1. TMCI Observations in AD
Summary of 1999 Observations
F. Pedersen, CERN/AB/RF
July 2006
TMCI Observations AD

2. Outline Modes of AD operation during 1998 – 2000 commisioning
Commissioning: original and revised plans
AD ring and beam parameters
Instability threshold and observations
Can studies be repeated?
July 2006
TMCI Observations AD

3. AD operation modes, Commissioning 1998 - 2000
Protons via loop, counter clock wise rotation, normal magnet polarity
N = 2 – 5x109 protons, optics set-up,
high beam density required for deceleration to 100 MeV/c (T = 5 MeV)
Failed due to TMCI instabilities
Protons direct via target area, clock wise rotation, reversed magnet polarity
N = 5x109 protons, initial stochastic cooling setup
Normal antiproton operation, clockwise rotation, normal magnet polarity
N = 2 – 5x107 antiprotons
Difficult beam diagnostics due to low intensity
July 2006
TMCI Observations AD

4. AD Modes: pbar mode Normal operation, low intensity pbars: 2 – 4x107
Difficult beam diagnostics due to low intensity (tunes, intensities, orbits..)
Each phase plane (EH, EV, EL) compressed 5000 [EH, EV] to [EL] (six-dimensional phase space density increased 6x1011 !!!)
With improved beam diagnostics (tunes, intensity, Dp/p, orbits), most setting up was done in this mode from 2000 onwards
Access to experimental hall OK
July 2006
TMCI Observations AD

5. AD Modes: protons direct
Used for initial setting up of stochastic cooling and electron cooling at 300 MeV/c
Intensity typically 2x109 (limited by stochastic cooling speed)
DC beam transformer works (marginally)
Magnetic fields does not re-produce when inverted (ex. coupling)
Large blow-up (EH, EV, EL), due to 40 m air path in ‘dog-leg
No access to hall
July 2006
TMCI Observations AD

6. AD Modes: protons via loop
No cooling possible with CCW rotation: very dense injected beam required
Intensity typically 3-5x109 (limit: transverse mode coupling instabilities)
Tune measurements at low energy impossible due to large Dp/p required to control TMCI, unable to decelerate to 100 MeV/c (Dp/p = 0.7% at 0.3 eVs)
Difficult to correct and maintain small injection oscillations
Used for set-up of ‘dog-leg’ transport
No access to experimental hall
July 2006
TMCI Observations AD

7. Commissioning Original and revised plans
6 months (1998/99) scheduled for commissioning
Set up optics, orbits, deceleration and tunes with protons via loop
Initial cooling set-up in protons direct mode, reverse polarity
Final cooling set-up with antiprotons, use protons via loop as necessary to measure orbits and tunes
After 1½ years we were still unable to decelerate protons from loop to 100 MeV/c due to either transverse (TMCI) or longitudinal blow-up (needed to combat TMCI)
In 1999/2000, a major RF shielding effort of RF and BPM systems made it possible to measure tunes (BTF) and orbits with 2 – 4x107 pbars
Commissioning completed successfully in July 2000 (design emittances or better, 4 times design intensity) using pbars only.
Protons via loop used last time in GeV/c Loop and one injection kicker module decommissioned since then…
July 2006
TMCI Observations AD

8. AD Ring and Beam parameters
Circumference, frev 185 m, MHz
Momentum range 3.57 –> 0.1 GeV/c
Gamma transition gtr 4.8
Slip factor, h , ginj = 3.94 (3.57 GeV/c)
Longitudinal Z/n ~1600 ohms, K-S crit., 12 SC tanks [AC]
RF Voltage VRF 3 kV, h = 1
Synchr. freq. fs kV, ns = 5.4x10-5
Intensity, Nprotons 3 – 5x109, longit. and transv. shaving in PSB
AD Acceptance, AH, AV 180 pi mm.mrad
Emittances, EH, EV 0.4 – 0.6 pi mm.mrad [95%, in PS]
Expected emittances 3.5 pi mm.mrad [95%, AD, Al 0.1 mm windows]
Longit emittance EL –> 0.3 eVs [controlled blow-up in PS]
Measured AD emittances 2.5 – 3.5 pi mm.mrad [95%, below TMCI threshold]
Measured AD emittances pi mm.mrad [95%, above TMCI threshold]
July 2006
TMCI Observations AD

