DESY, May 14th, 2004
The Electron-Proton
Collider HERA
Joachim Keil
DESY, MPY
Contents
May 14th, 2004
❍
Introduction
❍
Layout of HERA
❍
Luminosity
❍
HERA luminosity upgrade
❍
New interaction zone
❍
Synchrotron Radiation
❍
Polarization
❍
Summary
J. Keil: The Electron-Proton Collider HERA
p. 2
Overview of HERA
Hadron-Elektron-Ring-Anlage (HERA):
❍
Two independent rings
circumference 6336 m
❍
Energy electrons: 27.5 GeV
❍
Energy protons: 920 GeV
❍
Collisions of electrons with
protons (H1 and ZEUS)
❍
Internal gas target (HERMES)
❍
Internal wire target (HERA-B)
(Data collection ended 2003)
❍
Longitudinal polarization of e±
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 3
Main Features and Parameters of HERA
Proton
Electron
Circumference
6335.826 m
6335.822 m
Top beam energy
920 GeV
27.5 GeV
Injection energy
40 GeV
12.0 GeV
Operating beam currents
100 mA (180 b.)
50 mA (189 b.)
Dipole field
5 T (s.c.)
0.15 T (n.c.)
RF systems
2 × 52 MHz, 4 × 208 MHz
8 × 500 MHz 86 n.c. 7 cell, 16 s.c. 4 cell
RF voltage
≈ 1 MV
130 MV
spin polarization
—
54 %, 3 rotator pairs
IRs
North, South (c.b.)
North, South (c.b.), East (gas target)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 4
Protons
H− -source
18 keV
e− -source
150 keV
RFQ
750 keV
LINAC II
450 MeV
LINAC III
50 MeV
PIA
450 MeV
DESY III
7.5 GeV
DESY II
PETRA II
40 GeV
PETRA II
HERA-p
May 14th, 2004
Electrons
920 GeV
HERA-e
7 GeV
12 GeV
27.5 GeV
J. Keil: The Electron-Proton Collider HERA
p. 5
HERA Milestones
1981
Proposal for an e/p collider
1984
Approval and start of construction
1987
Tunnel ready
1988/1989
e-ring put into operation
1988-1990
Installation of p-ring
Jan.-Apr. 1991
Oct. 1991
1992
1993/1994
May 14th, 2004
p-ring put into operation
First e/p collisions
H1 and ZEUS put into operation
Installation of spin rotators for HERMES
1994
Switched to e+ /p operation
1995
HERMES put into operation
1996
Installation of HERA-B
1997
NEG-pumps, more RF-stations
1998
Increase of p-energy (820 GeV → 920 GeV)
1998-1999
e− /p operation
1999-2000
e+ /p operation
1999
Reached design luminosity of 1.5 · 1031 cm−2 s−1
2000/2001
Luminosity upgrade; spin rotators H1 and ZEUS
Mar. 2003
50 % longitudinal polarization with 3 rotators
J. Keil: The Electron-Proton Collider HERA
p. 6
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 7
Construction of ZEUS Hall
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 8
Luminosity
Luminosity is scale factor between counting
rate Ṅ and cross section σ:
L=
1
Ie Ip
· ? ?
ef0 Nb σx σy
Note: only valid for σe = σp !
★
Ie and Ip are the total currents of e/p
★
Nb is the number of bunches
√
σ ? = β ? is spot size at IP
→ depends on emittance and
beta-function β ? at IP
★
May 14th, 2004
How to increase the luminosity ?
❍
Increase the beam currents Ie and Ip !
❍
Reduce the beam sizes σ ? of e and p!
J. Keil: The Electron-Proton Collider HERA
p. 9
Luminosity Constraints & Limitations
For HERA it is better to write the luminosity formula in a different way:
L=
N p Ie γ p
p ? ?
4π e n,p βx,p
βy,p
Constraints:
❏
?
?
Beam size equal for e and p at IP: σx,y
p = σx,y e
❏
Beam-Beam tune shift ∆Qy,e < 0.045
Limitations:
❏
Np /n,p : p-beam brightness = 1 · 1011 / 5 mm · mrad (DESY III, BB tune shift for e± )
❏
γp : proton relativistic factor = 981 (max. field in s.c. dipoles, ρ)
❏
Ie : total lepton current = 60 mA (rf-power, BB tune shift for p)
❏
?
βx,y
p : beta functions for protons at the IP = 2.45 m, 0.18 m (IR layout, σ p )
→ Reduce proton IP beta-functions for higher luminosity!
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 10
Luminosity Upgrade: Motivation
❍
❍
❍
HERA collected L = 180 pb−1 from
1993 to 2000
?
Raised number of bunches, smaller σx,y
for protons at the IPs, more efficient
operating
Design luminosity (1.5 · 1031 cm−2 s−1 )
exceeded in 2000 with 2 · 1031 cm−2 s−1
luminosity production curve growth linear in time with 100 pb−1 /year
HERA luminosity 1992 – 2000
Integrated Luminosity (pb-1)
❍
70
2000
70
60
60
50
50
40
40
1999 e+
30
30
1997
20
20
99 e1996
❍
Goal of HERA physics
1000 pb−1
→ luminosity upgrade
May 14th, 2004
program:
10
1995
1998
10
1994
1993
15.03.
50
100
150
200
Days of running
J. Keil: The Electron-Proton Collider HERA
p. 11
Beam Dynamics: Transversal Plane
❍
Linear optics: Only dipoles (bending) and quadrupoles (focusing)
1
e
=
B(s)
ρ : bending radius
ρ(s)
p
e ∂By (s)
k(s) =
k : focussing strength
p ∂x
❍
Reference orbit: closed path passing through centers of all magnetic magnets
❍
Closed orbit: distorted path due to field errors and misalignments
❍
Particles with deviations from reference orbit make transversal oscillations
❍
Hill’s differential equation (z = x or y):
1 ∆p
1
z 00 (s) + ∓k(s) +
z(s)
=
ρ(s)2
ρ(s) p
❍
ρ(s) and k(s) are periodic functions for circulating particles!
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 12
Beam Dynamics: Solution of Hill’s Equation
❍
Solution of Hill’s equation are pseudo-harmonic oscillations:
p
z(s) = a βz (s) cos(µz (s) + µ0 )
Z s
1
µ(s) =
dσ
β(σ)
σ=0
with beta function β(s) and phase advance µ(s)
❍
❍
❍
a and µ0 depend on initial conditions
p
Beam size is σ = β(s) (Gaussian distribution with emittance )
Betatron Tune Q =
µ(C)
2π
is number of betatron oscillations per turn
n
m
❍
Avoid rational tunes Q 6=
❍
Liouville theorem: Density of phase space volume is preserved, if conservative forces acting
→ fulfilled for protons, but not for electrons (no SR!)
May 14th, 2004
with n,m ∈ N → resonance!
J. Keil: The Electron-Proton Collider HERA
p. 13
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 14
Beam Dynamics: Longitudinal Plane
❍
Focussing also needed in longitudinal plane:
RF-resonators: harmonic electric field in cavities
Synchrotron radiation: Constant energy loss in bending magnets
→ Energy/phase oscillations in rf potential relative to stable phase φ s :
φ̈(t) + (sin φ(t) − sin φs ) = 0
❍
❍
Stable phase φs depends on SR (changes with energy!)
√
Sychrotron tune Qs ∝ Vrf depends on RF-voltage
❍
RF system produces the bunched beam
❍
HERA-e: 500 MHz cavities, Vtot = 132 MV at 27.5 GeV
❍
HERA-p: Double RF system
52 MHz-system used for capturing of long bunches during injection
208 MHz-system used for longitudinal compression of bunches
❍
Bunch-to-bunch distance for both beams: 96 ns, 220 rf-buckets possible
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 15
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 16
Resonance Diagram
❍
Resonances Q =
❏
❏
❏
❏
n
m
with n,m ∈ N:
Dipole field errors (m = 1)
→ Integer resonances
Quadrupole field errors (m = 2)
→ half-integer resonances
Sextupole field errors (m = 3)
→ third-integer resonances
etc. . . .
