Measuring the Electron Beam Energy in a Magnetic Bunch ... - DESY

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super conducting cavities’ controllers would be sent commands from the central controller on an intra-bunch-train basis with a moderate bandwidth, while the normalconducting RF cavity amplitude could be sent commands on a fast, bunch-to-bunch basis. Each super-conducting cavity would have its own, independent, fast, vector-sum controller feedback loop using multiple cavity pickups as the diagnostic references. All of this would be accomplished with a newly developed μTCA crate system; this is the crate system that will replace the existing VME infrastructure in the coming years. The advantages of the system from Fig. 3.3.1(a.) include • the ability to use a high-level system identification algorithm to tune and stabilize the entire machine, • built in cross-checks and redundant measurements that use different techniques to measure the same quantities, • robustness afforded by distributed cavity controllers that do not require the central controller to operate. The disadvantage is that with a centralized decision making process, the latency of the signal transport increases due to the multiple digital processors and long cables to and from the central decision making crate. The latency problems can be offset with the use of a normal conducting cavity with a low quality factor in which the accelerating field can be changed quickly. The alternative, more expedient architecture using existing hardware and distributed control loops, shown in Fig 3.3.1(b.), requires fewer digital processors and cable lengths that are shorter, however, the high quality-factor of the super-conducting cavities used as actuators limits the speed with which the energy of each bunch can be adjusted. This means that the first 10 or more bunches of the train are un-stabilized. In Fig 3.3.1(b.), the photo-injector laser phase is synchronized to the Master Laser Oscillator (MLO) in the same manner depicted in Fig 3.3.1(a.), but that is where the similarity ends. The injector RF phase is stabilized with a cavity controller that incorporates beam-based feedback from a beam arrival-time monitor downstream of the first accelerator section, but upstream of the first chicane. The first accelerator section amplitude is stabilized with a cavity controller that incorporated beam-based feedback from an arrival-time monitor located directly after the first chicane. The first accelerator section phase is stabilized with a bunch-length monitor located after the second chicane. The second accelerator section amplitude and phase are stabilized in a similar fashion with an arrival-time monitor and a bunch length monitor after the second chicane. The system shown in Fig 3.3.1(b.) has the following disadvantages • If an upstream feedback loop fails to deliver acceptable beam stability, the downstream loop will start to feed back on noise that is generated some where other than in the cavity it is controlling. • The downstream feedback loop has to be slower than the upstream loop in order to avoid instabilities. • It is not able to make use of cross-checks, monitors that measure the same quantities in different ways. • The energy changes due to arrival-time jitter from upstream of a bunch compressor are not subtracted from the energy changes measured after the bunch compressor. 30
• The lack of a normal-conducting cavity limits the speed with which the beam energy can be changed. As a result, the first 20 bunches in the train will have a different energy than the stabilized bunches that follow Because the scheme depicted in 3.3.1(a.) will take 2-3 years to be realized, the scheme shown in 3.3.1(b.), to be commissioned in the coming months, will be the focus of the following sections. 3.4 Injector Jitter The injector needs to be stabilized on two fronts: the laser timing jitter and the cavity RF phase jitter. The present infrastructure synchronizes both devices to an RF reference and the delivery of this reference is subject to drifts and noise. The infrastructure described in the previous section synchronizes both devices to an optical timing reference for which drifts have been actively compensated. The injector laser is actively mode locked and has an electro-optical device within the laser cavity that regulates the arrival-time of the laser pulses at the cathode. These electro optical modulators (EOMs) are driven with the 1.3 GHz reference from the master oscillator. Temperature changes and noise picked up by the cable and amplifier that bring the master oscillator signal to the EOM have, of course, an impact on the phase stability and drift of the laser. Temperature changes and noise picked up on the cables involved in cavity field regulation will likewise have an impact on the phase stability of the field in the cavity. The jitter of the RF phase relative to the phase of the laser was most accurately measured in [28]. In this measurement, the phase of the laser was changed with a vector modulator that acted on the master oscillator signal feeding the EOM such that the beam arrived on the falling slope of the cavity RF signal. At this phase, the changes in the RF phase relative to the laser phase produced a change in beam charge that could be measured with a downstream toroid. By scanning the phase of the laser relative to the RF phase, a calibration of the beam charge dependence on the phase relationship between the RF and the laser could be determined. Multiplying this calibration by the charge fluctuations measured at the toroid gave a measurement of the phase jitter between the laser and the cavity RF. This jitter was larger than 0.5 degrees from pulse to pulse, a quantity that requires significant improvement. Nevertheless, it is of little use to the machine if the laser and cavity are locked together if they are drifting or jittering relative to a downstream reference. Relative to the optical timing reference, the injector laser timing jitter can be measured by optically cross-correlating the injector laser pulse with a pulse from the optical reference (Fig. 3.4.1) [29]. This measurement can then be used to feed back on the phase of the RF signal sent to the EOM in the injector laser cavity and thereby stabilize the injector laser timing relative to the optical reference. This can be done at a speed of 100 kHz, thereby counteracting much of the noise produced in the amplification of the reference RF signal. The RF module phase jitter can then be stabilized by measuring the arrival-time of the beam relative to the optical reference and then feeding back on the phase setpoint of the RF module. It does not, however, make sense to stabilize the cavity phase in this manner without first stabilizing or measuring the laser jitter because any beam-based feedback would respond to both RF module phase jitter and laser jitter. 31
