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last update: 10/03/09
|KEKB crab cavity may
help LHC upgrade
In 2009, the KEKB accelerator set a new world luminosity record with the help of a newly installed device called a crab cavity, and the integrated luminosity of KEKB reached 1,000 inverse femtobarn. This is the world’s first successful installment of a crab cavity. KEK scientists’ expertise in this new technology will help in an upgrade of the Large Hadron Collider at CERN. Here is why.
On Thursday February 18, prominent particle physicists and Ministry staffs gathered in one room at KEK to celebrate the great achievement of KEKB: the integrated luminosity of 1,000 inverse femtobarn. Nobel laureate and Professor Emeritus of KEK, Makoto Kobayashi, said of the particle collider that experimentally confirmed his Nobel prize winning theory: “KEKB has brought out its best and beyond.”
An inverse femtobarn is a measure of the rate of particle collisions per area. When KEKB began operation in 1999, the target integrated luminosity was 500 inverse femtobarn. Indeed, Kobayashi was surprised by the speed at which upgrades to KEKB were improving the luminosity. In 2003, then a head of the Institute of Nuclear and Particle Science, he reset the target integrated luminosity at 1,000. “To achieve the world record in luminosity is not a simple task, but is instead the result of hard work, brilliant ideas, and good luck,” says KEK’s Director General Prof. Atsuto Suzuki. “Everyone who has contributed in to achieving this milestone is to be celebrated.”
One important milestone was the successful installment of a crab cavity in each ring of KEKB. KEKB is well known for continually setting and breaking the instantaneous peak luminosity world record since 2001. It has achieved a peak luminosity of 2.11 x 1034 inverse square centimeters per second (21.1 inverse nanobarn per second), over twice as much as the original design peak luminosity. The installation of crab cavities in KEKB gave the final and vital kick to produce this luminosity record.
A particle accelerator drives particle beams that consist of a series of particle bunches. At KEKB, there are two separate beams, an electron beam and a positron beam. The beams cross at a horizontal angle in the center of the Belle detector. The bunches are ribbon shaped, around 5 millimeters long, 100 micrometers wide and a micrometer thick. Because the bunches are so narrow, not all region interacts when they collide. To increase the number of particle collision events, accelerator scientist Prof. Robert Palmer came up with a concept called a crab crossing more than thirty years ago. A device called crab cavity gives a tiny sideways kick to each particle bunch so that bunches collide head on, rather than crossing in an angle.
“The idea was already there, but the technology to make it work is very difficult,” says Prof. Andrew Hutton, the head of the accelerator division at Jefferson Laboratory, who chaired the KEKB review committee. Because of the thinness of the two bunches, the slightest error in positioning can cause them to slip right past each other, without interacting at all. It is the team at KEK that took the complex design behind the crab cavity and made it work.
The LHC is a ring, 27 kilometers in circumference, and 100 meters underground. The LHC creates a pair of proton beams, travelling in opposite directions, and smashes the beams together. It smashes protons at four major experimental sites—ATLAS, ALICE, CMS, and LHCb—to look for new elementary particles and evidence for new physics. The LHC successfully completed a test run late last year at a beam energy of 1.18 TeV, producing a wealth of data. This made it officially the world’s most powerful particle accelerator. This year, the LHC is starting with actual physics experiments at an energy of 3.5 TeV, half the designed energy of 7 TeV.
Because of the intense radiation from the particle collisions, detector hardware corrodes very quickly. The hardware damage would only increase the time the hardware takes to process a signal without an error. In order to keep up the good performance, one thing scientists can do is to increase the rate of particle collisions per area. To investigate the feasibility of using crab cavities at the LHC, the LHC crab cavity team at CERN is joined by the accelerator scientist who developed the crab cavity at KEKB, Prof. Kenji Hosoyama of KEK.
A cavity that contains electromagnetic waves inside
Hosoyama and his team have worked on various superconducting technologies, including both superconducting magnets and superconducting cavities. When he began working as an accelerator scientist over 30 years ago, superconducting cavities were just emerging. He illustrates the miracle of superconducting cavities using an example from his childhood.
Hosoyama carried his disappointment until the day he learned about superconducting cavities. In a superconducting material, electrical current can flow without any resistance. This characteristic allows for extraordinarily perfect reflection of electromagnetic waves (light) at the surface of a superconducting material.
The quality of a superconducting cavity is measured by the number of cycles (one cycle is a singe back-and-forth reflection) for which an electromagnetic wave can be contained inside. This is called Q value. Using a normal conducting mirror, microwaves can be contained for about 104 cycles before the light is reduced to half it's original brightness. On the other hand, using a superconducting mirror, the light can be contained for 109 cycles before it gets reduced to half. This means that, if one were to use superconducting mirrors, the microwave would last for an order of seconds, a hundred thousand times longer than for normal conducting mirrors. It also means a hundred thousands times less heat generated, and a hundred thousands times less power required to generate the electromagnetic waves that drive particles (though it costs power to cool the cavities to the superconducting temperature).