❍
Coupled motion in longitudinal plane:
Qs sidebands; position depends on VRF !
❍
Tune has spread ∆Q due to energy spread
and beam-beam effect (depends on Ip !)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 17
βx (m), βy (m)
HERA-p FODO Cell
90.
FODO cell of HERA-p
βx
MAD-X 1.12 12/05/04 16.37.01
βy
80.
70.
60.
50.
40.
30.
20.
10.
1998.0
2008.4
2018.8
2029.2
2039.6
2050.0
s (m)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 18
βx (m), βy (m)
Optical Functions of HERA-p
2000.
HERA-p, luminosity optics
βx
1800.
MAD-X 1.12 12/05/04 16.36.59
βy
1600.
1400.
1200.
1000.
800.
600.
400.
200.
0.0
0.0
1267.2
2534.4
3801.6
5068.8
6336.0
s (m)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 19
Dx (m), Dy (m)
Optical Functions of HERA-p
3.0
HERA-p, luminosity optics
Dx
MAD-X 1.12 12/05/04 16.37.00
Dy
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
0.0
1267.2
2534.4
3801.6
5068.8
6336.0
s (m)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 20
βx (m), βy (m)
Interaction Region of HERA-p
2000.
HERA-p, IR ZEUS
1800.
βx
MAD-X 1.12 12/05/04 16.37.01
βy
1600.
1400.
1200.
1000.
800.
600.
400.
200.
0.0
1464.
1560.
1656.
s (m)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 21
Mini-Beta Principle
20
❍
Beta-function in IR (=drift space):
s2
β(s) = β + ?
β
?
→ small decrease in β ? leads to big increase
in β-function of mini-beta-quadrupoles!
❍
❍
To avoid big β-functions in IP quadrupoles
→ quadrupoles as near as possible to IP!
m
15
10
β?
5
Otherwise: Aperture limitations, increased
chromatic effects of mini-beta-quadrupoles
?
0
May 14th, 2004
-3
J. Keil: The Electron-Proton Collider HERA
-2
-1
0
sm
1
2
3
p. 22
Concept of Luminosity Upgrade
❍
?
Goal: smaller beam spot σx,y
of e and p at IP
❍
Realization:
❍
❏
Move proton low-beta-quadrupoles as near as possible to IP
❏
Combine focussing and bending in the low-beta-quadrupoles
❏
Fast separation of both beams
Challenges:
❏
Superconducting magnets inside experiment
❏
Synchrotron radiation through experiment (and more than before!)
❏
Magnets in solenoid field can move!
❏
Longitudinal Polarization at H1 and ZEUS
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 23
Luminosity Upgrade: Parameters
HERA I
Parameter
HERA II
e±
p
e±
p
27.5
920
27.5
920
50
100
58
140
189/174
180/174
189/174
180/174
Emittance ε [π·nm·rad]
41
5.1
20
5.1
Emittance ratio εy /εx
0.1
1
0.17
1
Bunch length σs [mm]
11.2
191
10.3
191
Beta function βx? [m]
0.9
7.0
0.63
2.45
Beta function βy? [m]
0.6
0.5
0.26
0.18
Spot size σx? [µm]
192
189
Energy E [GeV]
Beam current I [mA]
Total / colliding bunches nt /nc
Spot size σy? [µm]
112
50
30
BB tune shift for 2 IPs ∆Qx
0.024
0.0026
0.068
0.0031
BB tune shift for 2 IPs ∆Qy
0.060
0.0007
0.103
0.0009
Polarization P
60 %
0%
55 %
0%
Spec. Luminosity Lsp [cm−2 s−1 mA−2 ]
0.67 · 1030
1.79 · 1030
Luminosity L [cm−2 s−1 ]
1.72 · 1031
7.44 · 1031
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 24
New Interaction Region
❍
Stronger focusing of protons: p-magnets nearer to IP (GM)
❍
Beam separation of electrons with super-conducting magnets (GO, GG)
→ Synchrotron radiation through experiment!
