Test Facilities Group High Power RF Equipment Safety Program
RF Safety Program (RFSP) for
End Station B (ESB)
Scope
End Station B supports a variety of accelerator development programs, including a 650 MeV (1.2 GeV safety envelope maximum) accelerator.
All ESB high-power RF systems operate in a pulsed RF regime at frequencies of 1.3, 2.856, and 11.424 GHz (L-band, S-band, and X-band respectively hereafter). All RF operations in ESB are covered under this RFSP.
Two discrete programs exist at ESB.
· Accelerator Systems
Operation of the accelerator uses S-band and X-band klystrons to power the laser gun and various standard accelerating cavities. (See note in L-band Development Systems, below). The RF power for the accelerator sections derived from klystrons that are, for some accelerator sections, combined in pairs and time-compressed to generate peak powers between 100-200 MW.
All RF fields are contained in metal enclosures like the klystron body, rectangular waveguide or resonant cavity. The klystron, waveguide systems, RF cavity, and RF loads (i.e.: all of the high-power RF components) are vacuum vessels and do not leak RF fields if they are vacuum tight. All work on RF systems is done under an End Station B High Power RF Equipment Lockout Procedure which insures that system is complete and vacuum tight prior to allowing energization of the systems.
· L-band Development Systems
The RF system of the L-band test in End Station B consists of a klystron, associated RF waveguides, high power circulators and/or an RF accelerator cavity. The klystron is driven by a pulsed modulator powered by a 100 kW DC power supply. The maximum available klystron beam power is 100 kW which at 44% (typical) efficiency limits the RF power to a maximum of 44 kW (average). The klystron is capable of generating pulses of up to 5 MW peak power. The klystron is driven by a solid-state amplifier working at 500 watts (pulsed) and the operating frequency is 1.3 GHz.
All RF fields are normally contained in metal enclosures like the klystron body, rectangular waveguide or resonant cavity. The klystron and RF cavity are vacuum vessels and do not leak RF fields if they are vacuum tight. Excessive levels of RF leakage out of the non-vacuum waveguide and waveguide components is normally limited to joints that are either defective, improperly assembled or insufficiently tightened. Leakage to free-space from open coaxial connections is typically small due to the poor coupling between the connector and free-space transmission.
1. RF Emission Limits
A personnel hazard is considered to exist when RF radiation (Non-ionizing Radiation or NIR) above a Maximum Permitted Exposure (MPE) is leaking from any RF system. The MPE are established by the IEEE industrial consensus standard[1].
MPE – Maximum Permitted Exposures[2], time average for pulsed sourcesBand / Frequency (MHz) / MPE Power (mW/cm^2) / Wavelength (λ) (inch)
L-band / 1,300 / 0.86 / 9.09
S-band / 2,856 / 1.90 / 4.14
X-band / 11,424 / 2.00 / 1.03
· RF emission MPE is average of free-space power over a 10 inch by 10 inch area[3]. Free space power is measured at distances exceeding the RF wavelength from any metal surface.
· The MPE values shown above have been derated by a factor of 5 in accordance with the IEEE standard.[4]
2. Non-ionizing Radiation Safety approach
The NIR integrity is being assured by the following methods:
§ Careful assembly of all parts
§ Inspection of high-power components
§ Labeling of high-power components
§ Pressurization of waveguide network tied to an interlock for pressure failure
§ RF field survey of the fully assembled system
3. Very Low power RF systems:
Milliwatt-class RF systems are used throughout the facility for measurement systems. These systems typically use detector diodes or mixers for the measurements and operate at less than 0.1 watt. These systems usually use cables and connectors which, due to their small size (size much smaller than the free-space wavelength) are very poor transmitters. Occasionally, waveguide systems are used (e.g.: network analyzers or mechanical filter networks). The users of open waveguide systems should recognize that much of the RF power can be released into the general laboratory environment.
The general and intuitive safety protocols of avoiding direct contact with the RF source and avoiding placing the connector or waveguide to your eye is recommended.
4. Low power RF systems
Power systems greater than approximately one watt (or 1/5 watt average power for pulsed power systems) can create non-ionizing radiation hazards. These systems typically include RF distribution amplifiers.
Low power systems can be safely used if the following precautions are observed:
· Turn off amplifiers while connecting or servicing cables or loads.
