design and test of a prototype cavity for a stern-gerlach polarimeter peter cameron - bnl
TRANSCRIPT
Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter
Peter Cameron - BNL
2
Design and Test of a Prototype Cavity for a Stern-Gerlach Polarimeter
P. Cameron1, M. Conte4, N. D’Imperio1, W. Franklin6, D.A.Goldberg3, A. Luccio1, M. Palazzi4, M. Pusterla5,
R. Rossmanith2, W. MacKay1, T. Zwart6
1Brookhaven National Laboratory, Upton, NY 11973, USA2Forschungszentrum Karlsruhe GmbH, D-76021 Karlsruhe, Germany
3Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA4Universita and Sezione INFN di Genova, 16146 Genova, Italy5Universita and Sezione INFN di Padova, 35131 Padova, Italy
6MIT-Bates Laboratory, Boston MA 01949 USA
3
Outline
• What this is about• Stern-Gerlach History• Derbenev and the transverse pickup• Conte et al and the longitudinal kicker• Conte et al and the transverse kicker• The Bates polarimeter• The RHIC polarimeter• What next?
4
What this is about
• Polarization measurement as beam instrumentation rather than a scattering experiment
• The essence of the problem• Enhance interaction of with pickup• Dynamic range – accomplish the measurement in the
presence of the electric charge background
• The approach• Resonant pickup• Magnetic dipole has geometry – take advantage of relativity• Mode suppression and filtering
• First electrons, then protons
5
Brief History
1896 Zeeman splitting - fine structure E = h = B
1922 Stern/Gerlach splitting - ‘space quantization’ F = grad(B) kicker
1927 Pauli proposes spin S = sqrt(s(s+1)) = 1.732/2 s = 1/2 observable
1983 Barber & Cabrera and Michigan/AGS - Squid pickup
1985 Niinikoski and Rossmanith- transverse splitting in a synchrotron kicker
1993 Derbenev - RF Resonance Polarimeter - transverse moment pickup
1995 Conte et al - longitudinal spin splitting kicker
1996 Argonne BIW Cameron et al - Squid Polarimeter - longitudinal pickup
1998 RHIC Note Cameron et al - MIT-Bates Cavity - longitudinal pickup
2000 LANL preprint server - Conte et al - transverse 2 pickup
2001 PAC - poster and paper based on MIT-Bates meeting pickup
2002 September - Spin 2002 MIT-Bates, RHIC, LHC,… pickup, kicker
6
Derbenev - Transverse
• Hamiltonian approach with spin motion as described by BMT.
• Potential confusion - BMT lives in more than one reference frame
• Requires TM cavity mode, which couples strongly to beam charge.
• Excitation to move spin away from stable spin direction (spectral separation) also drives the cavity
• No gamma dependence - small signals, no advantage to working at high energy
• Requires extremely high-Q (~1010) resonant cavity
7
Conte et al - Longitudinal
• Longitudinal magnetic moment transforms as – Jackson, Hagedorn,…
• First proposal for a longitudinal spin splitter • Proposal for polarimeter at MIT-Bates• Nature conspires against observation - contribution due to
space and time gradients of magnetic field cancel to order 1/
• Squid polarimeter should still work for electrons (non-linear device, energy comes not from beam but rather from junction bias) but if it doesn’t work for protons, why bother?
8
Outline
• What this is about• Stern-Gerlach History• Derbenev and the transverse pickup• Conte et al and the longitudinal kicker
• Conte et al and the transverse kicker• The Bates polarimeter• The RHIC polarimeter• Conclusions
9
Conte et al - Transverse
• Reference – LANL preprint 0003069• Transverse magnetic moment is invariant• BUT - interaction of moment with appropriate TE cavity mode
goes as 2
• analogous to inverse Compton scattering, FELs,???…
• Second proposal for a longitudinal spin splitter – kick ~ 2
• Second proposal for polarimeter at MIT-Bates - signal ~ 4
• Cheap, fast, accurate, non-destructive polarimeter• Possibility of calibration from first principles (straightforward
EM calculations, comparison with signal from charge) • We learn a lesson - the Italians (Waldo MacKay is an honorary
Genoese) are both smart and tenacious
10
TE011 on-axis Fields
E
B
11
TE011 Fields0
00
0
1
1
1
0
bkg problem
0
m=0 n=1 p=1
12
SG Force in lab frame
also Heinemann
13
Bates S/N
Bates
rad
bkg
signal -60dBm
dBm
dBm
• TE011 mode• Signal strength is good
• Schottky ~ -150dBm• Charge background requires alignment at the level of a few rad• First choice is motion control, cheapest is beam steering
14
Prototype Cavity• Refine frequency calculations to include beampipe perturbation• Determine probe length for optimal coupling• Determine optimal coupling for TM mode dampers• Investigate need for tuners
15
S21 – Closed and Beampipe
blue – closed boxred – with beampipe
TE0112.735GHz
TM1113.211GHz
TM1214.348GHz
TE0315.496GHz
TM1315.762GHz
new modes with beampipe?
16
S21 – with and w/o short
blue – with beampipered – with short
TE011TM111 TM121
TM131
modes attenuatedby short
17
Mode Strengths
18
Ratio of to q Power
100dB - Bates
200dB - RHIC
19
More Mode Strengths
Contamination by finite Q – TM mode damper? • TM111 mode ~ 500MHz away• Lorentzian lineshape down by ~ 120dB at this distance• Ratio - amplitude ~ 100dB above TE011 signal (4 helps)!• 20dB margin is not comfortable, argues for damper
Beam Stability – TM mode damper?
Conclusion – effect of TM damper on TE modes is weak, to be conservative we will add damper to next iteration.
20
Block Diagram
TM Mode Coupler
beam
Cavity FFT Box
Mix
Filter Filter
21
What about RHIC?
• Ideally one would avoid a superconducting cavity. Signal strengths appear to permit this.
• Signal power can be enhanced by high frequency, stimulating coherence with longitudinal kicker.
• Problem is charge background (foreground?).• Impossible alignment tolerances? Signal mode• Contamination due to finite Q – TM modes
• Measure only when spin is away from stable spin direction? Dynamic range (excitation of TM)
• Another is implementation of the 1/2 suppression of charge interaction. Careful study is necessary.
22
RHIC S/N
RHIC
rad x 104
bkg
signal -130dBm
• Bates cavity in RHIC• 1% longitudinal bunching at 2.7GHz to provide coherence• Signal strength is adequate• Charge background requires alignment at sub-nanoradian level• We need (at least) one more good idea
• free precession?• 1/ suppression of charge?
• else?
23
What next?• Add mode dampers to prototype cavity• Cut to frequency and measure• Decide whether or not to add tuners• Design filter• Design Review – at Bates? November?• Build vacuum compatible cavity and measure• Follow on with 1/2 suppression?• RHIC polarimeter• Polarization at full energy – LHC?
24
TE201 Fields
Advantages• longitudinal moment transforms as –Jackson,…• 2nd order (position and angle) cancellation of electric charge interaction due to geometryBUT
• contribution due to space and time gradients of magnetic field cancel to order 1/
25
TE201 Fields
0
00
0
1
1
1
0
No bkg problem 0
m=2 n=0 p=1