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 25
Beam Spot at IP
Design
before Lumi-Upgrade
after Lumi-Upgrade
0.4
0.2
0.2
0.2
70 Μm
ymm
0
-0.2
0
ymm
0.4
ymm
0.4
50 Μm
-0.2
192 Μm
-0.4
112 Μm
-0.4
-0.2
0
0.2
30 Μm
-0.2
270 Μm
-0.4
0
0.4
-0.4
-0.4
-0.2
xmm
0
0.2
0.4
-0.4
-0.2
xmm
❍
Spot size at IP: σx? = 112 µm, σy? = 30 µm
❍
Increase of Luminosity by a factor of factor 4.7 by:
❏
Increase of specific luminosity due to beam size reduction by factor 2.8
❏
Increase of currents by a factor 1.6 to reach design currents
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
0
0.2
0.4
xmm
p. 26
Specific Luminosity
29
Specific luminosity
2
15
2
Ls / mA cm /s
x 10
10
ZEUS
H1
5
0
29
2
15
2
Ls / mA cm /s
x 10
10
29
2
15
2
Ls / mA cm /s
01/04
02/04
03/04
04/04
Date
5
x 10
10
06/04
07/04
08/04
09/04
10/04
11/04
Date
ZEUS
H1
5
0
29
x 10
2
15
2
Ls / mA cm /s
31/03
ZEUS
H1
0
10
13/04
14/04
15/04
16/04
17/04
18/04
Date
ZEUS
H1
5
0
19/04 29
x 10
2
15
2
Ls / mA cm /s
30/03
10
20/04
21/04
22/04
23/04
24/04
25/04
Date
ZEUS
H1
5
0
26/04
27/04
28/04
29/04
30/04
01/05
02/05
03/05
Date
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 27
Superconducting Magnets
GO
❍
Combined function magnets (GO, GG) installed inside experiment
❍
Main function: horizontal dipole, quadrupole
(Correction fields: vertical dipole, skew-quadrupole, sextupole)
❍
superconducting magnets (air-coil)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
I
@
@
@
GG
p. 28
Separation e/p-Beam (1)
Two superconducting magnets (GO, GG) installed inside
the experiment (for separation and focusing of e and p)
→ compact design
→ p-magnets nearer to IP
May 14th, 2004
Magnets on bridge act as quadrupole and dipole!
Common vacuum system for e and p-beam
Accurate alignment of quadrupoles necessary!
J. Keil: The Electron-Proton Collider HERA
p. 29
Separation e/p-Beam (2)
p t t e±
GM (magnetic septum magnet):
Half quadrupole magnet with thin
plate; electrons in field-free region
split coil to pass synchrotron radiation
May 14th, 2004
Beamline downstream of the separation of the
vacuum system (between GM and GN magnet)
J. Keil: The Electron-Proton Collider HERA
p. 30
Synchrotron Radiation
❍
Early beam-separation of e± → strong magnetic forces on e-beam
❍
Emitted sychrotron radiation (SR) power (negligible for protons!):
E4
e2
1
Ps =
c (m0 c2 )4 ρ2
→ for high energies use big bending radius ρ to reduce SR loss!
❍
Energy loss per turn, which has to compensated by RF-system:
e2
E4
∆E =
3ε0 (m0 c2 )4 ρ
→ Limiting factor for electron current!
❍
Typical frequency of emitted radiation:
fc =
3cγ 3
4πρ
for HERA-e at E = 27.5 GeV in IR: Ec = 45 keV . . . 88 keV!
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 31
Synchrotron Radiation Fan
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 32
Main absorber at 11 m
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 33
Background issues
❏
Severe background problems at H1 and ZEUS since startup of HERA II in 2001
→ Reduction of beam currents necessary for operation of ZEUS/H1!
