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Electron energy stability (observations after the HV regulation repairs)

Electron energy stability (observations after the HV regulation repairs). Recycler Meeting April 30, 2008 A. Shemyakin. Introduction. In 2008, HV stability – related issues were the main reason for a downtime Jan 19-20 – shutdown to repair HV regulation circuitry in the terminal

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Electron energy stability (observations after the HV regulation repairs)

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  1. Electron energy stability(observations after the HV regulation repairs) Recycler Meeting April 30, 2008 A. Shemyakin

  2. Introduction • In 2008, HV stability – related issues were the main reason for a downtime • Jan 19-20 – shutdown to repair HV regulation circuitry in the terminal • Apr 2-3 – shutdown to repair broken HV readbacks in the terminal • Apr 10-12- shutdown to repair GVM (bearing replacement) • Right now there are no known with HV regulation problems • However, we need to keep adjusting time to time the energy manually to provide optimum cooling • This report is based on data from high-dispersion BPMs • I looked at several sets of data showing various aspects of the energy stability • Long run with the e-beam on axis on 12-Apr-08 • 15 and 24 April, when temperatures were changing • April 29: snapshots of 720 Hz BPM signals

  3. Energy reconstruction from dispersive BPMs • Beam position in return line Y-BPMs as a function of GVMVLT • 12-Apr-08 • MI was not ramping • 0.1A • Y-BPMs in the return line can be used for the purpose of energy tracking • Generally, the problem is sensitivity of beam position to MI ramps

  4. Long run on 12-Apr-2008 • E-beam was left running on axis for 13 hrs • MI was off, no pbars • Software HV regulation loop was off as well • R:GVMVLT and return line dispersive BPMs were recorded from D44 • Initial values were subtracted • BPM mm values were converted to energy using coefficients from the previous slide • In plots below, R:BYR0# refers to a such value, in keV • GVM reading is much more stable than the energy calculated by BPMs

  5. Long run details: response to a corona current • There were 3 nearly identical events when both GVM and BPM-restored energy moved at the same time • Most likely, those are response to an increased corona currents • Happen time to time, more frequently after opening the tank • Usually are mitigated by software regulation loop 3 min

  6. Long run details: energy fluctuations in a quiet period • In a “quiet” state, • BPM signals are highly correlated • no correlation with GVM • GVM scatter probably represents primarily ADC noise • It is not clear what goes into the HV regulation circuitry • HV noise is ~100V rms (0.001 - 1 Hz) • Main component of energy scatter is a slow (hours) drift Quiet period

  7. Long run details: energy jumps • There was one 8-keV jump and many (~10 per hour) ~1-keV jumps • No corresponded response on GVM • Therefore, GVM is to blame for generating them • 2-3 sec length • Different from what was caused by broken GVM bearings (several minutes-long fluctuations) • Should not affect operation noticeably

  8. Energy deviation parameter • To track the energy deviation with BPMs in a high-dispersion region, a new ACNET parameter R:DENRGY was created • Calibrated in keV of electron energy • Takes into account the e-beam position in the cooling section • The parameter shows the energy deviation from a nominal if • Trajectory upstream the 180-deg bend is stable • The magnetic field in the 180-deg bend is stable • This is questionable, because NMRs do not work in the magnet • coil currents are stable, but the magnetic field still may drift (temperature, stray fields, radiation…) • Kermit hopes to have new amplifiers and sensors in a couple of weeks • The parameter seems to work in hours - scale • It is difficult to use other Y-BPMs in the return line for energy tracking • The problem is sensitivity of beam position to MI ramps • Attempts to sum them with “good”coefficients to subtract effect of MI ramp were unsuccessful • Time dependence of various BPMs seem to be different

  9. Energy dependence on the Pelletron temperature • The energy changes by several keV after turning the Pelletron on • ~0.5 keV/C, but not linear with any of temperatures (at start, DENRGY changes faster) • Thermal expansion should ive ~30 V/C • Mechanical motion of the column? SF6 temperature GVM 1 keV Tank temperature MI-31 temperature 2 C Energy deviation 3hr

  10. Energy dependence on air temperature • When temperatures are close to an equilibrium, energy oscillates with the frequency of MI-31 air temperature (~0.3 keV/C) but out of phase • Can it be electronics? • Greg: GVM amplifier, TPS box, reference signal… GVM SF6 temperature Energy deviation Tank temperature 1 keV 5hr MI-31 temperature

  11. 7MeV Indications of 180-deg bend drift • On 22-Apr-08, R:DENRGY was close to zero, trajectory upstream the 180-deg magnet was as before, but the momentum distribution became flat • Increase of the Pelletron voltage by 1.5 kV made it peaky again • The only explanation that I have is a drift of the bend field

  12. Energy ripple at frequencies above 1 Hz • Snapshots of several BPMs were recorded outside of MI ramps (with MI in energy conservation mode), and data were e-mailed • 720 Hz, 2046 points, “Fast” signals, 29-Apr-08 • (BYR01F, BYR04F, BYR05F), (BXR01F, BXR04F, BXR05F), (BYC00F, BYC10F, BYC20F), (BXC00F, BXC10F, BXC20F) +I:IB • In low-dispersion BPMs, main components are 30, 60, 180 Hz, set of peaks at < 10 Hz, and, what’s new in comparison with 2005 data, peaks around 40 Hz • In high –dispersion channels, there is a strong enhancement of signals below 10 Hz and 40 Hz lines • Chain ripple ~1.8 Hz

  13. Summary • Presently, HV stability is most likely at its best level • With infrequent adjustments of the HV set point, the stability is satisfactory for operation at present level • HV readback by R:GVMVLT has the noise level of 200 V rms. • Resolution of the energy measurement with R:BYR01S is ~20 eV rms. • The absolute value of energy offset calculated with R:DENRGY drifts up to 1 keV, most likely because of drifting 180-deg bend field. • There are seconds-long energy jumps caused by HV-regulation circuitry • Temperature dependence of the electron energy is caused by both mechanical changes and electronics drifts. • It would be useful to look in details at energy fluctuations at frequencies above 1 Hz • Paul Joireman can record all BPMs at 300 Hz (with MI not ramping) • Alexey can trace signals from the cooling section to the return line to exclude everything but energy • It looks like there is a 40 Hz perturbation that was not in 2005 spectra

  14. Summary table

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