1. An Ultra Low Noise AC Beam Transformer for Low Intensity Beams – for AD (& Elena?)
For Diagnostics and Deceleration
Flemming PEDERSEN
[Carlota GONZALEZ VAZQUEZ ]
Alan FINDLAY
Maria Elena ANGOLETTA

2. Ultra Low Noise AC Beam Transformer
Outline
CERN AD (Antiproton Decelerator)
CERN ELENA (Extra Low ENergy Antiprotons)
Beam Transformer and Head Amplifier
Transformer and Amplifier Noise Sources
Low Noise Amplifier Design
Feedback Resistor and Circuits
Low Frequency Version
Transformer in Beam Transfer Line
LTI Coax Cable developed
Can a broad band electrostatic device (à la AD ring PU S signal) achieve similar or adequate performance?
April 2012
Ultra Low Noise AC Beam Transformer

3. CERN AD (Antiproton Decelerator)
Collection, Cooling, and Deceleration of antiprotons
Momentum 3.5 GeV/c down to 100 MeV/c
Intensity: few 107 charges: current mA
DC beam transformers (Unser-trons) are not sensitive enough
Bunched : use RF current (at two harmonics) for intensity, fRF = MHz,
Bunched: Use full bunch bandwith signal for bunch length and longitudinal emittance measurements (tomoscope)
Bunched: Use fundamental component for RF system phase loop
Unbunched: Use longitudinal Schottky scans for intensity and momentum distribution
For more than 13 years, this device has been the workhorse of longitudinal diagnostics and intensity measurements in the AD
April 2012
Ultra Low Noise AC Beam Transformer

4. CERN ELENA (Extra Low ENergy Antiprotons)
Cooling, and Deceleration of antiprotons
Momentum 100 MeV/c down to 13.7 MeV/c
Intensity: few 107 charges: currents comparable to AD: b and circumference lower by similar factor
DC beam transformers (Unser-trons) are not sensitive enough [..but SC DCCT developed in several labs outside CERN]
Bunched : use full bandwith signal for RF based intensity measurements (integration of bunch areas, CERN BE/RF receivers now available)
Bunched: Use full bunch bandwith signal for bunch length and longitudinal emittance measurements (tomoscope)
Bunched: Use fundamental component for RF system phase loop
Unbunched: Use longitudinal Schottky scans for intensity and momentum distribution as in AD
The initial conceptual design (2007 ELENA report) was based on the use of a similar ultra low noise beam transformer in ELENA
April 2012
Ultra Low Noise AC Beam Transformer

5. Applications of Low Noise Transformer
Applications of this new PU ( MHz) in AD:
i) RF phase PU during deceleration for RF system
ii) Intensity during deceleration (using two rev. harmonics)
iii) Bunch lengths
iv) Intensity and momentum distribution while debunched (Schottky)
S/N ratios at optimum Schottky harmonic is about 10 – 50 dB at different points in the AD cycle (measured and calculated)
It is more than 30 dB better than the old 42 MHz Schottky PU: Noise current density near 1.5 MHz:
v) Single shot bunch intensity measurement in transfer line using LF version (0.02 – 3 MHz). Resolution few 104 charges; calculated but not achieved due to current analog signal integration.
April 2012
Ultra Low Noise AC Beam Transformer

6. Low Noise Longitudinal Beam Transformer
April 2012
Ultra Low Noise AC Beam Transformer

7. Doubly Shielded Copper Cavity
April 2012
Ultra Low Noise AC Beam Transformer

8. Ultra Low Noise AC Beam Transformer
Cavity Assembly (LF)
April 2012
Ultra Low Noise AC Beam Transformer

9. The Low Noise Feedback Principle
Make the pick-up resonant with as high a Q as possible to reduce the thermal noise current of the transducer itself
Use a low noise high impedance amplifier (Si JFET's) and regain broad band properties by strong active feedback WITHOUT deteriorating the signal to noise ratio (inject feedback current through a small capacitor)
The transimpedance amplifier made this way has an extremely low 'noise temperature' in a certain frequency range (< 0.5 K)
April 2012
Ultra Low Noise AC Beam Transformer