❏
Problems are almost solved now:
❍
Direct synchrotron radiation
IR design, SR collimation, sophisticated beam steering procedures (beam based
alignment)
❍
Backscattered synchrotron radiation
improved masking in ZEUS
❍
Positron background
improved vacuum conditions, addition of a pump in a critical region
❍
Protron background
improved with regular beam operation; (almost) no issue for H1 and ZEUS
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 34
Working point issues
❍
❍
Injection tunes:
❏
Dynamic Aperture is sufficient
❏
High specific luminosity
❏
Tune footprint limited in collision by strong resonances
❏
Polarization poor
Collision tunes:
❏
Dynamic Aperture small (6 − 7σ)
❏
Reduced specific luminosity (10 %)
❏
Non-reproducible orbit effects
❏
Tune footprint limited between 2Qs and 3Qs satellite resonances
❏
Polarization good (50 % in collisions with 4 rotators)
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 35
❍
Space charge of one beam acts as a defocusing lens on the
other beam
❍
Round beam: Force Fr on particle depends on impact
parameter r
1
Fr
0.5
0
-0.5
-1
-10
-5
0
rΣ
5
10
❍
linear force inside bunch (r < σ); very non-linear outside!
❍
Distribution of impact parameters leads to a tune spread
∆Qx,y
?,e
βx,y
re N p
=
?,p
2πγe σx,y
(σx?,p + σy?,p )
❍
For positrons σx σy , therefore ∆Qy is the important one!
❍
Beam-beam effect depends on intensity (of the other beam)
❍
Beam-beam changes optical functions at IPs
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 36
Polarization: General remarks
❍
❍
N −N
Polarization is defined as P := N↑↑ −N↓↓
N : number of particles in up/down-spin state
~
Precession of polarization vector around B-field
~
(E-field
negligible!)
❍
HERA-e has mid-plane symmetry: stable spin direction ~n0 is
parallel/anti-parallel to vertical bending fields in arcs
→ Polarization in longitudinal direction is not preserved
❍
Emission of SR e → e + γ: Spin flip possible
❍
Probabilities for spin-flip P↑↓ → P↓↑ is different from
P↓↑ → P↑↓ : self-polarization of e-beam (Sokolov-Ternov effect)
❍
Uncompensated longitudinal solenoid fields → spin rotation
necessary
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 37
Polarization: Spin direction
❍
Spin direction in HERMES, H1 and ZEUS longitudinal; in arcs vertical
❍
Two polarimeters available: TPOL near HERA-B; LPOL near HERMES
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 38
Polarization: Spin Rotators
❍
Principle: Interleaved horizontal and vertical bending magnets
❍
Change of helicity in IR by mechanical movement of the vertical bend
In arc: Spin vertical
↓
May 14th, 2004
In IR: Spin longitudinal
↓
J. Keil: The Electron-Proton Collider HERA
p. 39
Polarization buildup
❍
❍
Polarization buildup due to self-polarization:
t
P = P ∞ (1 − e− τ )
Equilibrium polarization P∞ :
Fast build-up time for low polarization;
slow for high polarization
For HERA with three rotator pairs:
P ∞ = 60 % (?), τST = 36 min
70
60
50
P%
❍
40
30
20
10
0
❍
Time constant of polarization τ depends on
effects off depolarizing resonances
(depend on tunes, energy, beam-beam effect)
❍
Good orbit correction necessary
❍
Empirical correction using harmonic bumps
→ polarization optimization has become more
difficult
May 14th, 2004
0
20
J. Keil: The Electron-Proton Collider HERA
40
60
80
tmin
100
120
140
p. 40
Summary
❍
Luminosity upgrade of HERA: Achieved by complex interaction zone
❍
Problems with luminosity upgrade of HERA underestimated
❍
Main problem of machine was background → solved now!
❍
Beam size reduction successful, specific luminosity (almost) achieved
❍
Current Ie and Ip not at limit, depends on performance of pre-accelerators
❍
Many parameters of HERA have been pushed to its limits
→ Operation of HERA has become more difficult now
❍
Biggest problem now: Reliability of hardware components
May 14th, 2004
J. Keil: The Electron-Proton Collider HERA
p. 41