked and virtually vaporizes it. These higher power pulses also help in producing a contrasting color when marking plastics.
The Diode Pumped Nd:YAG lasers are generally water cooled via small refrigerated chillers to maintain laser output power stability and cool the diodes and laser rod. The solid state Nd:YAG or Vanadate laser rod (crystal) is the lasing medium and will last indefinitely if cared for properly. The laser pumping diodes will generally last from 10,000 to 20,000 hours before replacement is required. The most frequent maintenance required is a change of water, water filter, and anti-algae compound in the closed loop refrigerated chiller’s water circuit every 3 months. Laser optics will also have to be cleaned periodically.
The “Q” Switched Nd:YAG and Vanadate lasers can also be frequency doubled to 532 nm (green), frequency tripled to 355 nm (ultraviolet), and frequency quadrupled to 266 nm (deep ultraviolet). The shorter the wavelength the smaller the spot size that the laser beam can be focused to. However, for each conversion to a shorter wavelength, the laser power is significantly decreased and the laser price tag is significantly increased. These shorter wavelengths are generally needed when processing micro-electronic devices that require the finest detail or resolution that can be achieved.
The Pulsed Nd:YAG Laser with a wavelength of 1,060 nm is suited to intricate metal welding, cutting, and drilling applications. The average power level of these lasers range from 15-watts to 600-watts and are flash lamp pumped. The pulse rates are generally 1 to 25 pulses per second for lasers with power levels up to 50-watts and 1 to 1000 pulses per second for the highest power lasers. The main feature of the Pulsed Nd:YAG laser is its high energy per pulse which can be up to 80 joules for the highest power laser. This laser functions by using overlapping precisely controlled laser pulses to control the progression of the cut or weld, and in the case of drilling several pulses can be delivered in the same location. For cutting and drilling applications, a high pressure small orifice coaxial gas assist nozzle is used to help remove the molten material from the cut path or drill hole. In the case of welding, a larger orifice lower pressure coaxial nozzle is used to deliver a blanket of inert gas cover to the molten weld area to prevent oxidation.
Because the Pulsed Nd:YAG laser uses a broad spectrum flash lamp for laser pumping (excitation), it is fairly inefficient at converting the electrical input power to laser power. It has an efficiency of approximately 3%. These lasers are water cooled at the low end via air/water heat exchangers and at the higher power levels by refrigerated chillers. The flash lamps require changing every 500 to 1000 hours of operation. Higher maintenance costs are associated with changing flash lamps, aligning and cleaning optics, and maintaining the water purity in the closed loop chiller systems. All of this said, it is still the only laser that can be used for certain applications.
The Fiber Laser, with a wavelength of 1,060 nm, is the newest laser on the block. It is diode pumped and has power output levels from 5-watts to 10 plus kilowatts. From 5 to 20-watts, it can be “Q” switched for marking and scribing applications including plastics. At the higher power CW mode of operation, it can be used very effectively for metal cutting and welding applications. The beam quality of the Fiber laser is generally better than other 1,060 nm lasers so it will process parts faster for any given power level. The laser beam is delivered to the work piece via small diameter fiber cable with a focusing head so it is very flexible when trying to fit it into tight quarters or moving the laser beam at high speeds on an X-Y-Z Axis Gantry System. One laser can also share its laser beam with several workstations by switching the beam from one fiber optic cable to another.
The Fiber laser has a wall plug efficiency of nearly 30%. This means it will take the least amount of electrical power to operate for any given power level. Pump diode lifetimes in excess of 100,000 hours are projected. This is truly a maintenance free laser. Fiber lasers from 5-watts to 100-watts are air cooled and over 100-watts are water cooled. Because of the higher efficiency of the fiber laser, the refrigerated chiller size can be smaller than other types of lasers operating at similar power levels.
Cutting, Welding and Marking capabilities of the several laser types mentioned above:
1. Low power CO2 lasers are used for marking and engraving wood, plastic, paper, fabric, and other organic materials. These lasers can also cut paper and wood up to 1/8” thick.
2. High power CO2 lasers are used to cut wood up to 2” thick and steel up to 5/8” thick. They can also weld steel up to 1/2” thick.
3. “Q” switched Nd:YAG and Vanadate lasers are used for marking and scribing metals, plastics, ceramics and other dense materials.
4. Pulsed Nd:YAG lasers are used for intricate cutting and welding applications and also used for hermetically sealing small electronic device enclosures and medical implant devices. They can also be used for the intricate cutting of blood vessel stents. Other higher power applications include the drilling of cooling holes in high temperature turbine engine blades and vanes.
5. Low power “Q” Switched Fiber lasers are used for marking metals and plastics. They can also be used to scribe solar cells and ceramic materials.
6. High power Fiber lasers are used to cut and weld metals. They can cut metals up to 1” thick and weld metals up to 5/8” thick.
There are many other applications that these lasers can be used for other than the ones listed above. You should now be prepared to approach either a laser manufacturer or laser systems manufacturer that produces the laser type you feel would be required for your application. Most all manufacturers will process evaluation samples at no charge to verify the results you are looking for. Other information that they will need is your desired production rate, your part handling preferences, and automation required. Information about various laser systems and laser component manufacturers can be located at http://www.allthelasers.com .
Robert Holland is the Technical Advisor for allthelasers .com. Allthelasers.com is a depository for information and links to help users find the appropriate laser systems and component vendors to fill their laser requirements. Mr. Holland has held various laser related management, engineering, and sales positions for laser and laser systems manufacturers since entering the industry in 1972. He has a thorough knowledge of industrial high production laser systems and their applications.
what's the temperature of laser dot that stings your hand?