10. Transformer and Amplifier Noise Sources
N is transformer step-up ratio
Ib is the desired signal
Vna [V/Hz1/2] is the amplifier voltage noise
Ina [A/Hz1/2] is the amplifier noise current
Inth = [A/Hz1/2] is the thermal noise of the pick-up shunt impedance Rp, so make Rp as large as possible (high Q)
Transform Ca, Vna and Ina to primary of transformer
Increasing N lowers voltage noise, but increases current noise and capacity
April 2012
Ultra Low Noise AC Beam Transformer

11. Transformer and Amplifier Noise Sources
The amplifier voltage noise Vna/N can be transformed into an equivalent current noise by the relation: Ivna = Vna/N Zpu
The total equivalent, input-noise current is then:
if the 3 noise sources are uncorrelated (not quite true for Ina and Vna)
If the amplifier noise is sufficiently small, the signal to noise ratio is dominated by the noise from the shunt impedance and is constant over a wide frequency range
April 2012
Ultra Low Noise AC Beam Transformer

12. Equivalent, Input-noise Currents (gap)
April 2012
Ultra Low Noise AC Beam Transformer

13. Amplifier input stage considerations
Bipolar transistors may have low voltage noise, but current noise much larger than for FET transistors due to the shot (Schottky) noise:
where Ibase is the base bias current
A FET transistor has a voltage noise: where gm is the transconductance [A/V] and a current noise induced in the input circuit by the thermal noise of the channel through the distributed gate channel impedance:
where Cc is the coupling capacitance: ~ 2/3 of total gate-channel capacity Cgs.
FET noise quality is thus characterised by the gm/Cgs which is also the high frequency figure of merit fT: best ratio between current and voltage noise is found in FET transistors with high fT
April 2012
Ultra Low Noise AC Beam Transformer

14. Choice of FET transistors
GaAs FET: very high fT, but also high 1/f (flicker noise) corner frequency: > 10 MHz
Si MOSFET: medium fT but somewhat better 1/f corner frequency: 1 - 5 MHz
Si JFET: similar fT to Si MOSFET, but much lower 1/f corner frequency: ~10 Hz for Philips BF861C . By far the best choice in the interesting frequency range: MHz
Monolithic amplifiers with JFET inputs are available, but not with very low voltage noise and a high gain-bandwidth product (feedback!)
April 2012
Ultra Low Noise AC Beam Transformer

15. Ultra Low Noise AC Beam Transformer
Amplifier Design
16 parallel Si JFET's with high gm and ID to obtain low voltage noise (0.25 nV/Hz1/2) - bipolar cascode to obtain low miller capacity and high bandwidth. Total delay ~ 5 ns.
Bipolar 2nd stage to further amplify with increasing equivalent input noise and prevent the rather noisy 3rd stage from adding noise
Class B output current mode feedback video op-amp for high slew rate, high output swing and wide bandwidth (> 100 MHz, AD811)
April 2012
Ultra Low Noise AC Beam Transformer

16. Feedback Resistor and Circuits
Although the resonant high Q transformer has a good signal to noise ratio, it has drawbacks:
i) Highly resonant response
ii) Amplifier saturates at very low current near resonance
The response can be made broad band if a negative current feedback resistor is introduced around the amplifier
This converts the high input impedance amplifier into a low resistive impedance Rfb/G : the response becomes broad band
If Rfb and G are sufficiently large, the equivalent input noise current due to Rfb can be made negligible compared with the Rp noise current: Rfb > 16 N Rp
April 2012
Ultra Low Noise AC Beam Transformer

17. Feedback Resistor and Circuits
There are several disadvantages from such a high feedback resistor (400 kW):
i) difficult to implement at high frequencies (stray capacity)
ii) limited dynamic range: Iin,max = Vout,maxN/Rfb
large amplifier gain (10'000!) required, short delay difficult
Much better performance and easier implementation is obtained by injecting the feedback current through a small capacitor combined with a 90 degree phase shift (integrator)
By making Rint small and Cint >> Cfb the effective noise temperature of the feedback resistor is dramatically reduced
April 2012
Ultra Low Noise AC Beam Transformer

18. Ultra Low Noise AC Beam Transformer
Low Frequency Version
To extend the Bandwidth towards lower frequencies (limited by Vn and Lp):
Use higher m ferrites: 200 -> 1200
Use higher step-up ratio: N = 2 -> 4, this lowers
Transformer installed adjacent to high frequency device and signals combined (crossover 1 MHz)
April 2012
Ultra Low Noise AC Beam Transformer