Page 1 and 2: Measuring the Electron Beam Energy
Page 3 and 4: Abstract Within this thesis, work w
Page 5 and 6: 9 Synchrotron Light Monitors 9.1 Pr
Page 7 and 8: List of Figures 1.0.1 Magnetic bunc
Page 9 and 10: 5.5.10 Dependence of the slope of p
Page 11 and 12: 8.3.6 RF phase measurement drift wi
Page 13 and 14: where α is the bending angle intro
Page 15 and 16: R 16 ( eBl / 2) d ⎛ ⎞ 1 eff 2 p
Page 17 and 18: 2 Free-electron Lasers The facility
Page 19 and 20: arrival-time and energy prior to a
Page 21 and 22: electrons travel faster than low-en
Page 23 and 24: efore bunch compressor energy chirp
Page 25 and 26: corresponds to terms that are linea
Page 27 and 28: spread will be larger and the trans
Page 29 and 30: (a) (b) Figure 2.3.4 Single chicane
Page 31 and 32: 3 Beam Arrival-time Stabilization A
Page 33 and 34: changes in the amplitude of the kly
Page 35 and 36: The problem of cavity field measure
Page 37 and 38: creates a single point-of-failure s
Page 39 and 40: accelerator section and it was firs
Page 41: ACC1 ~Gun 3rd ACC2 ACC3ACC4 ACC7 Ac
Page 45 and 46: ( ) E − E ⎛ E − E ⎜ ⎝ 0 0
Page 47 and 48: 3.7 Second Accelerator Section Jitt
Page 49 and 50: 4 Beam Shape in the Bunch Compresso
Page 51 and 52: There has been some debate over the
Page 53 and 54: V T = V cos( k ⋅ z + = V T 0 rf
Page 55 and 56: Upstream of the first accelerator s
Page 57 and 58: Positive bump 5 8 deg off-crest in
Page 59 and 60: E (MeV) 4.75 4.74 4.73 4.72 4.71 4.
Page 61 and 62: leads one to suspect that as in the
Page 63 and 64: vacuum feedthroughs, the buttons we
Page 65 and 66: J image − ⎡ ⎛ ⎞ ⎤ = ⎢ +
Page 67 and 68: sensitivity of 8 and 20 mm diameter
Page 69 and 70: 1 v phase = . (5.1.21) LC Finally,
Page 71 and 72: some locations. Nevertheless, it is
Page 73 and 74: BPM BAM More power at lower frequen
Page 75 and 76: arrival-time monitor with and witho
Page 77 and 78: 5.2 Cavity BPM Cavity BPMs (Fig. 5.
Page 79 and 80: t=0 Z 0 t=L/c Z 0 2L Figure 5.3.1 L
Page 81 and 82: 0.2 nC beam charge and can handle b
Page 83 and 84: T1-T2=T3 X=T3*c T1 T2 Figure 5.5.2
Page 85 and 86: None of the designs suffered from a
Page 87 and 88: pickup functions appropriately over
Page 89 and 90: 35 Measured BPM Vertical amplitude
Page 91 and 92: 2.5 Beam position 2 position (cm) 1
Page 93 and 94:
L( x0 , t) U 0 ( t) = λ( x0 , t) (
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1 x beam = ) xdtdx Q ∫∫λ ( x,
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0 BPM beam width dependence arrival
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100 Vertical y position sensitiviy
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Such an x-y tilt was measured with
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10 5 head x [mm] 0 tail -5 -5 -4 -3
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3.5 x 10-3 Horizontal Charge Distri
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The problem that could arise due to
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around their zero-crossings in orde
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v i ( i i i t) = A sin(2π f t + θ
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strengths of the peaks seen in Fig.
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algorithm required to select the ph
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Downmixers for EBPM BPM-L BPM-R BPM
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While this option was not built and
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A Peltier element acts as a heat pu
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For the measurement shown in Fig. 7
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2 1.5 1 BC2 x position (mm) 0.5 0 -
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0 BC2 arrival time (ps) 1.5 1 0.5 0
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eality check BC2 PMT EBPM and PMT p
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EBPM and PM positions (mm) 6 5 4 3
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If a limiter or nothing at all is u
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Depending on the arrival-time of th
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20mW to BAM FARADAY 8m 80/20 60mW W
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can be placed anywhere prior to the
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Insulation Peltier element on metal
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using the RF limiter. This might be
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compared. The length of the cable c
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eplaced with fiber. The advantage o
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Photodetector Photodetector Two las
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RF-Lock Box for FLASH MLO Synchroni
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In Fig. 8.3.4, the spectral noise d
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Mixer Output (fs) 40 20 0 -20 MLO R
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For a frame of reference concerning
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(Fig. 9.1.2). This method proved to
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N λ ∞ ∫ ω / ω Δω 9 3 ( ω)
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Finally, we can use this, together
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The highest energy resolution (ΔE/
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Good agreement between the RF chica
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10.3 Out-of-loop Vector Sum The out
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out. This was done in two ways: by
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1.8 1.6 stripline button 1.4 Upstre
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10 Conclusion and Outlook Six disti
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Appendix A The derivation of Eqs. 3
Page 179 and 180:
Solving Eq. 8 for E 1 ’ provides
Page 181 and 182:
Higher order terms and the 3 rd -ha
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∂t ∂V 2 2 ⋅ ΔV 2 1 = T1 '( E
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References [1] P. Castro. “Beam t
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[28] H. Schlarb et al., “Beam bas
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[55] C. Gerth, personal communicati
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Measuring the Electron Beam Energy in a Magnetic Bunch ... - DESY

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