By Heather Rock Woods
SLAC is creating radically new modulators to power the
next linear collider accelerator. Modulators convert wall-plug energy from
PG&E into a high-voltage pulse that fuels the klystrons, which in turn
propel particles down the linac.
The next linear collider project (NLC) aims to operate at
10 times the energy of SLAC’s accelerator. This will require a 20-mile
accelerator and enormously better modulators and klystrons, which are the
heart of the radio frequency (RF) system that accelerates particles to
nearly the speed of light.
In collaboration with LLNL and Bechtel-Nevada, SLAC has
designed a prototype solid state modulator that recently met two major
milestones: driving four XL4 (50 megawatt X-Band) klystrons at full peak
current and voltage, and creating the ideal shape pulse required for
constant RF power. Richard Cassel (ESD) is the project leader at SLAC.
The Solid State Induction Modulator team beside the
8-Pack Modulator. Front row, left: Richard Cassel (ESD), Dan Moreno (ESD),
Chris Pappas (ESD), Minh Nguyen (ESD). Rear row, left: Marc Larrus (ESD),
Brad Hickman (LLNL), Ray Larsen (TD), Piotr Blum (ESD), Ed Cook (LLNL),
Jeff de Lamare (ESD), Craig Brooksby (Bechtel)
(Photo by Diana Rogers)
The old, shed-size modulators, which have served SLAC
since its inception, use switch tubes that are now obsolescent and would
last just over a year of full-time running in the NLC. When triggered, the
switch tube sends a short high-voltage pulse from a pulse-forming network
(PFN) to a transformer, where it exits at 350 kiloVolts (kV) to drive the
klystron cathode. These modulators are sufficient for operating the
roughly 240 klystrons on SLAC’s linac.
But the old design won’t work for NLC. Just the thought of
procuring enough of the short-lifetime switch tubes for over 4,000 more
powerful klystrons sent the R&D team to the drafting board.
"The old modulators are totally impractical for NLC,
they’re not flexible and reliable enough. We’d worry whether we could keep
the machine running," said Ray Larsen, NLC Program Manager. "They’re also
expensive, and we challenged ourselves to build a more reliable,
energy-efficient and cheaper design."
The new modulators put solid state switches on standard
circuit boards. The prototype currently being tested at the lab’s NLC Test
Accelerator (NLCTA) employs 76 boards stacked in a cabinet with a unique
3-turn, tri-axial transformer that steps up the stack’s voltage by a
factor of three. Each board handles 2.2 kV, enabling the modulator to
generate the 500 kV the NLC needs to feed its new 75 megawatt klystrons,
which were also designed and built at SLAC.
"The prototype has a lot of beautiful features," said
Larsen over the modulator’s buzzing noise. Each board handles relatively
low voltage, so is easier to insulate. The solid state switches—Insulated
Gate Bipolar Transistors (IGBTs)—turn on and then off to make the short
pulse. The pulse length and shape are easily adjustable, a major advantage
over the PFN design. The modulator is powerful enough to drive eight
klystrons at a time. It runs even if a few boards aren’t working.
And, the prototype has successfully pumped four XL4
klystrons to provide the required RF peak power for testing the critical
RF distribution and accelerator beamline components.
The modulator team’s next challenge is to achieve reliable
24/7 operation to support the NLC full-power demonstration program
scheduled over the next one to two years. Meanwhile, the team has just
manufactured the first test section of a new prototype geared to run two
klystrons instead of eight, because the latest design calls for a more
spread out RF system. The cylindrical ‘2-Pack’ modulator relies on 12
boards with fewer switches operating at 4.0 instead of 2.2 KV, and 11
turns in the transformer to generate the 500 kV.
The group continues to improve the IGBT switch to make it
faster and better suited for short pulse work. Originally designed to
drive electric trains in Europe, the switches will drive even faster
objects wherever the new machine gets built.
For more information, see: