New Materials and Designs for High Power Fast Phase Shifters

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A Fast Chopper for HINSRobyn Madrak, Tech.
Division
High Intensity Neutrino Source
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FNAL HINS

• 60 MeV Linac under construction at Fermilabs
  meson building would be a Front End for an 8
  Gev linac
• (proves technical feasibility of an 8 GeV Linac,
  aka Proton Driver)
• G.W. Foster and J. A. MacLachlan, Proceedings of
  Linac02, Gyeong, Korea

3
Unique Aspects/Challenges

• Solenoidal focusing ? cleaner beam
• Use of SC spoke resonators
• Fast ferrite phase shifters
• will allow all cavities (and RFQ) to be driven
  by a single 2.5 MW, 325 MHz klystron gt
  large cost savings
• Fast Beam Chopper

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HINS Chopper

• Should the HINS be extended to an 8 GeV Linac,
  output beam would be transferred to Fermilabs
  Main Injector, with 53 MHz RF frequency
• HINS Linac Bunches are spaced by 325 MHz (3.1ns)
• In MI, RF frequency is 53 MHz (19ns)
• Dont want bunches in the 53 MHz separatrix
• ? Chop out 1 of every 6 bunches
• Additional complication 325 ? n G 53
• ? Sometimes chop 1, sometimes 2

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Traveling Wave Chopper Structure

• beam is deflected by traveling pulse (electric
  field)
• b(beam)0.073 gt must slow down pulse
• Use traveling wave meander structure
• 50 cm long
• 16 mm between chopper plates
• 2.4 kV per plate
• deflection of 6mm at end of plates
• Based on design and prototype by
• F. Caspers (CERN)

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Chopper in MEBT
length of chopper plates 50 cm drift space
downstream 30 cm
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Deflection
length of chopper plates 50 cm drift space
downstream 30 cm plate separation
16 mm
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Pulser Development

• We need
• Two pulsers to drive the 50 O meanders /- 2.4
  kV
• Max 5.5 ns pulse width (including rise and fall
  time)
• 53 MHz rep rate
• burst of 3ms _at_2.5Hz, or 1ms_at_10Hz
• Programmable pulse width
• (may sometimes chop 1 bunch, sometimes two)
• ? Specs do not lead to an obvious solution

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Similar Choppers
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Using one or two single power switches
min width pulses _at_20MHz
20ns/div
1ms/div
? cannot get to a narrow enough pulse
4ns/div
With two switches Pulses are narrower,
but Still not narrow enough
from DEI/IXYS RF
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Using one or two single power switches
With a slightly lower current version of the
switch
20ns/div

• The DEI FETS can be used to make a very narrow
  pulse by charging cable in the drain
• But in this case we cannot attain the desired 53
  MHz rep rate

4ns/div
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DEI FETS for H- Source

• DEI FETS were useful for beam notching in H-
  source for the current linac
• 40ns pulses 2.2ms spacing
• Burst of 15 pulses, repeat at 15 Hz
• Two pulsers 800V

single pulses
toroid response shows notched beam
20 pulses
trigger signal (time offset)
Doug Moehs
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Combining lower voltage pulses?
t_rise 1.4 ns t_fall 2.2ns width 4.1ns
repeat for 10ms
2.5ns/div
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Combining lower voltage pulses?

• This scheme can be extended to a larger number
  of FETS (driving smaller impedances) combined to
  attain larger output voltages
• Kentech Instruments, Ltd are experts at this

Kentechs Mission Statement

• They had successfully built a pulser (but at
  1/20th rep rate) for RAL/ESS

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500V Prototype Pulser Scheme
10 pulse cards 50V?5O
5 FETS/card (in parallel) each FET drives 25O
pulse control cards
25O semirigid cable with ferrite
5O
5O
5O
5O
5O
25O
five 25O semirigid cable in parallel, with ferrite
25O
outer conductor
5O
5O
5O
5O
5O
center conductor
output 500V?50O
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500V Pulser
before fully assembled
trigger and power dist cards
pulse control cards
PSU
one side of combiner
five 25O semirigid cable in parallel, with ferrite
pulse cards
output
25O semirigid cable
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500V Pulser Output
5.5 ns
520 V pulse
1 ms of pulses _at_ 53 MHz
3 ms of pulses _at_ 53 MHz
(repeats _at_ 10Hz)
(repeats _at_ 2.5Hz)
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Next Steps

