RFQS IN GENERAL · Web view1998. 11. 19. · J. Bosser, P. Bourquin, M. Brouet, B. Couturier, G....
Transcript of RFQS IN GENERAL · Web view1998. 11. 19. · J. Bosser, P. Bourquin, M. Brouet, B. Couturier, G....
J. Bosser, P. Bourquin, M. Brouet, B. Couturier, G. Gelato, M. Giovannozzi,F. Grandclaude, J.-Y. Hémery, A.M. Lombardi, U. Mikkelsen, S. Maury, D. Möhl,
F. Pedersen, W. Pirkl, U. Raich, H.H. Umstätter and M. Vretenar.
What is a Radio Frequency Quadrupole?
Decelerator system design
Beam characteristics as seen from the users
Construction status
Conclusions/questions
The Radio Frequency Quadrupole is a linear accelerator which • focuses • bunches • accelerates a continuos beam of charged particles.
The focusing, bunching and acceleration are all performed by the electrical RF field!
RFQ HIGHLIGHTS
alternate-gradient, velocity-independent focusing
high bunching efficiency (>90% capture for accelerations)
easy-to-operate machine
cavity loaded with 4 electrodes electric field between the electrode’s tips(TE21 mode)
alternating gradient focussing structure with period length (in half RF period the particles have travelled a length /2)
longitudinal radius of curvature
beam axis
aperture modulation X aperture
)2
1(2
designing an rfq is defining aperture, modulation and phase all along the structure. Vane profile defines the beam dynamics
every rfq is a “special case” because it delivers beam that is customized to the end-user need
For accelerating rfqs there are “design” recipes (high/medium/low space charge) that have been experimentally tested
Decelerator RFQ is NOT an accelerator the other way around
longitudinal dynamics can not be reversed
longitudinal critical point is located at the end of the machine
physical emittance increases during deceleration
MAXIMUM ENERGY EXCURSION OF A PARTICLE MOVING ALONG THE SEPARATRIX
RFQ DECELERATOR DESIGN – Step one
MAXIMIZE BUCKET AREA AT LOW ENERGY END
Electrode voltage
Accelerating efficiency
Synchronous energy
phase around -160
Maximize voltage (sparking limit)
Maximize Accelerating efficiency (that’s when we run into problem)
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
0.10.25
0.4
0.55
0.7
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
A
modulation
aperture(cm)
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
2.7
2.9
0.10.25
0.4
0.55
0.7
0
20
40
60
80
100
120
longitudinal radius of curvature
modulation
aperture (cm)
Accelerating efficiency and longitudinal radius of curvature vs. aperture and modulation for = 3 cm (200MHz and200 keV or 100 MHz and 50 keV)
RFQ DECELERATOR DESIGN – General criteria
Highest possible modulation (compact structure)
Aperture as to keep focussing constant (to avoid transverse normalised emittance growth and to keep beam dimensions constant)
Minimise the surface electric field (sparking)
RFQ DECELERATOR DESIGN – Step two
From the high energy end
First decelerating cell :
modulation to a high value (2-3)
aperture to give the chosen focussing
phase around –160
Following cells :
Modulation of the preceding cell or as close as machining limit allows
Aperture as to keep focussing constant (tentatively)
Phase constant
decelerator rough design is generated
Optimise locally
phase advance per focussing period
max field on the vanetip
smooth abrupt changes in aperture and modulation
(this might require inserting transition cells in critical areas)
Longitudinal matching , i.e. find the “decelerating acceptance”
The separatrix at the last cell is traced backwards to the input of the RFQ. The points of the boundary are rotated counter-clockwise around the synchronous phase and synchronous energy with a cell-by-cell angular velocity given by the longitudinal phase advance.
