a fast chopping system for high intensity linac beams f. caspers, t. kroyer cern-ab-rf e. mahner...

A Fast Chopping System for High A Fast Chopping System for High Intensity Linac BeamsIntensity Linac Beams

F. Caspers, T. Kroyer CERN-AB-RFE. Mahner CERN-AT-VAC

CARE’06 Frascati, November 15 – 17, 2006

CARE06 Frascati, November 15-17, 2006

A Fast Chopping System for High Intensity Linac Beams

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Introduction CERN chopping scheme Layout Technical requirements

Evolution of the SPL chopper Modifications in 2005 Status by September 2006

Measurements & tests Electrical properties Vacuum & leak test Heat transfer test

Coverage factor Measurement Simulation

Remaining jobs

ContentsContents



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Superconducting Proton Linac (SPL) Fast beam chopping at Ekin = 3 MeV ; thus is about 8%

Fast chopper required to establish desired beam pattern

SPL LayoutSPL Layout

RFQ: Radio frequency quadrupole DTL: Drift tube linac CCDTL: Coupled cavity DTL SCL: Side coupled linac = 0.65, = 1.0: superconducting cavities



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8*2.84 ns ( 44 MHz)

Most demanding scheme for SPL operation: cutting out three bunches out of eight repetition rate 44 MHz bunch spacing 2.84 ns ; 10 to 90% rise and fall time required < 2 ns

CERN Chopping SchemeCERN Chopping Scheme



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Chopper off

Chopper line lattice designed such as to magnify the kick from the chopper; this reduces required kick field

The chopper plates have to be installed in the quads; this saves length and reduces space-charge related emittance growth

Chopper Line (1)Chopper Line (1)

Chopper on

Kick-magnifying quad



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Chopper Line (2)Chopper Line (2)

chopper beam dump

bunching cavities



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The chopper plates are no longer DC-wise floating (no more triaxial mode of operation)

Now we have a coaxial instead of a triaxial chopper structure

The triaxial version was meant for simultaneous dual mode of operation, i.e. 0 to 10 MHz: electrostatic deflector, above 10 MHz travelling wave mode

Removal of isolating units both in water cooling circuits and coaxial driving lines

Modifications in 2005Modifications in 2005



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Initially samples of the meander lines were produced at CERN While all parameters were basically ok (electrical, vacuum),

reproducibility of certain electrical parameters (electrical length, match) was not always perfectly satisfactory

After the accomplished proof of principle with CERN technology, a supplier that can well control all the process parameters was needed; a possible change in technology is not a problem as long as the key properties are preserved

Kyocera was eager to enter in a cooperation with CERN and willing to adapt their technology to our needs

The plate that was recently furnished compared well in all aspects with the best CERN samples and we hope that the promised good reproducibility will show up in reality

Meander linesMeander lines



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Technological differences between the CERN and Kyocera meander structures:

Meander line: TechnologyMeander line: Technology

CERN Kyocera

CERN orders from Wesgo (D) ceramic plates with fixation holes and 10 to 15 m thick homogeneous MoMn layer (fired at 1400 C in hydrogen atmosphere

Kyocera produces the alumina plates in-house. Then a meander pattern is created in thick film silver paste of about 10 to 15 m thickness. This thick film silver paste has after firing a much higher resistivity than bulk silver (factor 5 to 10)

The meander structure is etched into the MoMn layer. Afterwards a 1 to 2 m thin-film layer of Ag is attached by sputtering.

Onto the Ag thick-film layer 30 to 40 m copper are deposited electrochemically

In a final step this silver-coated MoMn layer gets another 30 m silver by electrochemical deposition

Finally 1 to 2 m of Au are applied for good contacts and protection against oxidation



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Metallize printing (Ag thick-film)

Firing

Cu Plating

Resistance Testing

Final inspection

Packing

Machining

Ceramic incominginspection

Au Plating

CERAMIC PLATE Process Flow CERAMIC PLATE Process Flow

RF Property Testing

Machining

Electrode for electrochemical deposition removed by grinding

Grinding Area

Courtesy: Kyocera



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First meander line plate from Kyocera were received in June 2006 but it turned out that the attenuation was too high

In the second iteration the technological parameters were properly adjusted and the last sample was very satisfactory

After extensive electrical tests this single plate was installed in the chopper tank

Vacuum, leak and heat tests performed successfully

Status by September 2006Status by September 2006



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Electric Measurements: Electric Measurements: Transmission AttenuationTransmission Attenuation

Measurements performed on single chopper plate with an image plane 10 mm above the line’s surface to simulate the presence of a second plate

