1. 6th CWRF 2010 Workshop - ALBA, May 4, 2010
The New RF Power Amplifier Unit for Berkeley 88” Cyclotron
Slawomir Kwiatkowski
Team Members: A. Hodgkinson, D. Byford, P. Casey, M. Kireeff-Covo, J. Julian, Ch. Reiter
Lawrence Berkeley National Laboratory 1 1

2. 88” Cyclotron Overview
The 88-Inch is a K=140 sector-focused cyclotron with both light- and heavy-ion capabilities. Protons and other light-ions are available up to maximum energies of 55 MeV (protons), 65 MeV (deuterons), 135 MeV (3He) and 140 MeV (4He). Most heavy ions through uranium can be accelerated to maximum energies which vary with the mass and charge state.
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3. 88” RF System
The RF system of the cyclotron is based on the quarter-wave cantilever type resonating structure.
PA configuration- 500kW 4648 RCA RF power tetrode operating in grounded cathode configuration.
The resonance frequency range 5.5MHz to 16.5MHz.
The course tuning of the RF resonator is accomplished by changing the characteristic impedance of the resonance structure from 5Ω to75Ω.
The fine frequency adjustments of the resonator is done with help of the capacitive trimmer located in the vacuum close to the tip of the dee.
The matching of the effective shunt impedance of the resonator to the required anode impedance of the power tube is done with help of the anode trimmer capacitor located inside the resonator.
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4. 88” Resonator Model
Due to the large size and very complex form of the resonator it could be extremely difficult to calculate its parameters with help of any 3-D finite element codes like MAFIA, HFSS or Microwave Studio.
The transmission line model of the resonator has been build and the simple MathCAD program has been written which allow us to determine with good enough accuracy the main parameters of the unit.
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5. Calculated Parameters of the Resonator
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6. Remarks about Resonator
General expression for the resonance frequency of the quarter wave coaxial structure loaded with tip capacitance:
Calculations of 88” indicates that
by taking away panels (maintaining Zo=75Ω) and introducing moving shorting plate the power dissipation at the higher frequency end (16.5MHz)would be 30kW (for 70kV dee voltage).
The length of the last section of the resonator would be 0.2m (2.134m for 5.5MHz).
Current density at the ID of the short<4A/cm.
The well design sliding contacts can handle current density by order of magnitude higher.
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7. Why new PA Unit
The cost of RCA 4648 tetrode become prohibitively high (>120k$)
Frequent problems with anode blocking capacitors.
88” Cyclotron likely to operate for >10 years.
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8. PA Tube and Config. Choice
For 88” RF frequency range (5.5MHz-16.5 MHz) and power requirements (Prf<120kW) the most economical choice for the final tube is CPI 4CW150000E water cooled tetrode (cost<20k$).
Grounded grid configuration is attractive since it creates better insulation between output and input networks due to the smaller coupling capacitance (0.35pF instead of 1.2pF for grounded cathode ).
Due to the smaller tube input capacitance and low input impedance PA unit operating in the grounded grid configuration could cover whole 88” cyclotron frequency range (5.5MHz to 16.5MHz) without the necessity for tuning of the input circuit of the power tube.
The disadvantage - lower power gain what could drive existing 2kW driver unit to the limit.
For this reason grounded cathode configuration has been chosen for new PA unit.
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9. Calculated PA Parameters for
Rinp=100 OHM
Freq.
Rsh
Vdee
Pa1 Ra
Anode
Pao
Iao
Ia1
Effic
Drive
[MHz]
[kΩ
[kV]
[kW]
[kΩ]
Trim kW A % W
5.5
316 75
8.9
8.2 17
0.94
1.47
52.4 40
6.5
213
13.2 8 21
1.16
1.82
62.9 70
7.5
151
18.6
6.9 7 27
1.48
2.3
68.9
100
8.5
110
25.6
5.3 15 37
2.03
3.1
69.2
140
9.5
83.2
33.8
4.02 25 48
2.7
4.1
70.4
190
10.5
64.2
43.8
3.11 30 62
3.4
70.7
245
11.5
50.5
55.7
2.44 78
4.3
6.75
71.4
300
12.5
40.3
69.8
1.94 97
5.4
72.0
340
13.5
32.6
86.3
1.57
119
6.6
72.8
380
14.5
26.6
92.1 50
126
7.0
11.2
73.4
410
15.5
21.9 65
96.5
1.41 47
131
7.3
11.7
73.7
440
16.5
18.1
60.6
101.5
1.34
137
7.6
12.3
74.0
470
CAP
Mod Anode
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10. Stability of PA Unit
P.A unit stability at the fundamental frequency is achieved if the ratio of the magnitude of the reverse attenuation (S12) to the magnitude of the forward amplification (S21) is greater than 1.
For PA unit in grounded cathode configuration and operation at 16.5MHz (worse case) and grid load resistor Ri=100Ω Ms is 1.5. Since the stability margin is marginal it has been decided to use in new PA with simple neutralization circuitry contain house made variable capacitor and ferrite loaded transmission line baloon.
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11. 6th CWRF 2010 Workshop - ALBA, May 4, 201011

12. 6th CWRF 2010 Workshop - ALBA, May 4, 201012

13. PA Anode Circuit
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14. PA Input Circuit
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15. PA Tube Socket
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16. Neutralization Performance
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17. Neutralization Performance
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18. Neutralization Performance
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19. Status All low level tests done. Installation 21-26 May 2010 19
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