#vacuumtech

The Webb Space Telescope Finishes Cryogenic Testing

Vacuum chambers come in many shapes and sizes, from small, hermitically sealed microelectronics to huge space systems test chambers to the 4km long LIGO chambers. On December 1st, the James Webb Space Telescope emerged from the large Chamber A test facility at Johnson Space Center, which is a great piece of vacuum technology.

Webb inside Chamber A 38544426831_7ba4e21f98_b.jpg

NASA’s James Webb Space Telescope sits inside Chamber A at NASA’s Johnson Space Center in Houston after having completed its cryogenic testing on Nov. 18, 2017. This marked the telescope’s final cryogenic testing, and it ensured the observatory is ready for the frigid, airless environment of space. Credits: NASA/Chris Gunn

The James Webb Space Telescope will be a large infrared telescope with a 6.5m primary mirror. The scheduled launch date is in the spring of 2019. It will be lifted on an Arian 5 rocket from French Guiana.

Before a space instrument is put into service, it must be rigorously tested in a space simulation chamber. NASA’s Johnson Space Center’s Chamber A was prepared. The vault-like, 40-foot diameter, 40-ton door was sealed shut on July 10, 2017, beginning the scheduled 100 days of cryogenic testing for NASA’s James Webb Space Telescope in Houston.

There are two important reasons that Webb needs to be tested in a space simulation chamber. First is that the telescope will be operating in a far orbit at cryogenic temperatures. Webb was designed so that the instrument will come to thermal and mechanical stability to known dimensions in the cold of deep space. Secondly, the infrared equipment must be tested in a cryogenic chamber because objects at room temperature emit infrared energy that would swamp the instruments, so putting the space telescope in a cryogenic chamber all but eliminates background infrared energy.

chambera_door_closing.jpg

Engineers watch as Chamber A’s colossal door closes at NASA’s Johnson Space Center in Houston.
Credits: NASA/Chris Gunn

The chamber evacuation sequence started on July 20, 2017. Engineers began to bring the chamber, the telescope and its science instruments down to cryogenic temperatures a process that took about 30 days. The cold gaseous helium shroud inside Chamber A is the innermost of two shrouds used to cool the Webb telescope down to the temperatures at which it will operate while in orbit. This shroud sits inside an outer liquid nitrogen shroud.  During cool down, Webb and its instruments transferred their heat to surrounding cold gaseous helium and liquid nitrogen shrouds. Liquid nitrogen reaches 77 Kelvin (minus 321 degrees Fahrenheit/minus 196 degrees Celsius), while the cold gaseous helium in the shroud gets as low as 11 Kelvin (minus 440 Fahrenheit/minus 262 Celsius).

Webb remained at “cryo-stable” temperatures for about another 30 days. The thermal sensors kept track of the temperature of the telescope, while the specialized camera systems monitored the physical position of Webb’s components as they moved during the cool down process. These tests included an important alignment check of Webb’s 18 primary mirror segments, to make sure all of the gold-plated, hexagonal segments acted like a single, monolithic mirror. This was the first time the telescope’s optics and its instruments were tested together, though the instruments had previously undergone cryogenic testing in a smaller chamber at Goddard.

webb_infographic_v6-final.jpg

NASA’s James Webb Space Telescope cools to cryogenic temperatures by radiating heat through the surrounding vacuum, to the liquid nitrogen and cold gaseous helium shrouds. The outside dimensions of Chamber A are 19.8m (65ft) in diameter and 33.6m (120ft) in height. The chamber is equipped with staged roughing pumps, valved turbo molecular and cryo absorption pumps, and 20 K (-424 oF) helium refrigeration units. The high vacuum pumps speeds are rated at 2 x 107 liters/sec condensibles and 3 x 105 liters/sec noncondensables at 1 x 10-6 torr pressure.

 

Systems testing continued and on Sept. 27, the engineers began to warm the chamber back to near room temperature, before pumping the air back into it and unsealing the door. This was done slowly and on the 18th of November, the chamber door was unsealed.

The Second Edition of Understanding Modern Vacuum Technology is Now Available!

2nd Edition FLAT REV.jpg

Cover art for the book Understanding Modern Vacuum Technology, 2nd Edition 

The second edition of Understanding Modern Vacuum Technology is now available! This book contains all the information found in the first book PLUS information that is not found in any other book to date.

What’s new? There is information on the cutting edge technology being done at NIST to develop new quantum based pressure standards. This is so new that the project is moving from the concept and feasibility phase to developing the standards that will replace the mercury manometers that are the primary standards used today.

There is an extensive section describing the Granville-Phillips VQM™  (Vacuum Quality Monitor). This is a revolutionary method for measuring the gas composition in a high vacuum system based on ion trap technology.

