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

More documents

Recommendations

Info

Feedforward is a term describing an element in a control system that delivers commands in a pre-defined way, without responding to how the system reacts. A fixed setpoint table that takes into account various calibrations done at an earlier time is an example of feedforward. After the effects of Lorentz-force detuning, cavity detuning, and imbalances in the actuator chain have been calibrated away over weeks of studies in the absence of beam production, a fixed feedforward (setpoint) table can be determined. When the beam is then added to the system, a slope on the RF pulse arises due to something called beam loading. Beam loading occurs when an electron bunch enters the accelerating cavity. The beam takes energy out of the accelerating field and this energy must be replaced by increasing the klystron’s output. If, in one bunch train, each bunch took a certain amount of energy out of the cavity, the same thing is likely to happen in a subsequent bunch-train, as long as the beam charge or orbit is not significantly changed. A fixed feedforward table may be appropriate for one set of beam parameters, but as soon as the machine operator changes the setup of the machine, the feedforward table will have to be manually tuned to compensate for changes in beam loading. In the absence of an expert to tune the feedforward table, an adaptive feedforward algorithm using “Iterative Learning Control” can automatically change the feedforward table in order to counteract the changes in beam loading. The control decisions of the adaptive feedforward algorithm do not take place within the bunch-train, but after averaging over multiple bunch-trains. The adaptive feedforward can not only remove slopes from the bunch train, it can also, in principle, remove ripples. If ripples are periodic and appear in bunch-train after bunch-train with the same phase, they can be removed through the feedforward. An adaptive feedforward control can identify patterns in the cavity signals during one klystron pulse and attempt to remove those patterns by applying a pattern of equal and opposite amplitude in a subsequent klystron pulse. This control option has not, however, been used for day-to-day beam operation due to the incompleteness of its implementation. The incompletely debugged failure modes of the controller have caused the superconducting cavities to quench. New versions of the controller are under development [11]. Feedback is different from feedforward in that it sets control parameters based on the reaction of the system to the control parameters. It utilizes measurements taken at the beginning of the pulse in order to change the settings of the klystron within the pulse. The current system can implement Proportional, Integral and Differential (PID) feedback control, but for typical operation, only the proportional feedback is used. Feedback is limited by measurement resolution and latency, how long it takes for a signal to be measured, interpreted, and converted into a control parameter. With Field Programable Gate Arrays (FPGAs), Analog-to-Digital Converters (ADCs), and Digital-to-Analog Converters (DACs) able to process signals at more than 100 MHz, or every 10 ns, and signal transport times that can be kept below 100 ns, bunch-to-bunch feedback within the FLASH bunch spacing of 1 μs and even the XFEL bunch spacing of 200 ns becomes a goal within reach. The latency of present systems is currently limited to ~3 μs, but in certain locations where cable lengths can be minimized, faster performance could be realized with future hardware. While the delivery of control decisions to the klystron between bunches is possible, making large changes in the field of the accelerating structure at that rate is physically limited due to the large quality-factor, or small bandwidth of the cavity. Large 20
