D. Sprehn

High-power klystrons for the Next Linear Collider

The Stanford Linear Accelerator Center (SLAC) version of the 1 TeV next linear collider (NLC) requires a 4:1 increase in drive frequency, from the 2.85 GHz of the 1 TeV Stanford Linear Collider (SLC) to 11:4 GHz for the NLC. More than eight years have gone into the development of a new 75-MW klystron for powering the NLC. The increase in power density and surface RF gradient at the higher frequency have rendered previous RF window and circuit designs unusable. Following numerous catastrophic gun, cavity, and window failures, new designs have evolved that solved the problems. As historys most ambitious klystron development enters its last year, the result includes a robust 75-MW peak power solenoid-focused, 50% efficient klystron. Not far behind is a 60-75-MW periodic permanent magnet (PPM)-focused 60% efficient version that will reduce the NLC electric power bill by tens of millions of dollars per year.

Design of a 11.4 GHz, 150‐MW, Sheet Beam, PPM‐Focused Klystron

The current baseline design for the 500‐GeV SLAC/KEK future collider requires approximately 5000 75‐MW, 1.6 μs, PPM pencil‐beam klystrons. A prototype is currently on test. Although the estimated cost of the klystrons is a small part of the total collider cost, this number of klystrons is at least an order of magnitude higher than the klystron population in any scientific or military system ever fielded. A back‐up sheet‐beam klystron design has been under study at SLAC for the last six years. It offers several advantages: If two sheet beams were employed in parallel, the current density at the two cathodes would be low, and the power density at the output cavity a fraction of that in the pencil‐beam klystron. Furthermore, because of significantly fewer vacuum parts, the 150‐MW SBK should have a substantially lower cost than the baseline 75‐MW pencil‐beam klystron. Finally, it is considered that because of the lower power density, a longer rf pulse (3.2 μs) could be employed. All this means is that, with m...

X-band klystron development at the Stanford Linear Accelerator Center

X-band klystrons capable of 75 MW and utilizing either solenoidal or Periodic Permanent Magnet (PPM) focusing are undergoing design, fabrication and testing at the Stanford Linear Accelerator Center (SLAC). The klystron development is part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). SLAC has completed a solenoidal-focused X-band klystron development effort to study the design and operation of tubes with beam microperveances of 1.2. As of early 2000, nine 1.2 (mu) K klystrons have been tested to 50 MW at 1.5 microsecond(s) . The first 50 MW PPM klystron, constructed in 1996, was designed with a 0.6 (mu) K beam at 465 kV and uses a 5-cell traveling-wave output structure. Recent testing of this tube at wider pulsewidths has reached 50 MW at 55% efficiency, 2.4 microsecond(s) and 60 Hz. A 75 MW PPM klystron prototype was constructed in 1998 and has reached the NLC design target of 75 MW at 1.5 microsecond(s) . A new 75 MW PPM klystron design, which is aimed at reducing the cost and increasing the reliability of multi- megawatt PPM klystrons, is under investigation. The tube is scheduled for testing during early 2001.

Periodic permanent magnet development for linear collider X-band klystrons

The Stanford Linear Accelerator Center (SLAC) klystron group is currently designing, fabricating and testing 11.424 GHz klystrons with peak output powers from 50 to 75 MW at 1 to 2 μs rf pulsewidths as part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). In order to eliminate the projected operational-year energy bill for klystron solenoids, Periodic Permanent Magnet (PPM) focusing has been employed on our latest X-band klystron designs. A PPM beam tester has operated at the same repetition rate, voltage and average beam power required for a 75-MW NLC klystron. Prototype 50 and 75-MW PPM klystrons were built and tested during 1996 and 1997 which operate from 50 to 70 MW at efficiencies greater than 55%. Construction and testing of 75-MW research klystrons will continue while the design and reliability is perfected. This paper will discuss the design of these PPM klystrons and the results of testing to date along with future plans for the development of ...

Accuracy of the equivalent circuit model using a fixed beam impedance for klystron gain cavities

Most small-signal calculations of the modulation in a klystrons gain cavity use an equivalent circuit which includes a fixed beam impedance. Comparing this calculation to the gain calculated self-consistently, we note there are appreciable errors in both the calculated amplitude and phase of the cavitys modulation. These errors may lead to large accumulated errors in determining either the tubes small-signal or large-signal gain. Both techniques are used in a comparison with an existing S-band klystron.

The design and performance of 150-MW S-band klystrons

As part of an international collaboration, the Stanford Linear Accelerator Center (SLAC) klystron group has designed, fabricated and tested a 60 Hz, 3 /spl mu/s, 150 MW klystron built for Deutsches Elektronen Synchrotron (DESY). A test diode with a 535 kV, 700 A electron beam was constructed to verify the gun operation. The first klystron was built and successfully met design specifications. This paper discusses design issues and experimental results of the diode and klystron including the suppression of gun oscillations.<<ETX>>

Sheet Beam Klystron Simulations Using AJDISK

AJDISK is a useful design tool for sheet beams. The code is capable of simulating sheet beam klystrons on modern computers in under a minute. The code can accurately and quickly predict saturated gain, reduced plasma wavelength, cavity voltages, and beam loading. Two of the output plots from the code are shown in figure 3. The gain predicted from AJDISK is in good agreement with measured results and is shown in figure 4

Pulsed rf breakdown studies

A series of experiments have been conducted to investigate the critical mechanisms involved in pulsed rf breakdown. This research has examined fundamental issues such as microparticle contamination, grain boundaries, residual gas, pulse duration, field emission, and the spatial distribution of plasma during a breakdown event. The motivation of this research is to gain a clearer understanding of the processes involved in breakdown and to determine methods to increase the breakdown threshold thereby increasing the available power in high power microwave sources and accelerator components.

Latest Results in SLAC 75-MW PPM Klystrons

75 MW X-band klystrons utilizing Periodic Permanent Magnet (PPM) focusing have been undergoing design, fabrication and testing at the Stanford Linear Accelerator Center (SLAC) for almost nine years. The klystron development has been geared toward realizing the necessary components for the construction of the Next Linear Collider (NLC). The PPM devices built to date which fit this class of operation consist of a variety of 50 MW and 75 MW devices constructed by SLAC, KEK (Tsukuba, Japan) and industry. All these tubes follow from the successful SLAC design of a 50 MW PPM klystron in 1996. In 2004 the latest two klystrons were constructed and tested with preliminary results reported at EPAC2004. The first of these two devices was tested to the full NLC specifications of 75 MW, 1.6 microseconds pulse length, and 120 Hz. This 14.4 kW average power operation came with a tube efficiency >50%. The most recent testing of these last two devices will be presented here. Design and manufacturing issues of the latest klystron, due to be tested by the Fall of 2005, are also discussed.

MAGIC code development for klystron applications at the Klystron Department at SLAC

We are currently addressing the lengthy simulation issue by operating parallel processor versions of MAGIC 3D. The Klystron department cluster at SLAC is a typical Beowulf Linux cluster running 24 processors in dual CPU boxes. Thanks to the cooperation of MRC, SLAC has been able to finish the parallel operation feature that MRC had operational and adjust it for accuracy against the single CPU version. We were also able to port Magic to Linux where parallel operation runs on our cluster. During parallel software development, it became obvious to us that the Magic user interface did not lend itself to the automation of numerical comparisons between runs.