Linac Low-Level RF Upgrade

1. All Experimenters Meeting April 20th 2009 Linac Low-Level RF Upgrade Presenters: Ed Cullerton (AD/RF Dept) & Trevor Butler (AD/Proton Source Dept) Contributors: Larry Allen, Fernanda Garcia (AD/PS), Brian Chase, Paul Joireman, Vitali Tupikov, Philip Varghese (AD/RF), Michael Kucera (AD/Controls)

2. Linac Low-Level RF (LLRF) Upgrade • Upgrade Motivation • Linac LLRF History • Design Goals • RF Simulation • LLRF System Design • Upgrade Status • Operational Effects • Summary Trevor Butler & Ed Cullerton

3. Linac LLRF Upgrade Motivation • Proton Plan Goals • Maximize proton delivery • Increase machine reliability • Reduce enclosure activation • Linac Proton Plan Goals for LLRF Upgrade • Decrease beam energy spread by improving both gradient amplitude and phase • This improvement allows for easier capture of Linac beam into Booster • Reduce enclosure activation by reducing the amount of unused beam accelerated through Linac Trevor Butler & Ed Cullerton

4. Linac LLRF History • 805 MHz Side-Couple Section (116 – 400 MeV) • Digital VXI based LLRF system designed in 1994 for 400 MeV Linac Upgrade • Uses both feedback & feed-forward to regulate amplitude & phase • 201.25 MHz Drift Tube Section (.75 – 116 MeV) • Originally designed in late sixties with a minor upgrade in 1994 • The LLRF system is a discrete, analog, component based module • Uses only feedback to regulate amplitude and phase • Lacks any type of feed-forward control • Without any LLRF feed-forward control, it takes 10 s after beam enters the cavity for the feedback system to stabilize the RF amplitude and phase. Since this unstable beam can not be injected into Booster, it is chopped and sent to the dump at the end of Linac, resulting in additional enclosure activation Trevor Butler & Ed Cullerton

5. Linac LLRF Design Goals • Generate 201.25 MHz RF from 805 MHz reference line to provide a phase reference for each station. • Improve RF Amplitude and Phase regulation during the beam pulse. • Improve beam injection into the Booster. • Expand working knowledge of the low energy Linac RF system. Trevor Butler & Ed Cullerton

6. Linac RF System Modeling • The first step in this project was to expand our working knowledge of the low energy Linac by creating a computer model of the existing system. • During the 2006 shutdown, measurements of the RF systems were taken for all 5 stations. • These measurements were used to create a computer model of the RF system. • This model accurately described all the working components of the RF system and guided the design decisions for the new LLRF upgrade. Trevor Butler & Ed Cullerton

7. Linac RF System Computer Model Trevor Butler & Ed Cullerton

8. Linac RF System Computer Model Results Blue = Measured Data Red = Simulation Data Trevor Butler & Ed Cullerton

9. New LLRF System • Digitally controlled phase feedback system that replaces the present analog RF phase feedback • Adaptive feed-forward for both amplitude and phase control to improve beam loading compensation • 201.25 MHz RF reference generated from the phase stable 805 MHz reference line • Phase regulation from the reference line replacing the existing inter-tank phase reference • Digital phase detection for the cavity resonant control system Trevor Butler & Ed Cullerton

10. New LLRF System Pictures Trevor Butler & Ed Cullerton

11. New LLRF System Upgrade Status • All 5 Linac LLRF Systems have been operational since 3/19/09. • The LLRF amplitude feed-forward loop is able to achieve design specifications for HEP beam gradient and phase flatness. • Using the new independent cavity phase settings at each RF station, adjustments are being made to improve HEP beam transfer to the Booster and minimize beam losses in the Linac. • The Modulator, 7835 PA, and Driver Amplifier are also being optimized for increased modulator response time and improved RF impedance matching. Trevor Butler & Ed Cullerton

12. New vs. Old LLRF System Comparison Beam Loading Compensation and Phase BEFORE - 7/13/05 AFTER - 4/16/09 Trevor Butler & Ed Cullerton

13. Operational Effects on RF Gradient 0.034% Gradient Flatness on Station 5 LLRF (Goal < ±0.2%) The LLRF amplitude feed-forward loop is able to achieve design specifications for HEP gradient flatness. Trevor Butler & Ed Cullerton

14. Operational Effects on RF Phase 0.06 Phase Flatness on Station 5 LLRF (Goal < ±0.5) Trevor Butler & Ed Cullerton

15. Operational Effects RF Summary Trevor Butler & Ed Cullerton

16. Linac LLRF Upgrade Summary • Installation of the entire LLRF system was done over a four month period, station by station, with minimal downtime. • The LLRF system is able to achieve design specifications for HEP gradient and phase flatness. • Future studies will involve tuning up both RF systems & LLRF parameters to increase gradient & phase flatness for all beam types (Including NTF). • Since the final LLRF system was installed, there has been encouraging beam performance • RF gradient and phase tilts in the High Energy RF systems, which were implemented to compensate for inadequate beam compensation of the previous Low Energy Linac RF system, have been removed Trevor Butler & Ed Cullerton

17. Operational Effects on Beam Stability A BPM, located in a high dispersion region of the 400 MeV beam line, shows the beam position variation. This variation is partially due to momentum shifts caused by instabilities of the Linac RF phase and gradient. After a combination of High Energy and Low Energy Linac LLRF work, a reduction of beam position fluctuation from 4mm to 2mm is seen in the BPM monitor plots below. Previous LLRF (11/30/07) ~4 mm variation New LLRF (04/13/09) ~2 mm variation 4mm 0mm 0mm -4mm -4mm -8mm Trevor Butler & Ed Cullerton