Frequency domain transmission A DC resistance of 1.1 was

measured, which agrees very well with the low-frequency limit of the measured attenuation

3 dB bandwidth 940 MHz. If there was no phase distortion the rise time would be

All rise times quoted are 10 to 90% values ns 355.0

3

1

BWtt

Attenuation over one chopper plate

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idth



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Transmission Step ResponseTransmission Step Response Response for 0 to 700 MHz low pass

step function (Kaiser Bessel weighting function with = 6)

Comparison between a measurement with and without the image plane. Due to the high electric field energy in the alumina the kick field does not change much when the symmetry is broken

Measured rise time trm = 1.771 ns, to be compared with tri = 1.407 ns of input pulse; structure rise time

This is a conservative estimate of tr since the tti is rather short and we get into the highly dispersive region of the response

ns 08.122 rirmr ttt



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Phase DistortionPhase Distortion

From the measured phase without image plate the electrical delay of 16.73 ns (linear term) was removed

With image plane (realistic configuration) the electrical length was 16.83 ns, within 0.1 ns of the required value

The remaining phase is not flat as for a dispersion-free line; thus we have phase distortion

The phase distortion is due to coupling between adjacent lines in the meander structure. This coupling increases quickly with frequency like in a microstrip directional coupler

In an ordinary first-order low-pass the 45 degree points coincide with the 3 dB points. Here they are at 375 MHz, i.e. much lower than the 3 dB points

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Phase without image plane



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ReflectionReflection

Very good impedance match of meander line to 50 : reflection in frequency domain of the order of -30 dB below 500 MHz

These data were measured on a test jig consisting of a single plate with SMA connectors fixed on either side

Gen

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Reflection Step ResponseReflection Step Response

Response for 0 to 700 MHz low pass step function

S11 very small, of the order of 0.02 which is another indication of good match

The line impedance is not perfectly constant over the meander length as can be seen from the bump at t = 10 ns.

Towards the end of the line an apparent increase in line impedance can be seen. This is an artifact caused by the lossy line; could be corrected numerically

Twice the line length of ¼17 ns



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Tuning of Electrical LengthTuning of Electrical Length

It was tried to adjust the electrical delay of a chopper plate by modifying the metal ground plane

Cutting a longitudinal groove into the ground plane reduces the effective and thus increases the group velocity on the line. Since the variation in line impedance is over distances much shorter than the wavelength, the other electrical properties should not be affected much

For two 5 mm wide and 3 mm deep grooves a 5% decrease in the electrical length was found on a CERN plate

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Vacuum & Leak TestingVacuum & Leak Testing Initially the obtained vacuum pressure was about

1.5e-7 mbar Leak rate smaller than 2e-10 mbar.l/s The ceramic plates were not baked before

installation and the entire tank cannot be baked due to the presence of the integrated quadrupole magnet

However, there is a rather larger thermal resistance between the ceramic plate and its aluminium support structure and the rest of the tank

Therefore an “in-situ bake-out” at about 150 degrees C was possible by passing current through the meander line with the water-cooling off

Now a vacuum pressure of about 3.5e-8 mbar is reached, which is within specs for the chopper line

water cooling



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Heat Transfer Test (1)Heat Transfer Test (1)

In order to settle question about the heat contact between the ceramic plates and the water-cooled aluminium support structure a heat transfer test was carried out

The temperature of the ceramic plate was monitored in two ways, by

Observation with an infrared camera through a window at one end of the tank

measuring the resistance of the meander line, knowing that the resistivities of the Cu, Ag and Au all have about the same temperature coefficient of 4e-3 K-1

In two runs the heating with and without water cooling was measured

The vacuum pressure was monitored, as well

water-cooling off

water-cooling

on



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Heat Transfer Test (2)Heat Transfer Test (2) The device under test…



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Heat Transfer Test (3)Heat Transfer Test (3)

The pictures on the right show a heating cycle without water cooling (Note the change in scale for the last picture!)

Without cooling the steady state conditions were reached at a temperature difference T = 128 K. The heating power was P = 28.4 W returning a thermal resistance Rth = 4.5 K/W.

Radiative heat losses were neglected here. This can be justified by the fact that inner surface of the chopper tank has very high reflectivity, thus reducing considerably the effective power flux. The measured temperature curves follow very closely the exponential curves predicted for heat conduction, indicating that this is the main process involved.

