High resolution beam position monitoring for linear accelerators

High resolution beam position monitoring for linear accelerators R. Apsimon University of Oxford 30/03/10 High resolution beam position monitoring for linear accelerators

Contents • • What is a BPM? How does a BPM work? Signal processing Calibration Resolution Current status Future development 30/03/10 High resolution beam position monitoring for linear accelerators 2

What is a BPM? • Beam Position Monitor – 4 metal striplines placed along beam pipe to measure the position of beam in horizontal and vertical direction • Different types of BPM – Stripline: ~10 cm long, 4 strips along beam pipe • Quick response, moderate resolution – Button: Like striplines, but shorter • Quick response, poorer resolution – Cavity: Uses antennae rather than strips as pickups • Slow response, good resolution 30/03/10 High resolution beam position monitoring for linear accelerators 3

FONT • Feedback On Nanosecond Timescales – Minimise bunch-bunch jitter • Need good BPM resolution – Time between bunches: 154 ns • Need fast BPM response • Stripline BPMs used as a compromise 30/03/10 High resolution beam position monitoring for linear accelerators 4

How does a BPM work? • Electron beam is ultra-relativistic • Electric field Lorentz contracted longitudinally • E-field forms a disc perpendicular to direction of motion • E-field now varies as r -1 NOT r -2 as it does classically • Signal induced in pickups by electric field • Signal amplitude proportional to bunch charge and angular width of pickup as seen from electron bunch (Gauss’ law). 30/03/10 High resolution beam position monitoring for linear accelerators 5

Cross section of a BPM 30/03/10 High resolution beam position monitoring for linear accelerators 6

The technical bit… Similar result in x 30/03/10 High resolution beam position monitoring for linear accelerators 7

Signal processing • BPM signals need to be digitised • BPM output ~1 GHz, too fast for digital system • Need to “slow” signals down to ~100 MHz • Process raw signals before digitisation • 2 main processing options: – Low-pass filter: • Easy to operate, robust • Cuts most of the signal so poor resolution – Local oscillator (LO) to mix down frequency • Keeps more signal so better resolution • More complicated, LO needs to be correctly phased 30/03/10 High resolution beam position monitoring for linear accelerators 8

LO scheme Raw stripline signals LO (714 MHz) Phase shifter 180 o hybrid Band-pass filter Mixer Low-pass filter Subtracts one signal from the other Cuts frequency components at 714 MHz ± 100 MHz Effectively multiplies two input signals together Cuts high frequency components Output Similar for the sum signal, except the hybrid adds the signals together, rather than subtracts. LO scheme used for optimal resolution. 30/03/10 High resolution beam position monitoring for linear accelerators 9

Mixer High frequency Low frequency The mixer produces a high and low frequency term, the low-pass filter then removes the high frequency term. Mixer is semiconductor device, only linear for ~± 50 m. V for our processors 30/03/10 High resolution beam position monitoring for linear accelerators 10

After processing • Processed sum and diff signals – Low enough frequency to be passed through ADCs – Beam position calculated from signals digitally – Digital system is clocked at 357 MHz with 14 bit resolution • 357 MHz clock generated from 714 MHz LO 30/03/10 High resolution beam position monitoring for linear accelerators 11

Calibration • C is the calibration constant of the BPM • 2 ways to calibrate the BPMs – Move the beam using an upstream dipole – Keep beam steady, move BPMs with movers 30/03/10 High resolution beam position monitoring for linear accelerators 12

Calibration methods • Moving the beam – Need well calibrated dipole magnet – Need to know where the beam moves to in BPMs • Transfer matrix from simulation, potentially unreliable • Moving the BPMs – Need to calibrate mover 30/03/10 High resolution beam position monitoring for linear accelerators 13

Resolution • Not a trivial problem – Beam jitter bunch to bunch • Correlated jitter – BPM resolution • Uncorrelated jitter • Have 3 BPMs – Use 2 to calculate the beam position in third • Use this to remove correlated jitter to leave uncorrelated jitter • Calculate resolution from residual 30/03/10 High resolution beam position monitoring for linear accelerators 14

A bit more maths… δV represents the noise on the processor signals Correlated jitter terms Uncorrelated jitter terms Σ is charge dependent Resolution is charge dependent and position dependent 30/03/10 High resolution beam position monitoring for linear accelerators 15

Current status • Calibration stable – LO phase stable • Were suffering phase jumps generating the 357 MHz clock • Phase drift with the LO so needed to recalibrate every hour • Both problems solved now – Processors working • Had several component faults with the processors, now fixed – This also affects resolution • Typical stripline BPM resolution ~ 10μm – Currently we can achieve ~ 2μm – Hope to achieve < 1μm with future improvements 30/03/10 High resolution beam position monitoring for linear accelerators 16

Future developments • Calibrate with movers rather than beam – Also allows beam position to be zeroed in all BPMs • Minimises resolution • Amplify signals – Use low noise amplifiers on output of processors • Improves signal to noise ratio, improving resolution • Develop new BPM processors – Use higher power mixers • Allows larger signals, better signal to noise ratio, better resolution 30/03/10 High resolution beam position monitoring for linear accelerators 17

Questions? • Please feel free to ask any questions now or later, and thank you for listening 30/03/10 High resolution beam position monitoring for linear accelerators 18