· Examine cable terminations for structural integrity.
o Specifically, if the shield connection on the coaxial cable is broken or loose, then the connector/cable assembly can become a very effective transmission antenna. Connectors must be in excellent condition for use with RF.
o Care must be taken when designing test fixtures and equipment to avoid free-space radiation. For example, a piece of coaxial cable with the shield removed for the last ¼ wavelength is a very effective antenna that can turn a one-watt instrumentation amplifier into a non-ionizing radiation hazard.
· Label all RF power connections that leave the chassis with an appropriate warning label that includes the frequency and RF power levels. Note: When using a coupler or power tap-off device, only the high power leads need to be labeled for safety.
· If any waveguide systems are powered at greater than 1 watt (CW) or 0.2 watt (average pulsed), the protocols for high-power systems must be used.
5. Medium power RF systems
Medium power RF systems consist primarily of drive amplifiers for klystrons. Generating up to a kilowatt of pulsed RF, these systems are protected by administrative controls consisting of the labels described below. No interlocks or other engineering controls are required.
6. Klystron power sources (High power RF sources)
Klystron Capabilities / L-band / S-band / X-bandTransport medium / Nitrogen 30 PSIG* / Vacuum / Vacuum / (*) – L-band power also transported through Nitrogen at ~0 PSIG and under vacuum
Beam Power / 100 kW / 100 MW / 100 MW / Max
RF power, average / 44 kW / 40 W / 10 kW / Maximum, time average
RF power, peak / 5 MW / 10 MW / 50 MW / Max
Pulse length / .1-1 ms / 2 us / 4 us / Typical
Frequency / 1.3 GHz / 2.856 GHz / 11.424 GHz
Pulse Repetition Rate / 10 Hz / 5 Hz / 10 Hz
Several of the X-band klystrons are combined and followed by a SLED delay-line pulse compression system. Average power of two times that of a single klystron are typical with peak RF power of up to 300 MW.
7. Waveguide Network
The high power RF is being transmitted in all-metal rectangular waveguide system. Appendix B is an illustration of the waveguide system envisioned in the mature system. The flange joints are of standard design and fitted with a gas seal to allow pressurization of the waveguide. The terminal connection of the waveguide may consist of a rectangular waveguide to another method (circular or coaxial) as required to power the RF cavities. The waveguide will be instrumented with directional couplers, arc detectors, and other instrumentation as required.
Prior to working on any high power system, proper consideration of workplace hazards [Control of Hazardous Energy (CoHE)] is required. High power RF sources fall within two classes:
· RF systems interlocked as beam stoppers (ionizing radiation sources) by an operational PPS system. The PPS interlocks and stopper enable controls provide the necessary protection to insure worker safety from RF hazards without the use of a CoHE Lockout lock and tag.
· All other RF systems. These systems must be controlled by a CoHE Lockout lock and tag. The “most exposed” workers must follow an approved RFSP ELP such as the End Station B High Power RF Equipment Lockout Procedure (02-03-35).
An RF leak tight waveguide network is assured by taking the appropriate steps during assembly and installation and checking after installation for proper assembly. A final RF survey is made using a radiation meter capable of accurately measuring pulsed microwave emissions. Measurements will be made for each joint accessible outside the accelerator enclosure[5]. Most of the proposed waveguide network is interlocked through a gas pressurization system which will automatically shut off the RF power if the waveguide loses its pressurization due to a gas leak assumed to be accompanied by a RF leak.
The following controls will be maintained on the high-power RF systems:
· Careful assembly of all parts
· Inspection of high-power components
· Labeling of high-power components
· Pressurization of waveguide network tied to interlocks for pressure failure. Exceptions to the pressurization requirement are a) low-power coupling ports, b) devices under high-vacuum and c) devices-under-test which are incapable of working under pressure.
· RF field survey of the fully assembled system
8. Device under Test
The Device-Under-Test (DUT) in the L-band system is generally the highest-hazard component in the L-band system. The L-band testing program includes testing of:
· High-vacuum components such as superconducting cavity coupler studies. These systems are entirely under vacuum or pressurized waveguides.
