Status of 500kV Low Emittance Electron Gun Test Facility for a Compact X-ray Free Electron Laser at
Paul Scherrer Institute
Martin Paraliev, Christopher Gough, Sladjana Ivkovic Large Research Facilities, Paul Scherrer Institute, Villigen PSI, Switzerland Abstract-The Paul Scherrer Institute (PSI) is developing an X-
ray Free Electron Laser (XFEL) [1] which relies on photo- assisted field emission and high gradient acceleration in a Low Emittance electron Gun (LEG). The first phase of the LEG design incorporates a sub-microsecond 500kV pulser, a high gradient acceleration diode structure and a diagnostic electron beam line. An air core (Tesla) transformer, in critically coupled mode [2], is generating 250ns FWHM voltage pulse to give a high gradient between the mirror finished surfaces of the cathode and anode. The emitted electron beam passes to the diagnostic beam line through a 1.5mm hole in the anode. The measured results are compared with numeric simulations. The timing and the amplitude stability of the pulser are discussed since they are sensitive parameters for this application.
I. INTRODUCTION
Since 2003 the Paul Scherrer Institute (PSI) is developing Low Emittance electron Gun (LEG), based on photo-assisted field emission [3]. This is part of the R&D activity to construct a compact X-ray Free Electron Laser (XFEL) at PSI. The first phase was 500kV pulser, which was successfully commissioned at the end of 2007. The current results and present status of the test stand will be presented.
Details of the 500kV pulse generator will also be presented.
II. OVERVIEW
The 500kV pulser (Fig. 1) consists of two main parts: sub- microsecond high voltage (HV) pulser and high gradient (HG) vacuum diode.
Fig. 1. 500keV test stand in the bunker.
Because high-energy photons can be generated, the 500kV pulser is located in a radiation protection bunker. A Nd:VAN
laser is located at the upper floor in a temperature stabilized environment. Gaussian light pulse (15.4ps FWHM) with maximum energy of 6 µJ at 266nm is delivered to the cathode through a vacuum transfer line. The size and the position of the laser spot on the emitting surface are controlled by a motorized optical telescope system.
Fig 2. Cross section of the 500keV test stand. a - SF6 pressure vessel, b – storage capacitors and primary switch, c- HV transformer, d – ceramic feedthrough, e – vacuum chamber, f – clean cubicle,
g – HG diode, h – diagnostics beam line, i – mover system.
The HV transformer (Fig. 2) generates a negative pulse with amplitude of up to 500kV and duration 250ns FWHM The synchronized laser pulse shines on the cathode through a 1.5mm anode iris. The emitted electrons are rapidly accelerated in the HG diode configuration and through the anode iris they enter in the diagnostic beam line. Focusing solenoids collimate the electron bunch and deliver it to charge and emittance measurement equipment.
The vacuum chamber is designed such that it could be opened and closed without generating dust. Two spring loaded Teflon seals and a differential vacuum volume between them assure the vacuum sealing. Once the vacuum is established the atmospheric pressure provides enough force to keep the vacuum chamber closed. A clean cubicle is built around the chamber opening in order to protect the vacuum chamber when opened. In this way the exchange of the electrodes in case of damage is relatively easy and quick. The separation between anode and cathode is controlled by motorized translation system. The whole SF6 pressure vessel sits on 5-axis mover. This allows positioning of the cathode with respect to the anode (and the rest of the machine) with micron resolution.
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III. THE PULSER
An air-core resonant transformer technology is used to produce high voltage pulses. The primary side storage capacitors are charged up to 50kV using 10kJ/s capacitor charging power supply (Maxwell Technologies CCS-10-050- P-1). Two hollow anode, air-cooled thyratrons (E2V CX1725A) are used in parallel as primary switch. The hollow anode thyratron type was a necessity in order to carry the reverse current during the resonant oscillation, without compromising the thyratron lifetime and performance. To ensure the current sharing a split primary capacitor bank is used. Since the voltage on the capacitors is close to their maximum rating and the polarity is reversing, it was found that their lifetime decreased dramatically. A T-shape circuit with two capacitors in series and a grounding resistor in the middle was adopted to prevent polarity reversal at the capacitors. A simplified equivalent circuit of the pulser is shown in figure 3.
