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 – shutdown to repair HV regulation circuitry in the terminal  Apr 2-3 – shutdown to repair broken HV readbacks in the terminal  Apr 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 ( 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 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? 1 keV 2 C 3hr GVM SF6 temperature Tank temperature MI-31 temperature Energy deviation

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 Tank temperature MI-31 temperature Energy deviation 5hr 1 keV

11 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 7MeV

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 ed  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 Frequen- cy, Hz Energy change keV Primary suspect Possible remedy Comments < (many hours) Up to 2Bend field+ adjustments by a human NMR repair Good value of R:DENRGY can be adjusted according to sharpness of the momentum distribution ~ (hours) ~3 (~0.5 kV/C) Mechanical distortion of the column, Adjust by DENRGY Drift when Pelletron heats up after being off ~0.001 (tens of min) ~1 (~0.3 keV/C) HV regulation electronics Thermal stabili- zation ? MI-31 temperature is usually stable +- 1C Corona, GVM, … ? 1 -10, 40 (30, 60 ?) 0.2Chain current fluctuations ?Plan to measure