Laser Wire Scanner test on CTF2 Laser Wire Scanner test on CTF2 - PowerPoint PPT Presentation

Laser Wire Scanner test on CTF2 Laser Wire Scanner test on CTF2 •Motivation •Experimental set-up •Time and space overlap •X-ray detection •Result of the scan •Future improvements and perspectives CERN Royal Holloway University of London J. Bosser H.H. Braun G.A. Blair E. Bravin E. D’Amico T. Kamps S. Döbert S. Hutchins T. Lefèvre R. Maccaferri G. Penn Laser Wire Scanner workshop Nanobeam 2002

LWS : Motivations LWS : Motivations CLIC Project : Main beam Beam size : 40-0.4 µ m For measuring for very small beam size at high energy Beam energy : 9 - 1500 Ge V Using the spatial performances of a laser (very small spot size : a few λ ) CTF 3 and CLIC Drive beams Beam size : 50-500 µ m For measuring beam profile on a high Beam current : 3.5- 35 A average current beam Beam energy : 50 MeV- 2 GeV Non - degradable detector compared to classic wire scanners or optical diagnostic (OTR and Cherenkov)

LWS : Experimental Set- -up up LWS : Experimental Set � � G H Total scattered photons �� J D Scattered photons on the detector U H Y D � �� � V Q R W R K � �� S � I R � U H �� E P X 3 GHz 3 GHz Focusing Current and Position 1 � Photo-injector Accelerating cavity triplet Monitor � � �� �� �� �� 3KRWRQ�HQHUJ\��NH9� Electron beam X-ray detector 100 µ m thick IR filter size of 160 µ m rms Al Window 50MeV, 1nC Spectrometer Laser focusing Laser 262nm, 10 µ J, 4ps & scanning systems virtual focus •600 photons ( 1 µ m resolution ) ( 30 µ m over 1cm ) IR to UV with 17keV doubling crystals averaged energy Laser shutter •Detection angle 26mrad Laser Beam dump (Can tolerate 5mrad Nd:YLF laser Photodiode (2.5 mJ ) misalignment) 1047nm, 3mJ, 4ps Remotely controlled delay line

LWS : Experimental Set- -up up LWS : Experimental Set 3 GHz 3 GHz Focusing Current and Position X-ray detector Photo-injector Accelerating cavity triplet Monitor IR filter Spectrometer Laser virtual focus IR to UV doubling crystals Beam dump Nd:YLF laser Laser shutter Laser Remotely controlled Photodiode delay line

LWS : Overlap technique LWS : Overlap technique Streak camera Electron Laser OTR Light 3 GHz 3 GHz Focusing Current and time Photo-injector Accelerating cavity triplet Position monitors Scintillator & OTR screen CCD camera IR filter Spectrometer IR Focusing 262nm, 10 µ J, 4ps & scanning systems IR virtual focus imaging CCD IR to UV doubling crystals Laser shutter ~3mm offset Laser Nd:YLF laser (~2mrad) Photodiode Doubling Beam dump 1047nm, 3mJ, 4ps Crystal In Remotely controlled delay line Electron energy 50 MeV (minimize energy dispersion)

LWS : Overlap performances LWS : Overlap performances Streak camera images Electron Laser Focus mode (2D) x y Laser Delay introduced by Streak mode 45ps the presence of the Sweep speed 10ps/mm doubling crystal time Electron y Estimated accuracy: ± 3ps and ± 300 µ m

LWS : X- -ray detection ray detection LWS : X Oscilloscope Expected signal 3.8 mV 3 GHz 3 GHz Focusing Current and Position Photo-injector Accelerating cavity triplet Monitor 600 photons with 17keV IR filter averaged energy Laser focusing X-ray detector & scanning systems IR to UV doubling crystals Laser shutter Beam dump Laser Nd:YLF laser Photodiode (2.5 mJ ) Doubling 1047nm, 3mJ, 4ps Crystal out Remotely controlled delay line

