Transcript Accelerator Physics Challenges Karlheinz Schindl Space charge Q-Diagram and Stopbands Unstable Beams Linac 2 and its impact Fast cycles Beams for p-bar production Ions: from light to heavy ISOLDE.

Accelerator Physics Challenges
Karlheinz Schindl
Space charge
Q-Diagram and Stopbands
Unstable Beams
Linac 2 and its impact
Fast cycles
Beams for p-bar production
Ions: from light to heavy
ISOLDE to PSB
PS(B) for LHC
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Space Charge
Q-shift
1 N
Q   2 
βγ ε *
 v/c
smaller at
higher energy
beam
brilliance
 External focusing (quadrupoles) tuned to QH,QV (betatron oscillations/turn)
 QH,QV are decreased by Q-shifts ∆QH, ∆QV for higher intensity
unbunched
constant line
density
bunched
variable line
density
∆Q spread
larger
 ∆Q-shift
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Q – Diagram and Stopbands
5.7
Stopband: m.QH+n.Qv = p (here 2Qv=11)
m+n order of stopband (here 2)
In PSB strong beam losses for order≤3
3Qv=17
QV
Q
h+
Q
v=
10
Injection
5.6
-7
v=
-2Q
Qh
2Qv=11
5.5
2Qv=11
3Qh=13
Qh
+2
Qv
=1
5
5.4
v=
Q
hQ
Necktie-shaped areas: ∆Q-spreads
which shrink during acceleration
-1
 many stopbands covered at injection
 much smaller after ~100 ms
 no stopbands at ejection energy
3
3Qv=16
2Q
h-Q
v=
-
5.3
Ejection
5.2
4
=1
Qv
h+
2Q
5.1
Stopbands are compensated by multipole
lenses (quadrupoles, sextupoles...)
6
v=-2Q
Qh
5.0
4.1
4.2
4.3
4.4
4.0
1013 protons in one PSB ring
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4.5
QH
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Dynamic Working Point
Move (Qh,Qv) during acceleration
to area clear of stopbands
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Unstable Beams
without
with
Magnani shaking: modulation of RF
phase by ~4 kHz after RF trapping
Decisive to reach nominal 1013 p/pulse
3 PSB buches at 800 MeV (1973)
Analytical study of longitudinal and
transverse (“head-tail“) instabilities
observed at the PSB by F.Sacherer
Spin-offs:
 "Standard model" of coherent
instabilities in accelerators
 PSB longitudinal feedback
system based on this theory
Bunch oscillation modes at PSB analysed by F.Sacherer
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Transverse feedback system added
in 1980
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Linac 2 and its Impact
 Construction started 1973
 operational 1977
 equipped with a (then novel) RFQ
 50 MeV as Linac1, but stable 150 mA
BUT
immediate improvement small because
PSB could only digest 120 mA at that time
BUT
Drift Tube
thanks to the huge improvement potential of
the PSB the 150 mA beam is fully used now
Acceleration
gap
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Fast Cycles
4-months shutdown (1980)
 conversion of computer control system
IBM 1800 (48 kwords.....) to NORD and CAMAC
 Faster magnet cycles (rise time ~600 to ~300 ms)
 faster cycle repetition rate to allow more flexible supercycles
 save energy!
Idea by F. Sacherer
Implemented by R.Gailloud et al.
Fast cycle
slow cycle
slow (normal) cycle (600ms)
dB/dt small and constant
 increasing bucket area
fast cycle (300ms)
dB/dt increases
 constant bucket area
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P-bar Production Beam: Vertical Addition
Installation (1980) of Vertical Addition “(10-bunch mode)" in PSB - PS line
20 – bunch mode




10 – bunch mode
thin double
septum magnet
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generates 10 bunches
1.7 x line density but
3 x larger vertical emittance
beam losses at PS injection
Improved after increasing
PSB energy to 1 GeV (1987)
 field levels +16%
 ∆Q in PS –25%
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Pbar Production Beam: Funnelling with RF Deflector
level 3
Sine-wave RF deflector (8 MHz)
bends bunches from level 2 downwards
from level 3 upwards
level 2
R. Corsini
Invented by
G.Nassibian
(1987)
RFdeflector
∆-signal
Σ-signal
zoom on
one bunch
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Spin-off: Inspired CLIC people to
generate the CLIC drive beam via
bunch frequency multiplication.