· Coupler test stand. The coupler test stand consists of a set of high-vacuum couplers (3 distinct vacuum volumes) and a pair of atmospheric-pressure mode-converters. The pressurized waveguides are isolated from the mode-converters by a pair of pressure-qualified RF windows. Components of the DUT between the two pressurized waveguides are operated without any system interlocks capable of detecting disassembled components. The unmonitored elements of the DUT extend for about 1 meter and are entirely within a soft-sided cleanroom.
· RF Distribution System: This distribution system is designed to mate to the superconducting test accelerator at Fermilab. The system consists of high-power RF components which operate with pressurized nitrogen, and somewhat lower power components which operate at atmospheric pressure.
· All work on DUTs is done in accordance with the End Station B High Power RF Equipment Lockout Procedure.
9. Waveguide Assembly and Installation
Assembly and installation checks are done in accordance with the End Station B High Power RF Equipment Lockout Procedure.
Whenever practical, non-vacuum high power RF joints will be labeled with the following label to provide an additional administrative barrier to personnel against unintended exposure to applicable hazards.
/!\ DANGER.CONTROLLED JOINT – DO NOT DISASSEMBLE
OR LOOSEN BOLTS. MICROWAVE AND GAS
PRESSURE HAZARDS.
CONTACT AREA MANAGER OR SAFETY
OFFICER TO REMOVE TAG
10. Coaxial Cables
The coaxial cables are used for instrumentation-level signals, monitor signals attached to coupling ports to the waveguide network, and for the drive to the klystron. Instrumentation cables will not be identified as NIR hazards. The klystron drive power and RF monitor cables coupled to the klystron output more strongly than -70 dB will be labeled with the following information:
Warning – Microwave Power Hazard.LOTO required before work.
Power level: ___ watts peak, ___ watts average. ____ on ____
11. RF Emission Survey
An RF emission survey consists must be done around each accessible flange.
· Joints which are physically difficult to approach (by, for example, their height) do not need to be surveyed if personnel are not expected to be working near the equipment during operation.
· Joints inside the accelerator enclosure do not need to be surveyed if the RF power source is interlocked to the PPS stopper controls.
· Joints consisting of metal-based vacuum seals do not need to be surveyed if the system passed an UHV vacuum leak check.
The preferred survey method consists of using a small antenna and a high-sensitivity detector such as an RF diode or spectrum analyzer. When surveys are preformed using un-calibrated high-sensitivity equipment, operation of the antenna/detector combination must in all cases be confirmed by probing a flange or coupler output to confirm the operation of the equipment.
If extraordinary levels of RF emission are detected, and the situation cannot be corrected, measurements must be repeated using a calibrated RF field. Equipment which exceeds the MPE exposure levels cannot be operated without specific authorization from the Non-Ionizing Radiation Safety Committee.
Survey methods and documentation requirements are described in the End Station B High Power RF Equipment Lockout Procedure.
12. Gas Pressurization System
The waveguide is pressurized to a pressure of 30 PSIG. A pressure monitor provides interlock inputs to the RF source.[6]
The supply line is equipped with a manual vent-valve (normally closed) which allows the bleeding of the system to allow interlock checks and for system maintenance. The valve is installed separately from the pressure detectors, allowing a true test of the interlock function under full gas supply scenarios.
13. Waveguide Pressure Interlock Check
Verification of waveguide pressure interlock must be performed upon initial commissioning and thereafter at least once per year. The interlock checks are part of a checklist signed, dated, and filed in the NLCTA Control Room.
14. Waveguide Removal
Removal and disassembly is done in accordance with the End Station B High Power RF Equipment Lockout Procedure.
15. System Reconfiguration
Following any modification of the RF system which changes the engineering controls will require a review by the NIR citizens’ committee.
An example of a change requiring review would be the installation of a SF6 gas system for the circulator or cavity input coupler. As an SF6 system would not be protected by the waveguide pressure interlock described above, a new engineering or administrative control would need to be developed and a NIR review would be required prior to operation.
16. Document Information
Background
This document is owned and authorized by the PPA/ARD/Test Facilities Department for control of Non-Ionizing Radiation (RF) hazards.
Source File Location:
SharePoint : PPA : ARD : TF : Shared Documents : Safety Documents : 02-03 Safety :
Document has been reviewed and approved by:
PPA/ARD/TF Department Head Date
NLCTA Operations Manager Date