longitudinal matching
high energy end low energy end
Need a front-end longitudinal matching section
“adiabatic” buncher system
discrete buncher system
Decelerating acceptance
BARE-BONES DECELERATING SYSTEM NEEDS RF CAVITY + RFQ
RFQ
Efficiency of deceleration Beam quality
Prepare the beam for deceleration
Decelerator
Drift
Gas target experiment : energy variability
smooth waist at the experiment, (beam collimated for the longest possible distance)
Trap experiment :mono-energetic beam : 605 keV
beam concentrated in a 1 mm radius inside the trap
Decelerate a 5.3 MeV beam to a variable energy in the range10-100 keV for gas target and trap experiments
ENERGY VARIABILITY :
Extra device (RF or electrostatic)
GOOD TRAP EFFICIENCY
minimize the output energy spreadaccurate study of the last rfq cellstransport to the trap simulation of the trap magnetic field
5.3 MeV 70 keV 30-130keV
5.3 MeV 40 keV 40-80keV
5.3 MeV 400 keV 70 keV 10-140keV,300-500keV
5.3 MeV 120 keV 80-130keV
5.3 MeV 5.3 MeV 0-100 keV
5.00 m
1
2
3
4
5
filled box = 100 MHz
RFQ
RFQ
RFQ
RFQ
RFQ
B
B
B
B
BB
B
B
B
B= rf cavity
rfq
rfq
the chosen solution
DECELERATING SET-UP:
1. LONGITUDINAL MATCHER : a coaxial TEM resonator loaded with double gap with an effective voltage of 47 keV
2. DRIFT : 615 cm , contains magnetic elements, monitors, steerers, energy corrector cavity
3. RFQ : a four-rod structure, 3.44 m long, decelerates to 60keV
4. ENERGY VARIABILITY DEVICE : the structure holding the electrodes can be raised to a potential (± 60 keV)
rfq sketch with cells, ladder and stuff
RFQD electrodes defining parameters
0
0.5
1
1.5
2
2.5
3
0 50 100 150 200 250 300 350 400
z (m)
a(cm
), m
0
1
2
3
4
5
6
W(M
eV)energy
modulation
aperture
transition cells
PRE-BUNCHER/CORRECTOR CAVITIESFrequency: 202.56 MHzEffective voltage: 50 kVAperture radius: 15 mm Length: 300 mm
RFQ Frequency: 202.56 MHzVane voltage: 167 kVMaximum electric field: 28 MV/m (1.7 Kilpatrick)Vane length: 340 cmNumber of beam dynamics cells: 75Power losses: 700 kW (rough estimate)Average radius of aperture: 0.79 cmMinimum radius of aperture: 0.4 cmVane modulation factor (max.): 2.9
BEAM RELATED PARAMETERS Transmission : 100 %Transmission in 60 5keV : 50 %Transverse design emittance: 10 mm mrad Transverse acceptance : 15 mm mrad Design energy spread: 0.5 10-3
Energy spread acceptance : 0.9 10-3
Transverse normalised-emittance growth: ~0
Computer simulation from the HO (DE1) point to the experiment
Areas to study : Transfer line to and from the RFQRFQTrap solenoid
TOOLS for the simulations
MAFIA/SFH 3d/2d : electromagnetic field PARMILA/PARMULT/PATH : multi-particles tracking
End-to-end tracking till the trap including the “real” solenoid field
Input beam
Beam at the rfq input
Beam at the rfq output
Beam at the gas target experiment (formwar window)
Beam at the trap
From HO (DE1) to the RFQ
in the RFQ
RFQ-IN5.3 MeV
RFQ-OUT60 keV
LEBT Set-up
SC solenoidRFQ2 type sol
Half length of SC solenoid: 1000 mm
Half length of magnet windings: 700 mm
227.5 mm
170 mm52.5 mm
RFQ inside wall (z=0)
Gas target experiment
60 keV
X (cm) , Xp (rad)
Y (cm) , Yp (rad)
X (cm) , Y(cm)
Gas target experiment
60 keV
energy Solenoid field Emittance Beam radius divergencekeV Gauss mm mrad mm mrad100 8300 77 1.6 4860 6500 100 2.2 4520 3750 170 3.2 6015 3250 200 3.2 6310 2660 250 3.5 70
RFQ outputExperiment
Trap
Combined field (dotted line) Nornal conductiong solenoid standalone (solid line)
Trap
Beam envelope in SC solenoid (foil at z=87 cm)(emittance 10 pi); W=63 keV then 10 keV
z=0 corresponds to RFQ inside wall
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
z(m)
r(cm
) Before foilAfter foil
Trap
34% in 1 mm
Matching-to-the-rfq design concerns
1.AD jitter : effective overall emittance
2.Energy variation from the AD
3.Steering
4.Diagnostic and monitoring during operation
Overall emittance error distribution
RFQ acceptance
Profile (mm)of the rfq acceptance from 2 meters upstream the rfq to the rfq input plane
.