LWS : X- -ray detector ray detector LWS : X Lead loaded plastic Scintillator Thin aluminized Mylar foil Detector assembly Mu metal Photo-multiplier tube Calibration curve done at ESRF on the Swiss Norwegian beam line Source: 8000 photons of 20 keV within 150ps � � 9 P � � H J D W O 600 photons with 17keV R Y � H averaged energy U ��� X W D Q J L V � Q R W R K 0.006mV S ���� � H O J Expected signal of 3.8mV Q L 6 (PMT high voltage : 1.65kV) �(�� ��� ��� ��� ��� ��� 307�+LJK�9ROWDJH��N9��

LWS : Raw signals LWS : Raw signals • Fast variations (20%) due to the shot to shot reproducibility in the UV laser pulse energy • Slow variation (30%) due to temperature changes in the laser room • Not correlated with a significant change in the bunch charge • Very sensitive to a steerer located along the accelerating cavity • Change in the position of the laser on the photo-cathode or Drift in the RF phase or in a power supply

LWS : Background subtraction LWS : Background subtraction Laser off values are used to evaluate the background signal ��� �� +LJK�EDFNJURXQG�OHYHO ��� ���������������������� �� σ : RMS error of the background subtraction �� technique �� �� � �� �� �� � � � � /DUJH�)OXFWXDWLRQV Compensated signal (mV) $FFHSWDEOH�FRQGLWLRQV Give an estimate �� of background level � ��� ��� ��� ��� ��� ��� �����������������

LWS : Statistics on the scans LWS : Statistics on the scans Expected signal to noise ratio � �� � � � R Background level ~ L W D U •8000 photons of 20keV � �� H •2000 photons of 1MeV V L R •1000 photons of 20MeV Q � R average W � �� O D Q J L V � G � H W F H S [ ( � � � �� �� �� �� 1XPEHU�RI�VFDQV • Signal to noise ratio changes between 1/8 and 1/30 • 11 scans are under the average value

LWS : Statistics on the scans LWS : Statistics on the scans Statistical noise : r.m.s value of the histograms of the compensated data � � 9 � P � � H V L R Q � � O D F L W V L W average � D W 6 � � � �� �� �� �� 1XPEHU�RI�VFDQV • Statistical noise changes from 0.3 to 3.5 mV • 9 scans are above the average value

LWS : Scattered photons measurements LWS : Scattered photons measurements Total of 9 scans with a S/N ratio better than 1/10 and a RMS error smaller than 1mV No scan : Acquiring data at fixed position 12 Laser on values Laser off values Laser on - Laser off values Detector signal (mV) 6 0 Overlap position Offset position 1mm -6 Averaged value Averaged value 1.04mV -0.06mV

LWS : Profile measurements LWS : Profile measurements Longitudinal profile : Scan ±18ps � � � �&RPSHQVDWHG�VLJQDO �&RPSHQVDWHG�DQG�DYHUDJHG�VLJQDO ([SHFWHG�VLJQDO������ � σ HOHFWURQ � σ ODVHU ��SV ����������� � � � � ��� ��� ��� �� �� �� � � � � �� �� ��������� 1.5ps offset compared to the overlap values (2ps offset maximum)

LWS : Profile measurements LWS : Profile measurements Vertical profile : Scan ± 250 µ m ��� �&RPSHQVDWHG�VLJQDO �&RPSHQVDWHG�DQG�DYHUDJHG�VLJQDO ��� ([SHFWHG�VLJQDO������ ����������� σ HOHFWURQ ��� µ P ��� σ ODVHU �� µ P ��� ��� ���� ���� � ��� ��� ������������������� µ �� 25 µ m offset compared to the overlap values (150 µ m offset maximum )

LWS : Conclusion and Perspectives LWS : Conclusion and Perspectives • Thomson photons have been detected • LWS profiles are in accordance with the beam dimension measured by optical means • Small offsets of maximum 2ps and 150 µ m have been observed which corresponds to the accuracy of the overlap technique. • The signal to noise ratio is still too low to allow an accurate measurement • Background consideration is a key issue in the use of LWS. The main source of background comes from the accelerating cavity (beam halo losses) •With our background subtraction technique we can tolerate a signal to noise ratio of 1/10. Possible improvements using a second detector in parallel for direct background measurement (gain a factor 2) • Concerning the CTF3 machine, a much higher laser power would be required to the obtain an adequate signal to noise ratio. Q-switched lasers do not deliver enough power Ti:Sapphire lasers must be foreseen (but very expensive)