# Presentation: 206029

## SNS SCL HISTORY

- Decision to switch to superconducting option was made in the 2nd year of the construction project based on conceptual design spearheaded by Yang Cho
- Design was a compromise forged between several partner laboratories that resulted in a less than optimized design

## COMPROMISE WITH PARTNER LABS

- There was no real incentive for partners to be bold, but there was a defacto incentive to minimize achievement risk at the expense of other partners
- Communications and decision making process was slow because of geographics
- Command and decision chain weak

## EFFECT OF SCHEDULE ON DESIGN

- No time to do cavity and cryomodule development – Chose existing JLAB cryomodule design to save design and drafting time.
- Physics studies, and possible design innovations, were hard pressed to keep pace with construction schedule since AE & CM were preparing bid packages.
- Choice of major parameters, like accelerating gradient, transition energy between medium and high beta cavities, size of microphonics and Lorentz detuning coefficient, were effected.

## EFFECT OF COST

- The Conceptual Design had a cost estimate based on one cavity per klystron topology that was not vetted by any outside review team prior to the decision to switch to the superconducting option. It was considered substantially too low when finally reviewed.
- The idea of using fast ferrite phase shifters to control phase and amplitude was formulated to get back to many cavities per klystron.
- There were insufficient R&D funds to provide for parallel study programs that might have helped strategic planning.

## SCU PARAMETERS DRIVING DESIGN

- Peak Gradient of 27.5 MV/m +- 10%
- Microphonics as large as 120 Hz at 6 sigma
- Lorentz detuning as large as 2-3 Hz/(MV/m)2  accelerating gradient
- Input coupler match up +- 20%

## FERRITE TUNER GOALS

- 60 degrees of phase shift (later expended to 80 degrees)
- Response of  >1 degree per microsecond
- Low insertion loss < 0.15 dB
- Input VSWR of 1.2

## TRANSIENT PERFORMANCE FOR 6 CAVITIES DRIVEN BY ONE KLYSTRON AND FERRITE TUNERS  

## SUMMARY OF FERRITE PROGRAM

- Small signal prototype with reduced height waveguide built using ferrite that was readily available
- Yoon Kang devised a two-slab, LH/RL circulated polarized device with dielectric slab input matching
- 40 degrees of phase shift was achieved with 6” long ferrite slabs. 80 degrees of shift could have been achieved with 12” ferrite slabs
- Tests were performed with a step response power supply that operated at 30 volts and 300 amperes.
- Optimization of the design and power supply driver were never done.

## TWO INCH HEIGHT WAVEGUIDE LOADED WITH YIG AND ALUMINA SLABS

## MEASURED PHASE SHIFT AND INSERTION LOSS VS BIAS CURRENT

## MEASURED PHASE SHIFTER RESPONSE

## REASONS FOR NOT ADOPTING FERRITE TUNERS

- Concern that ferrite tuners might not achieve their performance goals (especially whether they’d ever be fast enough)
- The LANL group believed that the ferrite tuners would make the system more complicated because of the extra  power supply connections, water connections, and low level controls that they required
- LANL felt it could simplify the one-on-one design to make the cost acceptable (11-12 klystrons per modulator unit, ganged water systems and interlocks, etc.)

## NEGATIVE ASPECTS TO FINAL TOPOLGY

- Ganged klystrons require all klystrons to operate at same gun voltage
- Replacing a single klystron is not easy
- Waveguide switching schemes ala APS is impractical

## REVISITING THE FERRITE TUNER ISSUE

- More than likely the basic cavity parameters driving the ferrite tuner design can be considerably reduced compared to what was specified for the SNS cavities and cryomodules.
- Piezoelectric and mechanical piston tuners can reduce the demand on the range and response of ferrite tuners.
- The added complexity of ferrite tuners is more than likely one of those “in the eye of the beholder” issues. The individual klystron approach requires water connections for each collector and focusing magnet, requires power connections for filaments and focusing solenoids, and requires low level controls and predriver and driver amplifiers for each klystron.

## ADVANTAGES OF USINGFERRITE TUNERS

- Single klystron per modulator allows each klystron to be operated at its optimum gun voltage and makes klystron replacement and handling much easier
- Fewer klystrons allows consideration of using waveguide switching to increase availability without increasing reliability.
- Believe it will lead to considerable cost savings even with ferrite tuners