changes in the amplitude of the klystron produce only small changes in the cavity gradient. When the corrections demanded by the feedback loop become small enough, then the large quality-factor of the cavity is no longer a problem and the cavity phase and amplitude can, in principle, reach a stability determined solely by the resolution and drift of the monitoring system used in the feedback. In Fig. 3.1.1, the current digital processing architecture is depicted [11]. The cavity field probe signals 1 through n are sent to the field detectors wherein they are down mixed from 1.3 GHz to 54 MHz and sampled with an 81 MHz ADC. The digitized signals from the ADC are sent into a digital phase and amplitude (I & Q) detection algorithm. This calculation requires a multiplication by a calibration constant. The phase and amplitude information from each individual cavity are then added together in a vector sum. The vector sum is compared to the setpoint values generated from a feedforward table. A correction to the cavity fields is then calculated from a gain setting and the difference of the measured signals from the setpoint. This correction is multiplied by calibration factors appropriate for each cavity and sent to a DAC which generates a 250 kHz signal. The 250 kHz signal goes to a vector modulator (labeled VM in the diagram) which shifts the phase and amplitude of the 1.3 GHz that is sent to the klystron. Accelerating Module beam mod klyst actuator controller Cavity field probes Detectors*n 1 2 … n VM DAC calib outt Setpoint, gain, feed fwd vect sum calib in I/Q detection ADC MO Figure 3.1.1 System for controlling the cavity fields of the accelerating module. The phase and amplitude of the fields are detected from cavity pickups, the difference from the setpoint is calculated and a correction is sent to the klystron [11]. The boxes labled: I/Q detection, calibration in, vector sum, setpoint, gain, feedforward, calibration out comprise the routines that take place on an FPGA. This routine is depicted in more detail below in Fig. 3.1.2. In Fig. 3.1.2, the phases and amplitudes detected from each cavity are added together in a vector sum and then subtracted from the setpoints generated by the feed-forward table. The feed-forward table is generated from a setpoint given by a user and an older feed-forward table [11]. 21
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: 3 Beam Arrival-time Stabilization A
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 and 42: ACC1 ~Gun 3rd ACC2 ACC3ACC4 ACC7 Ac
Page 43 and 44: • The lack of a normal-conducting
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) (
Page 95 and 96:
1 x beam = ) xdtdx Q ∫∫λ ( x,
Page 97 and 98:
0 BPM beam width dependence arrival
Page 99 and 100:
100 Vertical y position sensitiviy
Page 101 and 102:
Such an x-y tilt was measured with
Page 103 and 104:
10 5 head x [mm] 0 tail -5 -5 -4 -3
Page 105 and 106:
3.5 x 10-3 Horizontal Charge Distri
Page 107 and 108:
The problem that could arise due to
Page 109 and 110:
around their zero-crossings in orde
Page 111 and 112:
v i ( i i i t) = A sin(2π f t + θ
Page 113 and 114:
strengths of the peaks seen in Fig.
Page 115 and 116:
algorithm required to select the ph
Page 117 and 118:
Downmixers for EBPM BPM-L BPM-R BPM
Page 119 and 120:
While this option was not built and
Page 121 and 122:
A Peltier element acts as a heat pu
Page 123 and 124:
For the measurement shown in Fig. 7
Page 125 and 126:
2 1.5 1 BC2 x position (mm) 0.5 0 -
Page 127 and 128:
0 BC2 arrival time (ps) 1.5 1 0.5 0
Page 129 and 130:
eality check BC2 PMT EBPM and PMT p
Page 131 and 132:
EBPM and PM positions (mm) 6 5 4 3
Page 133 and 134:
If a limiter or nothing at all is u
Page 135 and 136:
Depending on the arrival-time of th
Page 137 and 138:
20mW to BAM FARADAY 8m 80/20 60mW W
Page 139 and 140:
can be placed anywhere prior to the
Page 141 and 142:
Insulation Peltier element on metal
Page 143 and 144:
using the RF limiter. This might be
Page 145 and 146:
compared. The length of the cable c
Page 147 and 148:
eplaced with fiber. The advantage o
Page 149 and 150:
Photodetector Photodetector Two las
Page 151 and 152:
RF-Lock Box for FLASH MLO Synchroni
Page 153 and 154:
In Fig. 8.3.4, the spectral noise d
Page 155 and 156:
Mixer Output (fs) 40 20 0 -20 MLO R
Page 157 and 158:
For a frame of reference concerning
Page 159 and 160:
(Fig. 9.1.2). This method proved to
Page 161 and 162:
N λ ∞ ∫ ω / ω Δω 9 3 ( ω)
Page 163 and 164:
Finally, we can use this, together
Page 165 and 166:
The highest energy resolution (ΔE/
Page 167 and 168:
Good agreement between the RF chica
Page 169 and 170:
10.3 Out-of-loop Vector Sum The out
Page 171 and 172:
out. This was done in two ways: by
Page 173 and 174:
1.8 1.6 stripline button 1.4 Upstre
Page 175 and 176:
10 Conclusion and Outlook Six disti
Page 177 and 178:
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
Page 183 and 184:
∂t ∂V 2 2 ⋅ ΔV 2 1 = T1 '( E
Page 185 and 186:
References [1] P. Castro. “Beam t
Page 187 and 188:
[28] H. Schlarb et al., “Beam bas
Page 189 and 190:
[55] C. Gerth, personal communicati
show all

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

Create successful ePaper yourself

Delete template?

Save as template?