9. Instability Thresholds, Observations
We monitor injection oscillations (H&V) by observing a low frequency betatron line (6 - Qx) x frev and minimize with vertical steering dipoles, septum and kicker
We look for signs of coherent instabilities by observing a fixed high frequency betatron line (39 - Qx) x frev near the bandwidth limit of the pick-up system: ~61 MHz
With large longitudinal emittance (>0.3 eVs), small enough injection oscillations, and intensities below 4x109, the expected transverse emittances can be achieved: 2.5 – 3.5 pi mm.mrad
With small longitudinal emittance (0.12 eVs) and small injection oscillations we observe much large emittances than expected (7 - 9 pi mm.mrad):
Noisy coherent signals (filamentation following instability) appears at high frequency betatron lines 100 – 200 msec after injection
From profile measurements we observe that the central density in transverse phase space is depleted relative to larger amplitude,
Filamentation signals and transverse emittance blow-up disappear when the longitudinal emittance is increased (PS longitudinal blow-up)
July 2006
TMCI Observations AD

10. Instability Thresholds, Observations
No beam loss results from the instability. Most likely tune spread from octupolar fields stabilize the blow-up before the aperture limit (~180 pi mm.mrad) is reached
If the aperture is limited artificially with scrapers to a value slightly larger than the expected emittance, a large fraction of the beam is lost on a time scale in the order of a synchrotron period (10 ms): the instability is amazingly fast considering the low intensity – not classical head-tail modes.
The AD is a ‘dirty’ machine in terms of coupling impedances: 4 stochastic cooling tanks with guts optimized to maximize the transfer impedance (12 tanks in the AC era), Z/n ~1.6 k ohms estimated from longitudinal coasting beam instabilities
The large transverse coupling impedance and the low synchrotron frequency (close to transition, low RF voltage and harmonic number) results in a low TMCI threshold:
From Chao and Tigner p. 136 (B. Zotter)
July 2006
TMCI Observations AD

11. Vertical beam profile, above instability threshold
Graph shows losses (scintillator counts) versus scraper position
Central phase space density only half of density a few mm’s from the centre when instability is present
Could also be result of poor injection steering, but injection oscillations were carefully monitored
July 2006
TMCI Observations AD

12. Horizontal beam profile, below instability threshold
Measured emittance close to expected value: 2 pi mm.mrad
High phase space density in center of beam
July 2006
TMCI Observations AD

13. Could TMCI studies be repeated?
The objective in 1999 was not to study the TMCI in the AD, but to provide some useful test beam (protons or antiprotons) at 100 MeV/c
The last operation with protons via loop took place in 2003 (beam intensity calibration of RF and Schottky pick-up)
Since then, the 3.57 GeV/c loop has been decommissioned, ADTST beam is no longer in regular operation, and one of the injection kicker modules has been removed from the machine
Protons direct (and using AD reversed polarity) suffer a large emittance blow-up (~80 – 100 pi mm.mrad) from the transport in air over ~40 meters, and the instability will most likely not be observed
With pbars the intensity is 100 times lower and the longitudinal emittance 16 times larger..
It would be difficult and very expensive to repeat these studies…
July 2006
TMCI Observations AD

14. Source of Information
AD Logbook III 15 July > December 1999 (paper log)
Injection oscillation studies, 21 October 1999, afternoon shift
TMCI studies, 6 November 1999, morning shift
Good transverse emittances injected: 8 November 1999, afternoon shift
In 2000, ‘protons via loop’ mode was abandoned in favor of normal antiproton mode thanks to improvements in low intensity beam diagnostics….
July 2006
TMCI Observations AD