(works better for short bunches)
RF deflector GHz rather than MHz
10 PSB bunches fill ¼ of PS
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Bunch Flattening with 2nd Harmonic RF Cavity
 ∆Q proportional to bunch peak line density (h=5 cavities with voltage V)
 Adding h=10, or second harmonic, with peak voltage ~ V/2 cancels the RF
focusing in the bunch centre and thus generates flat bunches
 with the same peak line density, 25 – 30% more particles captured
PSB bunch after RF capture
(G.Nassibian, test 1991)
h = 5 only (12 kV)
bunch peaked
 50 ns/div
h=5 (12 kV) + h=10 (6 KV)
Bunch flat-topped
More protons with same ∆Q
The h=10 cavities became operational by 1983  spectacular success
 3 1013 p/pulse in the PSB, with now 150 mA from Linac 2
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Ions – from light to heavy
Proposal (1981) to triple PS light ion intensity (d+, α++ , O8+ ) by a detour via the PSB
Challenge
solution
RF frequency swing (3-8 MHz) too small
debunch–rebunch on intermediate flat top
low intensity (O, S, Pb): 108 – 109 charges/ring
higher sensitivity electronics and diagnostics
Pb53+ (partially stripped) ions interact with rest
gas molecules
vacuum improved to 5 10-9 Torr
even faster acceleration, injection at dB/dt»0
DC current transformer
Debunchingrebunching
RF Voltage VRF
 Deuterons and alphas in 1983
magnet cycle B
intermed. flat top
 Alphas collided in the ISR!
 O8+ and S16+ produced 1986/7
RF frequency fRF
Acceleration of lead ions 1995
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 Pb ions (3 108 to PS) in 1995
(with Ion Linac3 operational)
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ISOLDE to PSB
Proposed in 1989 (R.Billinge et al.)
 PSB has reached 3 1013 p/pulse at 1 GeV
 The SC (Synchro-Cyclotron) at the end of lifetime (33 ys)
 ~ half of PSB cycles available
SC
PSB
Energy (GeV)
0.6
1
p/pulse
1011
3 1013
rep. rate (Hz)
200
~0.4
Current (μA)
2.8
2.1
pulse length (μs)
40
2.4
From PSB
100m line
GPS
 Higher isotope production rate
 more stress on the targets
HRS
Project (1989-1992) led by D.Simon
 New ISOLDE facility built near PSB
 transfer line (100 m) to ISOLDE
 R&D on targets for short beam pulses
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Operational 1992
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PSB for LHC: Challenges
Figure of merit: Luminosity (L) in the LHC
kb # bunches per LHC ring
ε* normalised r.m.s. emittance (≤3.75 μm)
Nb protons per LHC bunch (~1011)
L ~ kb Nb2 / ε*
Challenge:
LHC beam intensity (kbNb) easy to produce but
 unprecedented beam brightness N/ε* in both PSB (∆Q almost 1) and PS
Solutions:
 For PSB: filling the PS with two PSB pulses reducing ∆Q in the PSB to < 0.5
 For PS: reduction of ∆Q by increasing the PSB energy from 1 to 1.4 GeV
PSB magnet
field +26%
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PS Injection
energy (GeV)
B (PSB)
(Gauss)
(βγ2)/(βγ2)0.8 GeV
0.8
5924
1
1
6870
1.29
1.4
8670
1.97
2
11280
3.22
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PS ∆Q down by 36%
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PS for LHC: Two-batch Filling
Challenge:
Two-batch filling of PS with several
bunches per PSB ring not feasible
Solution:
 Accelerate one bunch per PSB ring
 Adjust kicker timing to fill ½ of PS ring
New RF systems for the PSB
 RF cavities for h=1 (0.6–1.8 MHz, 8 kV) (CO2) (new)
 RF cavities for h=2 (1.2-3.9 MHz, 8 kV) (CO4) (recycled)
Beam test PSB – PS (1993)
 use just PSB ring 3
 provisional hardware using spare parts or prototypes
 LHC beam with nominal transverse properties produced
 Project PS for LHC launched (1994-2000)
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PSB for LHC: PSB to 1.4 GeV
Magnet field
+ 26%
Magnet field
+ 26%
Canadian contribution in red
(~1/4 of total project cost)
TRIUMF/Vancouver
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PSB for LHC: Beam Tests 1998 -2000
Tests in 1998 revealed problems in the PSB
 beam loss due to impedance of vacuum flanges
(Michel‘s talk)
 Saturation effects in gaps 1 and 4 of the bending
magnets (field ~1% lower)
 New trim power supply for rings 1 and 4 in series
(proved compatible with cabling of the main coil!)
gap 4
gap 1
M.Benedikt
Project commissioning in 2000
The PSB and PS produced LHC bunches at 25 GeV with
 nominal intensity 1011 p/bunch
 transverse emittances (both planes) ε* ~ 2μm (3 μm specified)
BUT
 excessive bunch length due to debunching-rebunching in the PS
....... that is another story
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PSB25 (1997)
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