With the cooling of the aluminium support plate switched on, the steady state T decreases to about 18.2 K for P = 20.4 W giving a Rth = 0.89 K/W

Thus more than 100 W of heat power can be dissipated for an operation temperature 100 K above the cooling water temperature

t = 0 min, T = 0 K

t = 35 min, T = 20 K

t = 500 min, T = 120 K



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The chopper must be able to stand the 500 V pulses on the plates High voltage testing showed that a single chopper plate can stand

at least 2 kV to ground which is largely sufficient for the present requirements

High Voltage TestingHigh Voltage Testing



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The field coverage factor is defined as the ratio of the deflecting electric field on the beam path to that for the case when a continuous metal plate replaces the meander pattern

Time domain simulations presented at EPAC02 predict a coverage factor of 0.8 in the center of the structure [1]

In the electrostatic approximation it follows from Gauss’s law that the vertical electric field integrated over any horizontal plane above the chopper plate is constant. However, in dependence of the transverse position the field strength may change.

Simulations by M. A. Clarke-Gayther predict a coverage factor of 0.78 in the center of the chopper aperture; 10 mm to the side the coverage factor decreases to about 0.73

Since the beam dump is round, the maximum kick field is needed in the center of the structure

Coverage factorCoverage factor

[1] Caspers, F; Mostacci, A; Kurennoy, S; Fast Chopper Structure for the CERN Superconducting Proton Linac, EPAC02, Paris, 2002

Time domain simulation (MAFIA)



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In terms of field homogeneity the CERN structure compares favorably to the SNS meander, since the meander period to aperture ratio is considerably smaller

Field homogeneity (1)Field homogeneity (1)

+25mm

-25mm

+66mm

-66mm

SNS structure image: S.S. Kurennoy and J.F. Power, Development of a fast traveling-wave beam chopperfor the SNS project, LINAC’98, Chicago, 1998

CERN meander lineSNS meander line



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An electrostatic simulation with Ansoft Maxwell was run to determine the field homogeneity of the structure in the beam aperture

Very close to the conductor surfaces the fields peak, while the smoothen out as one approaches the beam axis

Field homogeneity (2)Field homogeneity (2)

Beam goes into plane of the “paper”



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Coverage Factor Measurements (1)Coverage Factor Measurements (1) Alumina plate with meander structure compared with full metal plate Metal plate made twice as wide to assure that its fringe field can be neglected Second metal plate used at the position of the beam (10 mm above ceramic

surface) as image plane In the center of the structure field measured using probe with 10 mm

diameter using a button pick-up-like set-up

Port 2

Port 1Port 3



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Coverage Factor Measurement (2)Coverage Factor Measurement (2)

Button as probe at Port 3

image plane

reference line

meander line



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Coverage Factor Measurement (3)Coverage Factor Measurement (3)

We have a 3-port, with ports 1 and port 2 being the input and output of the slow-wave structure and the probe at port 3 Full two-port calibration used, since for the reference measurement we have a 4 line thus strong mismatch Measured quantities

S21: Transmission along the plate with port 3 matched Reflection on ports 1 and 2 S31: Transmission from the plate to the probe with port 2 matched

These quantities were measured for the slow-wave structure (subscript DUT) and for the full metal plate as reference (index R) From these quantities the coverage factor CF in dB can be calculated from CF = S 31DUT - S21DUT/2 - (S31R - S21R); the S parameters have to be plugged in in dB This formula was derived under the assumptions

negligible losses for the reference measurement but heavy mismatch mismatch of the slow-wave structure small but certain losses



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Coverage Factor Measurement (4)Coverage Factor Measurement (4) CF = 0.78 at low frequencies Ripple with ¼25 MHz due to small mismatch of slow-wave structure For high frequencies (>50 MHz) standing waves strongly impact reference measurement Mechanical uncertainty of half beam gap (chopper plate to image plane) estimated as ¼0.1 mm;

effect on results determined by introducing a 0.2 mm offset ( cyan traces) At about 1 MHz comparison with modified technique: port 2 left open in reference measurement. At

low frequency S21R is then expected to double. The corrected result is plotted at one frequency (black star)



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Remaining JobsRemaining Jobs

Final assembly of both chopper tanks including outgassing, leakage and electrical tests

Power test with new pulser Test with beam

Static: A DC voltage applied at the plates, 50 termination removed Dynamic test with pulser

Whatever else you can suggest or dream up...



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AcknowledgementsAcknowledgements

We would like to thank the AB-RF workshops for assembling the tank, F. Wurster and M. Nagata from Kyocera for fruitful cooperation in development and implementation of the technologies for printing the meander structure and J. Borburgh for assistance with the heat transfer measurements

Thanks to R. Garoby and T. Linnecar for support

a fast chopping system for high intensity linac beams f. caspers, t. kroyer cern-ab-rf e. mahner...

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