• We have also ordered from Kentech a 1.2 kV pulser
• This is currently being fabricated
• We plan to combine two (1.2kV?50O) into one
  (2.4kV?100O) output
• This requires a combiner and a meander with 100O
  impedance

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Combiner

• MN60 ferrite three 11 OD, 4.5 ID, 1 thick
  cores
• 58 turns of superflex cable

ferrite
Test combiner by splitting and recombining (using
our 500V pulser) Vout 95 Vin
3 ms pulse
combined output
input
1 ms pulse

• Expect behavior to be better than this
• currently we have extra unneeded cable length
• matching resistors (100O to scope 50O) add extra
  inductance

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Meander Technology

• began studies w/ 19cm prototype from CERN
• Double meander of 50O (two 100O traces in )
• Ag /Moly-Mn on alumina
• Layout for 3 Mev (b0.08)
• Only 10-15mm of Ag (prototype)

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Microstrips in General
phase velocity and impedance are determined by
effective dielectric constant

• In our case, delay time and Z0 may be adjusted by
• Adjusting d, W, and also meander pathlength
• Using only one trace (as opposed to two in
  parallel)
• Adding an air gap beneath the dielectric
  (changes ee) can be used to tune b

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FNAL Fabricated Meanders

• We have pursued the following
• Use original double meander design with air gap
  between meander and ground plane (50O w/no gap,
  100O/w gap)
• Using single meander
• Material Rogers TMM10i, Cu clad e 9.8,
  18 long (46 cm)
• Meander is formed by routing out traces

single meander
double meander
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Meander Substrate
Terry Anderson
bakeout begins
nitrogen backfill

• Meander traces are generated by routing out
  traces on Rogers TMM10i, Cu clad
• Cheaper, faster than paste/firing/electrochemical
  deposition
• Test indicates acceptable vacuum properties

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Double Meander, Impedance
101 O with 0.062? spacers
55 O with no spacers
Impedance measurements from 1 500 MHz
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Double Meander, Delay
output delayed
input pulse

• Want b 0.073 to match beam speed
• Measure pulse delay in meander
• Varying distance between meander and ground
  plane shows sensitivity

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Single Meander Impedance and Delay
Delay For 18 ? (46cm) gt b0.073
95 O impedance _at_100 MHz
Ch1 input pulse Ref nominal
configuration Ch2 0.010?shims between meander
and ground plane ?t20.8 ns
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Pulse Behavior along Length

• Meanders are 18 long
• Look at pulse behavior along the length
• (high frequency scope probe)
• Single Meander displays much better behavior

single meander
double meander
input pulse beginning half way end
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Coverage Factor

• The electric field between the chopper plates is
  less than that for a structure in which the
  entire surface is conducting
• This must be accounted for in the chopper design
  when determining the voltage needed for the
  desired kick

conductor
dielectric
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Coverage Factor Measurements
network analyzer port 2
network analyzer port 1
High freqency probe Tip is at top ground plane
xy stage for position dependent measurements
meander
Ground top ground plane at beam height
above meander
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Coverage Factor Normalization

• Normalize to stripline with wide trace
• Use geometry for 50O convenient
• For striplines
  
• Z 120p 2/8(ln2 pw/4h)
• Use w 25mm, 2h 16mm
• R. Collin, Foundations of Microwave Engineering

input pulse
Probe pickup signal (S21), 50 150 MHz
_at_end
_at_ end
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Coverage Factor, Meanders
single meander signal to 500 MHz
Which gives
double meander (100O)around 100 MHz
Relative to normalization measurement After
correcting for impedance difference and reflected
power
single meander (100O) around 100 MHz
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Single Meander Two Types
20mm
40mm
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All Measured Coverage Factors

• Caveats
• Simulations indicate double 50O meander should
  be 80
• We are still refining the normalization
  measurement

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Position Dependence
beam RMS size
100
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Dispersion in New Single Meander
old single meander(low dispersion)
input pulse beginning half way end
new single meander (higher c factor)
input pulse beginning half way end
double meander
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Conclusions/Summary

• We have built prototypes for the necessary
  components for the chopper the pulser, meander
  structures, and combiner
• The prototype pulser from Kentech performed to
  specs and we have ordered the higher voltage
  version
• We have built a combiner suited for combining
  these fast pulses
• We have explored different layouts for the
  chopper plates (meander structures). The higher
  coverage factor single meander is most likely the
  best compromise for tolerable dispersion and
  acceptable coverage factor