Z(from RFQ in cm) alpha Beta (m.) Radius (mm)
-200 9.81 21.90 18.-175 8.70 17.27 16-150 7.59 13.20 14-125 6.48 9.68 12-100 5.37 6.72 10-75 4.26 4.31 8-50 3.15 2.46 6-25 2.04 1.16 4.20 0.92 0.44 2.5
2 m before the RFQ
RFQ input
RFQ-dynamics design concerns
1.Alignment ladder-electrodes-beam axis
2.RF stability
3.Mechanical stability
Transport- to-the-experiment design concerns
1.Beam longitudinal length
2.Beam steering
3.Beam diagnostics during running in and during operation
Beam length
Macropulse length is determined by the AD bunching system
Decelerator system energy spread acceptance : 0.1%, i.e. 10 keV
Trap acceptance : 400 nsec for 605keV
Transmission vs.AD bunching voltage
Bunching voltage [V]
t (ns) T (keV) Transmission (DE1 to trap)
20 219 0.5 30%100 146 1.5 40%500 97.5 2.3 43%
1000 82 2.7 42%2500 62.3 3.4 39%
Sensitivity to steering
Steering is an issue:
The overall length of the system (hand-over point to the experiment) is some 10 meters
The transverse acceptance of the trap is 1 mm
Steering can be corrected.
Steering becomes a problem only when it degrades transmission and/or beam quality
RFQ dynamics is not very sensitive to steering (up to 2 mm and 5-8 mrad do not change the RFQ performance)
The overall efficiency of the system is sensitive to steeringSteerers in the LEBT
Steering –an example
remanent error 0.5 mm and 1 mrad (upstream monitors sensitivity)
beam centre position through the system (for 60 keV case)
Dx (mm) Dxp (mrad) Dy (mm) Dyp (mrad)RFQ INPUT 0.5 1 0.5 1RFQ OUT 0 -20 0 -28NC SOL IN -1.4 -20 -2.5 -28NC SOL OUT -5 5 0.6 0.9SC SOL IN -3.7 5 0.7 0.9
Conclusion : We need steerers ( and monitors) in the LEBT, can’t do everything with upstream ones
Steering strategy
Setting up phase
Operation
In-house test with an equivalent electron beam (scaling energy like mp/me=1833)
1. Test the scaling laws : use RFQ2 (90keV to 750 keV protons) for which we have extensive data of the test stand
2. Test deceleration process and our codes : use RFQ2 in reverse mode (750keV to 90 keV) and compare with particle tracking code
3. Test and validate the design of the RFQD with electrons
Test with antiprotons from the AD in a dedicated test line from sept 99
Aarhus proton beam line
1.RFQ : model, real
2.BUNCHERS
3.DIAGNOSTICS
4.FOCUSSING ELEMENTS (QUADS UPSTREAM, SOLENOID DOWNSTREAM)
5.ELECTRON TEST STAND
RFQ model
scale 1/3
bead-pull measurements : field distribution
effect of the ladder at a potential : effect on the rfq mode, flaps, ladder enclosed (increase the capacitance ladder to the ground), resistance to terminate ladder, in real RFQ absorber . Few % extra power needed.
the RF is mastered
RF contacts
between electrode and stem, between 2 parts (ladder and electrodes will be machined ½ at the time)
standard berillium-copper contacts loose elasticity at 150-200 deg (bakeout temperature)
solution : “spiral contact” stainless steel gold/silver plated
test set-up for the spiral contacts : lambda/4 resonator, mesure Q and degradation due to contacts. Tested at low power (OK), now the all system will be put under vacuum and tested at high power
RFQ construction
Cylinder, ladder, electrodes are specified.
Cylinder and electrodes machined at CERN
Ladder farmed out (Cinell, Italy)
Diagnostics
Understaffed
Upstream diagnostics : proposal will be tested in the AD beam lines
Downstream diagnostics
Electron test stand
electron gun borrowed from the LHC vacuum group
2.88 keV electron beam (equivalent to 5.3 MeV) is feasible :
Lower energies (49 eV and 408 eV) difficult : 1.Beam dynamics at low energy requires high vacuum2.We need high current at low energy 3.the RFQ2 is made of magnetic material and can’t be properly
screened
Et cetera
Bunchers : scale from LINAC3 model, farmed out to Cinell
Magnetic elements and their power supplies are in house
RF equipment : (power group) buy from OCE (check with WP)
RFQ model shows there are no difficulties in mastering the ladder at a potential
RFQ ready and tested by the end of 1999