19. Equivalent, Input-noise Currents (gap)
April 2012
Ultra Low Noise AC Beam Transformer

20. Ultra Low Noise AC Beam Transformer
Performance
Longitudinal Schottky S/N ratios are typically 10 – 50 dB with N = 5E7 at best frequency (~ 1.5 MHz)
At higher energies, 300 MeV/c and above, we use much higher harmonics to reduce Schottky acquisition time and statistics
The massive EMI shielding works as intended, main EMI source is output cable
April 2012
Ultra Low Noise AC Beam Transformer

21. Calculated vs. Measured Noise levels
Measured noise currents are within 20% of calculated values (except low frequencies < 10 kHz for LF version)
April 2012
Ultra Low Noise AC Beam Transformer

22. Absolute Intensity Calibration
Absolute transconductance gain Y(w) = Vout/Ibeam = 100 W, 500 W, 4 k W, 20 k W (4 selectable gains, measured by network analyzer)
Cross check of Schottky intensity (prop Y2) vs. bunched beam intensity (prop Y)
Intensities measured in injection line (TFA7049) and ejection line, which can be charge calibrated by a capacitor discharge.
April 2012
Ultra Low Noise AC Beam Transformer

23. Single Pass Beam Transformer in Transfer Line
In CERN AD: Fast extracted antiproton bunch, few107 charges, length ~100 to 300 ns.
Integrate current output during 1 ms
Bandwidth required: 3 kHz (18 kHz) to 3 MHz [to avoid errors from response limits]. Use LF version
Charge noise given by (graph next slideP:
Expected RMS fluctuation: 1.9×104 charges (4 s is 7.6×104 charges)
April 2012
Ultra Low Noise AC Beam Transformer

24. Single Pass Beam Transformer in Transfer Line
April 2012
Ultra Low Noise AC Beam Transformer

25. Parasitic Noise Sources
The double Cu shield of the pick-up cavity and head amplifier worked as expected: no parasitic noise observed into input of head amplifier
Some un-explained 1/f noise observed below 10 kHz in the low frequency version: actually scales as 1/f 3 [AD811? Voltage regulator noise?]
Main parasitic noise is medium wave radio noise picked up on 100m OUTPUT CABLE from ring to ACR due to Zt (transfer impedance) of CERN standard coax C (Zt ~5 1 MHz). This in spite of 63 dB head amplifier gain!!!
New cable C developed in industry (Draka) as a consequence of this (Zt ~ MHz): worked as expected when installed in 2001.
April 2012
Ultra Low Noise AC Beam Transformer

26. Coax Transfer Impedance
April 2012
Ultra Low Noise AC Beam Transformer

27. New CERN (Draka) LTI construction – C50-6-2
Coaxial following
CERN Specification 461 REV. 5.1
S-02Y(St)C(mS)CH 2.6/7.3 (C )
April 2012
Ultra Low Noise AC Beam Transformer

28. New CERN (Draka) LTI construction
S-02Y(St)C(mS)CH
2.6/7.3
Inner conductor
copper wire, bare mm
2.6
Insulation
Foam-PE
7.25
Outer conductor
Al-PETP-Al-foil + copper braid, silvered
Wrapping
PETP-foil
Magn. Screen
1 layer of iron tape, highly permeable
Static screen
copper braid, silvered
Sheath
FRNC ( Flame Retardant Non Corrosive Copolymere)
11.5
Colour
brown or black
April 2012
Ultra Low Noise AC Beam Transformer

29. New CERN (Draka) LTI electrical properties
Transfer impedance at
10 kHz
m/m
nom. 0.8
 1.0
0.1 MHz
nom
 0.180
1 MHz
nom. 0.06
 0.08
10 MHz
nom. 0.2
 0.250
100 MHz
nom. 2.0
 2.5
Insulation resistance
Gxkm
³ 10
Test voltage
KVrms
3.0
Operating voltage
1.2
April 2012
Ultra Low Noise AC Beam Transformer

