Mitglied der HelmholtzGemeinschaft Beam based measurements 3 rd
Mitglied der Helmholtz-Gemeinschaft Beam based measurements 3 rd September 2015 BND school Dieter Prasuhn
Outline: What can be measured Ø Lattice properties - Closed orbit Betatron tunes Chromaticity gtransition Ø Properties of the beam - Beam intensity Beam profile Momentum spread Time structure 3. September 2015 Dieter Prasuhn 2
Lattice properties Mitglied der Helmholtz-Gemeinschaft
Closed Orbit measurements • What is the origin of closed orbit deviations? • How to measure closed orbit? • Why to measure and correct CO deviations? 3. September 2015 Dieter Prasuhn 4
The origin of closed orbit deviations Beampipe defocussing Quadrupoles 3. September 2015 Dieter Prasuhn 5
The center of mass of the beam Beampipe defocussing Quadrupoles 3. September 2015 Dieter Prasuhn 6
One quadrupole is misaligned Beampipe defocussing Quadrupoles 3. September 2015 Dieter Prasuhn 7
How to measure the closed orbit Make use of the image current of the beam induced in the outer vacuum pipe 3. September 2015 Dieter Prasuhn 8
Beam Position Monitors (Button type): mainly used in electron synchrotrons, electron storage rings and light sources etc. 3. September 2015 Dieter Prasuhn 9
Beam Position Monitors (capacitive pick-ups): mainly used in hadron synchrotrons and storage rings D S 3. September 2015 Dieter Prasuhn 10
Why do we measure (and correct) the closed orbit? • The centered beam has more space in the vacuum chamber • Quadrupole changes will not change the beam position • The beam - target overlap can be optimized 3. September 2015 Dieter Prasuhn 11
Optimizing the Luminosity Counting rate of the experiment Closed orbit bump Beam Intensity 3. September 2015 Dieter Prasuhn 12
Betatron tunes We follow 1 particle through the accelerator 3. September 2015 Dieter Prasuhn 13
Betatron tunes We follow many particles through the accelerator 3. September 2015 Dieter Prasuhn 14
The motion of each particle seen at one position follows the phase space ellipse: The betatron tune is the number of oscillations on the phase ellipse during one revolution in the storage ring 3. September 2015 Dieter Prasuhn 15
Magnet errors generate angle kicks x` x 3. September 2015 Dieter Prasuhn 16
Betatron resonances x` q = integer shows the effect of emittance growth and beam loss 3. September 2015 Dieter Prasuhn x 17
Resonances occur, if • • • q = integer 2*q = integer 3*q = integer 1 st order resonance 2 nd order resonance 3 rd order resonance • • qx + qy = integer qx - qy = integer 2 nd order sum resonance 2 nd order difference resonance In general: 3. September 2015 l*qx + m*qy = n Dieter Prasuhn 18
The resonance plot l*qx + m*qy = n 3. September 2015 Dieter Prasuhn 19
How to measure a tune BPM Stripline unit RF-output x` D signal of BPM Spectrum analyzer Beam path x 3. September 2015 Dieter Prasuhn 20
Frequency spectrum of the PU signal Deuterons pc = 970 Me. V Since fractional tune q > 0. 5: f- = (2 -q)f 0 f+ = (1+q)f 0 2 f 0 f+ = (2+q)f 0 3 f 0 4 f 0 5 f 0 = 570. 6 k. Hz b = 0. 459 horizontal Qx = 3. 65 Qy = 3. 56 Result with f- = (2 -q)f 0 and f+ = (1+q)f 0: vertical • revolution frequency f- + f+ = 3 f 0 • fractional tune q = f+/f 0 - 1 Measured with BPM 09 Green and red curves: stored spectra when cavity is ON to make revolution frequency visible 3. September 2015 Dieter Prasuhn Courtesy: Hans Stockhorst 21
Chromaticity x= 22
For Correction: Sextupoles 3. September 2015 Dieter Prasuhn 23
How to measure the chromaticity • The width of the betatron side bands depend on x and dp/p q = q 0 + x dp/p 3. September 2015 Dieter Prasuhn 24
or with electron cooled beam • Change the voltage of the electron beam • The energy of the proton beam follows • Measure the new tune 3. September 2015 Dieter Prasuhn 25
g transition (momentum compaction factor) • Beam particles have different momenta • Different momenta result in different velocities • and different paths and path lengths Ø Momentum spread leads to frequency spread =h 3. September 2015 Dieter Prasuhn 26