Fig. 3. Simplified equivalent circuit of the pulser.
A critically coupled resonance scheme [4] is used to produce a nonsymmetric output pulse waveform. The reduced amplitude of the positive oscillation limits cathode ion back bombardment and the breakdown probability. Maximum pulser repetition rate is 50Hz limited mainly by the charging cycle.
The HV transformer design was optimized for strong magnetic coupling without employing a magnetic core.
Pressurized SF6 gas is used to provide the necessary HV insulation. Small cameras are installed inside the pressure vessel to observe the ceramic feedthrough and the transformer for breakdowns. A parametric study [2] showed that some variation from the optimal magnetic coupling (K = 0.6) could be compensated by tuning the primary capacitance, keeping the output waveform close to the ideal one. This observation was very important since construction of strongly coupled HV air-core transformers is a challenge. Keeping the system linear (within the thyratrons’ linearity) made the pulser capable of delivering HV pulses, with stable parameters, in large amplitude range. This is an important factor especially for testing different types of electrodes and different electrode separations.
IV. STABILITY REQUIREMENTS
After acceleration across anode-cathode gap the electrons are further accelerated by a set of RF cavities. In order to compress the electron bunch properly (create energy chirp for velocity bunching), it should enter the cavity at a certain RF phase. According to the beam dynamics simulations the arrival phase acceptance is 0.1° rms of the fundamental RF (1.5GHz) corresponding to 200fs rms. This tight jitter budget required a detailed look at the jitter contributing factors.
There are two parameters that affect the bunch arrival time in the first RF cavity: bunch emission timing and the bunch transit time. The laser oscillator is phase locked to the main RF clock to ensure low timing jitter (the precision synchronization of the laser to the master RF is beyond the scope of this paper).
The transit time of the electron bunch tt has two components: the time needed to cross the accelerating gap ta
and the time to reach the cavity entrance td.
d a
t t t
t = + (1)
Both terms are dependent on the accelerating voltage in the HG diode at the time of the emission. At 400kV and 4mm separation, the sensitivity of ta with respect to the accelerating voltage UG is dta/dUG = 22.6fs/kV and respectively the sensitivity of td is dtd/dUG = 305.2fs/kV. Since they are correlated the total sensitivity of tt is given below:
8 .
=327 +
=
G d G a G t
dU dt dU
dt dU
dt fs/kV (2)
Using equation (2) and the required time stability Δtt of 200fs, it is easy to define the relative accelerating voltage stability ΔUG/UG (UG = 400kV).
1 1.22
= Δ
Δ =
G t
G t G
G
U dt t dU U
U ppt (@400kV) (3)
Two factors contribute to the accelerating voltage stability.
The first is the timing jitter of the pulser Δtp. The accelerating voltage sensitivity dUG/dtp to the timing is defined using time derivative of the analytical solution of critically coupled resonant Tesla transformer output signal [2].
( )
t U( )
tU dt dU
a a
G ωsinω ωsin2ω
2 +
−
=
,
(4)where ω= 5.65x106 rad/s and Ua = 400kV. The maximum negative voltage peak is at t = 555.6ns. As an example, jitter sensitivity can reach ±255V/ns in a typical timing drift interval of ±4ns.
The second factor contributing to accelerating voltage stability is the pulser amplitude jitter ΔUp. A direct amplitude stability measurement is presented and discussed in section VI.
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V. HIGH GRADIENT DIODE
The HV pulses are applied to a HG diode configuration.
The separation between electrodes is variable (from 0 to 30mm).