30. Adaptations required for ELENA version of AD LPU
Few modifications required to permit baking at 200 degr C: higher temperature solder 220 degr. C), ceramic SMA feedthrough?
Could perhaps be raised to 250 degrees if required: FFEP coax cable -> PTFE cable (100x less radiation dose [but not an issue in Elena], lower b = 0.7 vs for FFEP)
AD vacuum chamber ID is 146 mm, so either Conflat transition pieces (increased length) to Elena ID of 80 mm or a mechanical redesign will be required
In case of mechanical redesign due to smaller diameter, a shorter ferrite length should be investigated to save space
Should the HF & LF pick-ups be mounted back to back to reduce gap to gap separation?
Electronics obsolescence: A study has been made, and only a few (non-critical) components need to be replaced when rebuilding, but new PCB of both HF and LF versions required.
April 2012
Ultra Low Noise AC Beam Transformer

31. Broadband Electrostatic Device Optimum noise matching
Optimum FET amplifier noise matching for a capacitive current source
Applies also to ferrite loaded beam transformer at high frequencies
Signal source is a current source in parallel with a source capacitor Cs
FET amplifier with n parallel amplifiers. Each amplifier:
Equivalent input noise current of Ina [A/Hz1/2]
Input capacity Ca [F]
Equivalent input noise voltage density of Vna [V/Hz1/2]
April 2012
Ultra Low Noise AC Beam Transformer

32. Broadband Electrostatic Device Optimum noise matching
Convert the noise voltage into an equivalent noise current:
Total noise current:
There is an optimum value for n:
which for zero noise current implies nCa = Cs
April 2012
Ultra Low Noise AC Beam Transformer

33. Broadband Electrostatic Device Optimum noise matching
For the specific case of FET, equivalent input voltage and current noise are given by:
Cc is coupling capacity, Ca is gate capacity, gm transconductance
optimum found when amplifier input capacity nCa is slightly smaller (83%) than the source capacity Cs and independent of frequency.
April 2012
Ultra Low Noise AC Beam Transformer

34. Electrostatic broadband PU elementary charge wave shape
April 2012
Ultra Low Noise AC Beam Transformer

35. Ultra Low Noise AC Beam Transformer
Calculated Schottky S/N ratios for a broadband electrostatic longitudinal PU
PU length limited by desire to measure shortest bunch length accurately (about 20 cm?)
With no amplifier connected, voltage wave shape DC value above baseline and low frequency Fourier components independent of length (..but lower length reduces optimum noise matched voltage noise)
Unlike low noise beam transformer, there is no strong dependence on particle velocity bc
Optimum noise matching important to achieve useable S/N ratios
Best S/N ratio at lowest harmonics due to width of Schottky lines. This results in long acquisition times, poor statistics.
Typical S/N ratios for N = 1E7, and nominal ELENA longitudinal emittances are about 20 dB.
Marginal S/N ratios for N = 1E6 and/or larger longitudinal emittances
Proposed parameters very close to existing electrostatic AD S PU (L. SØBY)
April 2012
Ultra Low Noise AC Beam Transformer

36. Ultra Low Noise AC Beam Transformer
Calculated Schottky S/N ratios for an electrostatic long. PU (100 MeV/c)
April 2012
Ultra Low Noise AC Beam Transformer

37. Ultra Low Noise AC Beam Transformer
Calculated Schottky S/N ratios for an electrostatic long. PU (35.7 MeV/c)
April 2012
Ultra Low Noise AC Beam Transformer

38. Longitudinal PU options
Use existing AD LPU design modified as required, vacuum chamber diameter, bakeable, possibly shorter length. Solid operational performance and excellent S/N ratios
Replace low noise beam transformer with optimized electrostatic broadband PU (1 kHz – 20 MHz) with improved EMI shielding. Drawback is sensitivity to lost particles, and poorer S/N ratios in some scenarios (large EL and/or low intensity)
As 2) but supplemented by an low noise (normal conducting) resonant Schottky to improve S/N ratios and enable the use of higher harmonics for faster acquisitions and better Schottky statistics
April 2012
Ultra Low Noise AC Beam Transformer

39. Ultra Low Noise AC Beam Transformer
What’s next? Choices?
Side by side calculated S/N ratios should be made for AD LPU (low noise beam transformer) and existing AD S PU
Calculated S/N ratios should be confirmed with existing electrostatic AD S PU in AD
The possible use of noise optimized broad band electrostatic PU’s in the transfer line for intensity (..and position?)
A longitudinal PU design report should be written during the summer of 2012 comparing the various options (FP + collaborators)
April 2012
Ultra Low Noise AC Beam Transformer