How to measure gtransition • Switch off the RF to measure the free revolution frequency • Now introduce a change in B-field (corresponding to a momentum change) • Measure the new revolution frequency due to the new orbit length • The change of frequency due to magnetic field is proportional to the g 2 transition 3. September 2015 Dieter Prasuhn 27
3. September 2015 Dieter Prasuhn 28
with electron cooler • Have de-bunched beam • Change the electron cooler voltage • Measure the shift in the longitudinal Schottky spectrum 3. September 2015 Dieter Prasuhn 29
Why do we measure gtransition 1. If g=gtransition bunched beams become unstable 2. Stochastic cooling needs „mixing“ (Hans Stockhorst). Mixing is defined by the difference of g and gtransition. 3. September 2015 Dieter Prasuhn 30
3. And for experiments: to measure the target thickness Mean energy loss leads to a frequency shift 3. September 2015 Dieter Prasuhn 31
Result 3. September 2015 Dieter Prasuhn 32
Beam properties Mitglied der Helmholtz-Gemeinschaft
Beam Intensity • Beam current transformer • • Charged particles circulating with a frequency f 0 in storage ring are seen as a winding of a tranformer. The current I measured in a 2 nd winding is proportional to the number of circulating particles Ncirc I = Ncirc * f 0 * Z*e 3. September 2015 Dieter Prasuhn 34
One example of BCT Beam 3. September 2015 Dieter Prasuhn 35
One picture of the BCT signal Experiment counting rate BCT signal 3. September 2015 Dieter Prasuhn 36
Beam Profile Monitors • Thin fibers are moved quickly through the beam • Seconary electrons emitted from the target are measured as function of the fiber position Disadvantage: destructive measurement 3. September 2015 Dieter Prasuhn 37
Ionisation Beam Profile Monitor Advantage: non-destructive measurement 3. September 2015 Dieter Prasuhn 38
The IPM at COSY 3. September 2015 Dieter Prasuhn 39
Beam profile measured with the IPM Beam profile before and after cooling 3. September 2015 Dieter Prasuhn 40
Momentum spread • For experiments often the momentum resolution is of big interest =h 3. September 2015 Dieter Prasuhn 41
• Measure gtransition or h • Measure the width of the longitudinal Schottky spectrum 3. September 2015 Dieter Prasuhn 42
Time structure of the beam • Makroscopic time structure Defined by the cycle of the accelerator 3. September 2015 Dieter Prasuhn 43
Microscopic structure due to bunching • A de-bunched beam delivers a quasi DCbeam • In LINACS, Colliders, electron accelerators and in hadron machines with internal target bunching is mandatory. • Experiments will directly show the time structure of the beam 3. September 2015 Dieter Prasuhn 44
Different Bunch signals • Pure sinusoidal voltage on an integer harmonic of the revolution frequency - Colliders and synchrotron light sources work on high harmonics - Medium energy hadron accelerators work at low harmonics - At COSY usually h=1 is used for acceleration 3. September 2015 Dieter Prasuhn 45
Bunch signals during electron cooling 3. September 2015 Dieter Prasuhn 46
Barrier bucket Advantage: homogenious beam intensity in the bucket, short time without beam 3. September 2015 Dieter Prasuhn 47
Summary • Introduction to some measurements of lattice parameters and beam parameters • Exercises are planned during the afternoon excursion 3. September 2015 Dieter Prasuhn 48
Outlook: The afternoon excursion • We prepared three demonstration objects: - COSY control room - Magnetic field measurements - RF-cavity measurements • Walk around COSY 3. September 2015 Dieter Prasuhn 49
Map of Forschungszentrum Jülich Institute for Nuclear Research COSY test hall Main gate „face control“ 3. September 2015 Dieter Prasuhn 50
Thank you for your attention and enjoy the excursion 3. September 2015 Dieter Prasuhn 51
- Slides: 51