Measurements of stray field in the NLCTA area Josef Frisch, Peter Tenenbaum, Tor Raubenhemier

Field Effects ATF 1nm on NanoBPM ~5nT NLC (simulated effects) Maximum sensitivity ~1nT (in beam delivery) At High frequencies, structures and beam pipe provide shielding At low frequencies (for NLC) feedbacks reduce effect.

Magnetic field effects for NLC/JLC Primary effect is beam motion at the IP –Damping rings – strong focusing -> minimal problems –Pre-linac, Main linac <0.1 Meters/Tesla –Bypass lines up to 6 Meters / Tesla (39.4 GeV) –Most critical in Beam Delivery where peak response at IP is ~3 Meters/Tesla –Positron line – 65nT for 1 sigma deviation Emittance dilution sensitivity in Linac for fields >100nT –Beam Delivery emittance dilution still under study (but looks OK at first glance)

Field effect vs. wavelength

Low Frequencies – Feedback For NLC/JLC low frequency magnetic field effects are attenuated by the beam / beam deflection feedback at the IP. –High gain below few Hz. For ATF extraction line NanoBPM, low repetition rate makes feedback impractical –Fields down to ~.01Hz significant –Consider magnetic feedback – but problem is difficult: Need a sensor which responds like the beam (including shielding).

High frequency – Skin Depth For structure (~1cm copper), cutoff is ~50Hz For beam pipe (~2mm Stainless), cutoff is 50KHz –Aluminum beam pipe would reduce this to ~2KHz Linac is ~80% structure by length

60Hz (or 50Hz in Japan) Fields The NLC beam operates locked to 2X line frequency. SLC used feedback designed to correct 60Hz sources –Basically treat each 60Hz “time slot” independently. Requires 60Hz correction actuator –Easy at IP, difficult if required in Linac

Natural Background Fluctuations Geophysical sources –Primarily solar wind / terrestrial field interactions Data shown from Australian IPS radio and space services –Available on web –Important for oil exploration, etc. Probably much more data available. Note: geophysical magnetic unit Gamma = 1 NanoTesla

Approximate NLC Sensitivity (guide to eye Only) ATF nanobpm Senstivity (guide to Eye only)

Magnetic Field Measurements Simple Coil, amplifier, and digitizer –High impedance amplifier: V ~ dB/dT –Chopper amplifier – noise spectrum flat to <<1Hz. –Noise ~<0.1nT/sqrt(Hz) at 1Hz. Calibrated (roughly) with current loop driven at 1Hz, 1nT calculated field Single measurement done in SLAC end station B. –“Typical accelerator environment” –Modulators (2 line type, + 1 solid state) –Magnet power supplies. –Cooling fans, water pumps, etc.

Measurement system upgrades Improved search coil – better sensitivity / calibration New preamplifiers (low noise 1Hz – 100KHz). Portable (at least luggable) spectrum analyzer (measure up to 50KHz). –Maybe use LabView laptop? Dual coils for correlation measurements

Measurements (this summer) Measure spatial correlation vs. frequency. Lots of data – but have summer student. Linac (operating and non-operating sectors, in klystron gallery). End station B. Remote – away from technical sources.

Measurements at ATF – possible technical issues. Need very low frequency measurement. –Theoretical sensitivity of pick up coil 0.25M radius, 1000 turns is <1pT/sec/sqrt(Hz) (easily good enough) –Need low frequency pre-amp. –May be difficult to separate large 50Hz signal from small DC signal –In practice, DC measurement (flip coil, hall probe, or similar may be more appropriate) Want smaller DAQ (spectrum analyzer is ~20KG, large volume. –Laptop ideal. No fundamental difficulties with measurement.

Overall Magnetic Issues Geophysical magnetic fields are unlikely to be significant for NLC/JLC Tests in SLAC end station B give fields which are significant, but probably acceptable (assuming 60Hz feedback). Fields could be significant for NanoBPM Need more measurements.