Digital Self-Excited Loop for Tuner-Less Cavity Recovery

Solution Overview

Problem

Current digital systems lack the capability to emulate an analog self-excited loop (SEL) for field control of superconducting accelerating cavities, which is crucial for maintaining phase and amplitude control, especially at high gradients and during cryogenic trips, where Lorentz detuning can cause significant frequency shifts.

Innovation Solution

A digital self-excited loop (SEL) algorithm is developed that converts signals from the Cartesian to the Polar domain for simpler logic operations and perfect amplitude limiting, implemented using an Altera FPGA, allowing automatic tuning to cavity resonance without external tuners or numerically controlled oscillators.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Extent of automation

If a digital system is used to control RF for superconducting accelerating cavities, then the system can be implemented digitally using FPGA, but it lacks the capability to automatically tune to cavity resonance without external tuners or numerically controlled oscillators

Engineering Contradiction:

Improveautomatic tuning capabilityVSAvoidsystem complexity

Core Design Contradiction:

Extent of automationVSDevice complexity

Solution Approach 1:

The digital SEL algorithm enables the system to automatically tune to cavity resonance by using the cavity itself as the reference oscillator. The system extracts the cavity frequency from the reflected signal and uses it to generate the drive signal, eliminating the need for external tuners or NCOs. This self-service mechanism achieves automatic tuning while maintaining digital implementation.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The SEL algorithm implements a feedback loop where the reflected signal from the cavity is processed to extract frequency information, which is then fed back to adjust the drive signal frequency. This feedback mechanism enables the digital system to lock onto and track the cavity resonance frequency automatically, resolving the contradiction between automation and complexity.

Inventive Principle:

Principle #23Feedback

2Reliability

If a Generator Driven Resonator (GDR) algorithm is used, then the system can maintain field control up to 16.7 MV/m, but it requires hunting algorithms or numerically controlled oscillators to tune the cavity back after detuning

Engineering Contradiction:

Improvefield control capabilityVSAvoidcavity recovery complexity

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The digital SEL algorithm enables the system to automatically tune to cavity resonance by using the cavity itself as the reference oscillator. The system extracts the cavity frequency from the reflected signal and uses it to generate the drive signal, eliminating the need for external tuners or NCOs. This self-service mechanism achieves automatic tuning while maintaining digital implementation.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

Instead of using an external reference oscillator as in traditional GDR systems, the digital SEL inverts the approach by using the cavity's own reflected signal as the reference. This inversion eliminates the need for hunting algorithms and NCOs, simplifying cavity recovery operations while maintaining reliable field control at high gradients.

Inventive Principle:

Principle #13The other way round (Inversion)

3Productivity

If the cavity operates at high gradients like those needed for upgraded cryomodules, then the Lorentz detuning will be many bandwidths, but cavity turn-on becomes problematic without tuner based compensation or other algorithmic solution

Engineering Contradiction:

Improvegradient capabilityVSAvoidcavity turn-on ease

Core Design Contradiction:

ProductivityVSEase of operation

Solution Approach 1:

The digital SEL algorithm enables the system to automatically tune to cavity resonance by using the cavity itself as the reference oscillator. The system extracts the cavity frequency from the reflected signal and uses it to generate the drive signal, eliminating the need for external tuners or NCOs. This self-service mechanism achieves automatic tuning while maintaining digital implementation.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The digital SEL algorithm dynamically adjusts the drive signal frequency parameter to match the cavity's resonant frequency, which changes with gradient due to Lorentz detuning. By continuously tracking and adapting to frequency changes, the system maintains ease of operation at high gradients without requiring tuner-based compensation.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS8130045B1Digital self excited loop

Publication Date: 2012.03.06 JEFFERSON SCIENCE ASSOCIATES LLC

AI summary

A process that provides the ability to incorporate a self exciting loop (SEL) algorithm into a digital LLRF system. The present digital SEL provides for conversion from the Cartesian domain to the Polar domain, wherein most signal processing is accomplished, and back to Cartesian. By handling most signal processing in the Polar (phase & amplitude) domain, a perfect amplitude limiter can be realized and simpler logic operations can be used. When operational, cavity recovery from faults will be tuner-less. At high gradients, ˜20 MV/m, like those needed for the upgraded cryomodules, the Lorentz detuning will be many bandwidths, making cavity turn-on problematic with out some tuner based compensation or other algorithmic solution. The present SEL solves this problem and allows cavity recovery from cryogenic trips, wherein cavities have been known to detune 1000's of Hz. Other applications such has He processing can also be implemented in situ without additional electronics.