Designing a crab cavity
According to Professor Hutton, “Akai-san is the father [of the KEKB crab cavity], and Hosoyama-san is the mother.” Prof. Kazunori Akai of KEK, then on sabbatical at Cornell University, did the baseline design work for the B Factory crab cavity in 1993. Cornell University’s proposal for the CESR-B Factory required crab cavities. The design was also applicable to KEKB.
The difficulty with this scheme is that the high current beam can also excite tens of unwanted modes inside the cavity. The primary mode of concern is one large loop of magnetic field around the beam’s path, called an accelerating mode. The secondary mode of concern is a two-loop magnetic field 90 degrees rotated from the fields of crab mode with respect to the beam axis. This mode is called vertical polarization mode. (The crab mode is thus a horizontal polarization mode.)
Akai worked to design a crab cavity that eliminates these unwanted modes. After a few months of hard thinking, his final cavity design came to be a squashed, oval cell that continues to a beam duct that contains another coaxial beampipe inside. The ingenuity of this design is that it eliminates unwanted modes efficiently while maintaining the high Q value for the crab mode.
“However, a slight misalignment of the coaxial beampipe can create an electric monopole that can couple to the crab mode, which takes a portion of the crab mode away from the inside cell,” says Akai. So he also developed a filter called notch filter. A narrow notch of depth that’s one-forth of the crab mode wavelength goes around the beampipe to send the crab mode back to the cell if it ever arrives.
Akai also calculated various other parameters such as maximum electric and magnetic field strengths to determine the optimal shape for the crab cavity. He then took the completed design and actually built a miniature model cavity, cooled it to superconducting temperature, and measured the fields. “Two crucial requirements need be satisfied for a B Factory crab cavity. First is the sufficiently strong field of the crab mode, and second is the beam stability under the high current beam environment that B Factory generally requires,” explains Akai. His model crab cavity successfully demonstrated every feature required by B Factory experiment.
KEKB crab cavities
Starting from the baseline design Akai developed, Hosoyama drew a more detailed conceptual design with his pair of compasses, and then a technical design with CAD (mostly because his favorite compass broke). “Crab cavities were not of immediate importance for the KEKB team, and were neglected in some years,” says Hosoyama. KEKB did not require crab cavity to achieve the target luminosity. A detailed simulation study at KEK later showed that crab crossing actually improves luminosity greatly. This motivated the development of crab cavities. The team persevered for thirteen years. “Persistence paid off eventually,” smiles Hosoyama.
Even for Hosoyama, an expert on superconducting cavities, developing a crab cavity was difficult work. For one thing, the cavity requires an asymmetric shape, and this made the fabrication process extremely complicated. Second, the sheer size of the cavity, 1.5 meters long and 1 meter wide, was a challenge at the time of fabrication and when trying to rotate the cavity during electro polishing and rinsing.
Hosoyama and his team spent years on the design of the KEKB crab cavities, producing multiple prototypes, using a test-and-modify approach. The team developed every piece of equipment necessary to fabricate, rinse, and polish these complicated structures. Superconducting meant the whole metal structure, including inner pipe, bolts and holes, had to meet at superconducting temperature. At the temperature, material volume would be significantly reduced, because the material shrinks when cooled. This made it hard to have the pieces fit together tightly. “We made a couple of trials before we finally had them perfectly fit,” says Hosoyama.
Once the crab cavity was built, the focus shifted to concerns about the possibility of a helium leak. Liquid helium circulates around the system to keep the system cool. If something in this system were to go wrong, the damage could be catastrophic. “We spent some time preparing an assembly station that could fix a damaged crab cavity,” says Hosoyama. ”Fortunately, there were no problems with the cooling system.”
The world record luminosity
Prof. Hutton recalls: “it was a disappointment when the crab cavity started.” The luminosity did not go up, but rather went down. An investigation was conducted, re-examining the system from both the theoretical and the technical side. A group at KEK proposed the idea that particles with different energies may be the culprit. Generally particles’ vertical motion couples with the horizontal motion (x-y coupling). The effect was corrected for the particles of design energy. In reality, the energy of the particles in a bunch varies slightly. It turns out that the x-y coupling in different energy particles cannot be taken care of by the ordinary measure.
LHC crab cavity design
There are several different types of crab cavity. KEKB’s oval shape is one. Another type is a mushroom shaped cell. Another type uses a waveguide to take out unwanted modes. Right now the LHC crab cavity team is considering all available options for the baseline design.
“Because it worked here [at KEKB], they [the LHC team] are now brave enough to take on a different design,” says Prof. Hutton. The problem with the design of the KEKB crab cavities is that they are too voluminous. The beampipe separation at LHC is just 20 centimeters currently, while the KEKB type crab cavity has a widthwise dimension of 1 meter. Even though the KEKB type crab cavity is already proven to work, it is not applicable to LHC unless the size is reduced, or the beampipe separation is increased.
The LHC isn’t the only place that is hoping to make use of the superconducting crab cavity. From the International Linear Collider (ILC) to short pulse light sources around the world, the potential utility of crab cavities is great. Akai and Hosoyama are happy that their efforts over the past decade became fruitful. As Hosoyama says, “keep at it and be patient, and you will be rewarded.”
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