The second edition contains an introduction to leaks and their detection.

There are now knowledge check questions at the end of each chapter.

The book is available through Amazon.com and Amazon’s European websites including Amazon.co.uk, Amazon.de, Amazon.fr, Amazon.it, and Amazon.es. For larger quantities, it is recommended to order from the Understanding Modern Vacuum Technology book store.

A personal landmark is disappearing

I got a tweet this morning from UVM. That is where I earned my degrees in physics. The tweet said that there is a webcam feed to watch the Cook Science Building deconstruction. This is where I began my journey into vacuum technology. Below you can see bricks being torn off of the facade facing east towards Mount Mansfield.

Vacuum Technology, Cook Science Building.jpg

On the fourth floor, you see three big empty rooms. That is were I used to teach undergraduate physics labs to young aspiring engineers, scientists and pre-med students. Across the building  from the yellow arm on the fourth floor was my vacuum lab where I I had a glass and metal UHV system that was the heart of my thesis research. Off to the right on the fourth floor I can see the window to my office I had as a graduate student. So many memories.

The original idea was to renovate Cook, which was built in 1969. The deconstruction of Cook was priced at $4M, however the old building had a $28M maintenance cost. A new building makes good engineering sense. This is part of a master plan to increase STEM delivery at UVM.

As I look back in time, I could not see myself in my lab thinking about the day that Cook would come down. What I did know was that I was learning skills that I could take out into the work world. I can remember making the decision to dig deeper into vacuum technology as a way to stay connected with science and engineering after graduating…and putting some brass in pocket.

 

 

LIGO Vacuum Systems and Gravitational Waves

LIGO is an acronym for Laser Interferometer Gravitational-wave Observatory. The purpose of LIGO is to detect gravitational waves. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein’s mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that ‘waves’ of distorted space would radiate from the source. In the semiconductor industry, I am concerned with a final product on a microscopic, nanometer scale on microprocessor chips. LIGO is at the other far end of the curve dealing with massive object colliding in space.

The problem is that detecting gravitational waves is an extremely difficult task. The waves will cause distortions in space on earth that are shorter than the dimension of an atom’s nucleus. In fact the distortions are on the order of 1/10,000th of the diameter of an atomic nucleus. The detector has to be super sensitive and in a very quiet location. Vacuum technology plays a key role in this experiment.

ligo-livingston-aerial-03.jpg

LIGO Livingston. Courtesy Caltech/MIT/LIGO Laboratory

LIGO consists of two interferometers, each with two 4 km (2.5 mile) long arms arranged in the shape of an “L”. Each chamber encloses 10,000 cubic meters of volume. One interferometer is located in Hanford, Washington and the other in Livingston, Louisiana. The reason for two is that the earth is a very active place with lots of human hustle and bustle. There are earth quakes and storms. So if the detectors both capture the same signal, then that is strong evidence that the signal is a gravitational wave.

When gravitational waves pass through the system, the distance between the end mirrors and the beam splitter lengthen in one arm and at the same time shorten in the other arm in such a way that the light waves from the two arms go in and out of phase with each other. When the light waves are in phase with each other, they add together constructively and produce a bright beam that illuminates the detectors. When they are out of phase, they cancel each other out and there is no signal. Thus, the gravitational waves from a major cosmic event, like the merger of two black holes, will cause the signal to flicker, as seen here

Gravitational waves sent out from a pair of colliding black holes have been converted to sound waves, as heard in this animation. On September 14, 2015, LIGO observed gravitational waves from the merger of two black holes, each about 30 times the mass of our sun. The incredibly powerful event, which released 50 times more energy than all the stars in the observable universe, lasted only fractions of a second.

In the first two runs of the animation, the sound-wave frequencies exactly match the frequencies of the gravitational waves. The second two runs of the animation play the sounds again at higher frequencies that better fit the human hearing range. The animation ends by playing the original frequencies again twice.

As the black holes spiral closer and closer in together, the frequency of the gravitational waves increases. Scientists call these sounds “chirps,” because some events that generate gravitation waves would sound like a bird’s chirp.

Audio Credit: Caltech/MIT/LIGO Lab

The lasers are operated in a vacuum level on the order of 10-9 torr. This ensures that there are no air currents causing distortion of the laser beams either through transmission of sound or thermal energy. Also it lessens the chance of particle movement in the vacuum system.

BEAM_TUBE_MFGR_AND_WELD_PHOTO.jpg

Spiral welding a section of a vacuum tube. Courtesy Caltech/MIT/LIGO Laboratory

LIGO’s vacuum tubes were constructed of spiral-welded 3 mm thick 304L stainless steel. With its relatively low carbon content, 304L steel is resistant to corrosion, especially at the critical welded seams. The 1.2 m diameter beam tubes were created in 19 to 20 m-long segments, rolled into a tube with a continuous spiral weld. To prevent collapse, LIGO’s tubes are supported with stiffener rings that provide a significant layer of resistance to buckling under the extreme pressure of the atmosphere. The tubes must withstand these stresses for at least 20 years.

