Commissioning the Linac at NSLS-II

May 11, 2012

The commissioning of the National Synchrotron Light Source II’s linear accelerator, or linac, signifies the first step in the completion of NSLS-II — a multi-year, multi-million dollar project that will yield the most sophisticated and powerful synchrotron light source in the world. NSLS-II will provide both hard and soft x-ray light, so scientists may image, probe and analyze complex biological systems and the properties of materials. The 200 megaelectron volt (MeV) linac will be the facility’s electron source.

Commissioning is the final test with beam after the Linac and the beam transfer line that connects the linac with the booster synchrotron was assembled, installed and tested. Commissioning is a task that is being performed at NSLS-II by engineers from the Linac’s German manufacturer, Research Instruments, with help from Brookhaven staff.

“The significance of the linac’s being commissioned is that this is the first part of NSLS-II to come to life,” explained Raymond Fliller of the Photon Sciences Directorate, the linac commissioning coordinator. “So, during commissioning, we are getting the linac functioning for the first time, making sure that it meets all of our specifications. And then, once it’s commissioned, it will be used as the injector to the booster accelerator, which is the next piece of the machine to be installed. The booster accelerator takes the electrons from the linac and then gives them their full energy so it accelerates them further and then puts them into the storage ring.”

Once the facility is complete, the storage ring will circulate the electrons, and scientists will be able to use the light that is released from the ring.

None of this can happen, however, without a fully functioning Linac that can produce a well-tailored amount of electrical charge matching precisely the needs of the storage ring for injected beam intensity. One of the objects of commissioning is to achieve a very high total charge of 15 nanocoulombs of charge per electron bunch train (corresponding to about 10 billion electrons), which make up the beam, said James Rose, the Radio Frequency Group leader.

“The goal of commissioning is to meet these beam parameters, which are extremely stringent,” said Rose.

The impact of the successful commissioning of the linac has a farther impact than just the completion of one step and the advance to the next, which is the booster commissioning to be completed in December of this year.

“The linac is a necessary part of the accelerator chain and much of the quality of the beam that we can provide to our users depends on the quality of the beam that you can produce in the Linac,” said Ferdinand Willeke, Photon Sciences Accelerator Division director.

Beam performance is of paramount importance to NSLS-II’s success and is what distinguishes the forthcoming resource from the Lab’s first generation light source, NSLS. NSLS-II will produce brightness of four orders of magnitude greater than the NSLS, added Willeke.

Brightness is related to what makes up the beam: electron bunches. The beam itself is not a continuous stream of electrons, but a train of short beam pulses called electron bunches. The smaller the bunch cross-section for the maximum bunch intensity, the better the brightness and beam quality. To achieve this optimum, the linac has to inject a beam often and maintain consistent performance.

“The intensity and the uniformity of the intensity over the train of bunches of the linac are both important in establishing a uniform train of bunches in the storage ring,” said Willeke. “That’s why the linac contributes a lot to the quality of the service that we can provide.”

The linac’s ability to perform is linked to its power source. Feng Gao, a Linac project engineer, said that the solid state modulator, which generates the radio frequency power that feeds the accelerating structures that transfer the energy to the beam, is a fairly recent development that has several advantages.

“This is the first linac on site with solid-state modulators,” said Gao. “They are easier and safer to operate. They operate with 30 to 40 times less voltage and still get the same performance.”

The NSLS-II linac commissioning is led by the injector and radiofrequency groups but incorporates all of the groups of NSLS-II, including the mechanical and utilities groups, the diagnostics group, the controls group, and the vacuum group.

“We are currently in the middle of the linac commissioning,” said Timur Shaftan, the Injector Group leader. “The bunch trains produced by the Linac are being tested by the diagnostic transport lines that we developed at BNL for the machine commissioning.” Tests have been running in two shifts per day, for at least eight hours each shift, five days per week. The testing frequency has increased near the end of the commissioning process.

“Our linac is a high performance machine as compared with existing linacs for synchrotron light sources,” commented Shaftan. “Our next step is booster commissioning and we are looking forward to successful completion of the injector project next year.”

There are currently 18 beamlines, with 21 independent endstations, planned for the first few years of NSLS-II’s operation, which is set to begin, at latest, in 2015. Also, a number of proposed beamlines may be installed after NSLS-II begins its regular function, increasing the variety of the experiments that can be performed at the new light source.

“NSLS-II will actually open the door to a completely new class of experiments, which are the next logical step in photon sciences. With NSLS-II and the unique properties of its photon beams, the Laboratory will remain a very attractive place to carry out research and perform experiments,” said Willeke. “It is an important factor in maintaining Brookhaven as a first-class science facility.”

2012-3054  |  INT/EXT  |  Newsroom