Evacuating the chambers took 40 days of constant pumping to evacuate them to their optimal operating pressure. In that time, turbomolecular pumps removed the bulk of the air in the tubes while the tubes themselves were heated to 150-170 degrees C for 30 days to drive out residual gases.

The gases that remain in the system are primarily H2 and water vapor. There are liquid nitrogen cryogenic panels in place to capture the stray water molecule and ion pumps to capture H2 gas. There is so much more technology involved in the LIGO detectors. I encourage you to visit the LIGO website. Although LIGO depends on extreme vacuum engineering, the vacuum technologies involved are explained in Understanding Modern Vacuum Technology.

NOTE: On June 1st, 2017, LIGO made their third detection of a gravitational wave event from the collapse of 32 solar mass black hole and a 19 solar mass black hole forming one large black hole of 49 solar masses. The means that two solar masses of material were transformed into energy by the collision.

Working principles of a turbomolecular pump

Turbomolecular pumps are found in every semiconductor fab, the vast majority of helium leak detectors and research laboratories. Yet most never give them much thought, one simply roughs out the vacuum chamber and spin them up. In a few minutes, your chamber is in the high vacuum range.

Today I want to introduce to you the working principles of these pumps. First is a video that shows how a Pfeiffer HiPace 2300 moves gas molecules from the chamber to the exhaust port of the turbomolecular pump. As you watch gas flow through the pump in the video, get the idea that the pumping principle is to increase the probability that the gas molecules will be impelled by the rotor (the spinning blades) into the stator (stationary) blades towards higher concentrations of gas. Pumping is achieved by directing the gas molecules from the low pressure inlet to the higher pressure exhaust port.

Turbomolecular pumps do not have liquid or contact seals. The rotor blades are typically spinning a few tenths of a millimeter from the envelop of the pump and the rotor shaft is a few tents of a millimeter from the stator blades. These gaps are necessary for the operation of the pump, however they do allow a small fraction of the gas to backstream (flow backwards, if you will) through the pump. This is one of the reasons that turbomolecular pumps must have a mechanical pump providing a rough vacuum pressure on the exhaust port. In other words, the turbomolecular pump cannot provide sufficient compression to move gas to atmospheric pressures and they need to be “backed” by a roughing pump.

The video below was produced by Agilent. It shows how a typical turbomolecular pump is built.

I hope this gives you some basic understandings of turbomolecular pumps. Understanding Modern Vacuum Technology has a section devoted to turbomolecular pumps. It covers the development of the pumping principles, the operation and safety considerations when implementing turbomolecular pumps. UMVT  is available for $59 at Amazon.com. You will find information UMVT that is not available in any other book to date.

Get an Intuitive Feel for Gas Properties with this Simulator

Today I want to bring attention to the PhET Gas Properties Simulator. This is a wonderful tool that allows a learner to interact with all of the levers in a gas system. The tool is set up so that the learner can add gas particles into a chamber and then see the results as the system’s parameters (temperature, volume, etc.) are changed.

In the figure below, I introduced equal amounts of a heavy gas and a light gas (60 of each) and then opened the lid on the top of the chamber a bit to see what happens. After letting the simulation run for a short while I could see that the light gas was escaping faster than the heavier gas, the temperature was dropping in the system along with the pressure.

Capture PhET1.PNG

60 heavy gas molecules and 60 light gas molecules were introduced into the chamber. The lid was cracked open and gas was allowed to escape. At the time of the screen shot, 55 heavy gas molecules and 50 light molecules remained in the chamber.

There are also some graphs that give insight of the molecular speeds and energies of the molecules. Lessons and exercises are available on the PhET website.

As a vacuum technologist, I think in terms of molecular flow where the molecules interact with the walls of a conduit rather than collide with each other. When opening the lid to allow that gas to expand out into space, it occurred to me that this is also a good tool to help convey the idea that in order for a pump to remove a gas molecule from a chamber, the molecule has to enter the pump’s inlet. Since the lid has a variable aperture, it can be use to introduce the concept of how a high vacuum pump’s pump speed is dependent on the inlet diameter .

The Gas Properties Simulator is suitable for high school, college, and continuing education students. I have loaded it onto my work laptop and will be using this in my corporate vacuum lectures in the future. I hope you will find it useful too.

Understanding Modern Vacuum Technology discusses the gas properties that can be explored with this simulator. UMVT also discusses vacuum pumps and pressure measurement.