Two types of sparks were observed: “hard” and “soft”
ones. They both are accompanied by a rather large high energy photon emission. The type could be identified using the HV output waveform signal. A “hard” spark dumps rapidly the energy accumulated in the secondary side of the HV transformer and stops the oscillations rapidly. The energy is dissipated in the spark. It melts locally the electrodes’
surface creating crater-like formations. Such a spark is usually followed by a large decrease of the HG holding capability of the electrodes. Some further spark conditioning could partly recover the electrodes’ gradient capabilities. A
“soft” spark does not change the output waveform significantly. After such a spark the HG holding capability of the electrodes is improved. An automated conditioning procedure gives possibilities to study the gradient equilibrium in the spark conditioning process.
0 400 800 1200
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Time (μs)
0 6 12 18 24 30 36
Fig. 4. Typical output voltage waveforms (thick line) and scintillator activity (thin line) at relatively low voltage, in case of: a – normal
operation, b – “soft” spark and c – “hard” spark.
An x-ray scintillator system is used to monitor the high energy photon emission associated with dark current and breakdowns. If the scintillator signal exceeds a certain threshold level the pulser is stopped. In this way, after a spark, the control system or the operator should decide if the test stand should be further in operation and under what conditions.
Fig. 4. shows typical HV output waveforms and scintillator activity during normal operation and spark events.
VI. MEASUREMENTS AND NUMERIC MODELING
Due to the strict requirements of pulse amplitude stability, care was taken to minimize the electromagnetic noise emissions. The thyratron switches are located in electrically sealed enclosures. Coaxial cables were used for the HV charging. To reduce the emitted electromagnetic noise, due to the reversely injected fast transient into the cables, two layers of additional screening braid were added. For all low signal level measurements double screened ½ inch coaxial cables
were used. In order to further improve the measurement stability the pulser repetition frequency is mains synchronized.
A calibrated capacitive divider is used to monitor the output pulse voltage. A high input impedance repeater circuit is used to protect the measurement equipment in case of parasitic discharge.
To study and optimize the pulser performance a detailed equivalent circuit was developed. The measured waveforms showed very good agreement with the numerically simulated waveforms (PSpice®). A typical output voltage waveform (measured and simulated) is shown below.
-400 -300 -200 -100 0 100 200 300 400
0.0 0.5 1.0 1.5 2.0 2.5 3.0
t (μs)
U (kV)
Measured Simulated
Fig. 5. Typical output voltage waveform - measured and simulated.
The measured time jitter of 100 consequent pulses was Δtp = 0.79ns rms. Fig. 6. shows the histogram of the measurement. Using the amplitude sensitivity analysis, done in section IV, the resultant relative amplitude jitter due to the pulser timing is:
1 500
= Δ
Δ =
G p G
Gt Gt
U dt t dU U
U ppm (5)
0 0.2 0.4 0.6 0.8 1 1.2
-5.50 -4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50 5.50
Pulser Jitter (ns)
Normalized frequency Pulser jit ter
Gausian fit
Fig. 6. Time jitter histogram of series of 100 pulses measured at 300kV.
Fig. 7. shows the measured amplitude jitter distribution of 110 pulses (at 300kV). It is easy to notice two distinctive distribution bands compromising the overall jitter performance. A more detailed look at the problem showed strong correlation between the charging voltage Uch and the output voltage Uout. Figure 8. shows the individual values for a series of 20 consequent pulses samples.
The used pulsed capacitor charging power supply was not capable of delivering the needed voltage stability. The two distinctive distribution bands were associated with quantized structure of its output voltage due to the finite amount of charge delivered by one single pulse of the power supply.
a b c
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0.0 0.2 0.4 0.6 0.8 1.0 1.2
303.
0 303.
2 303
.4 303.
6 303.
8 304
.0 304.
2 304.
4 304.
6 304.8
305.
0 305.
2 Uout (k V)
Normalized frequency
Fig. 7. Amplitude jitter histogram measured at 300kV.
In our case the effect is even stronger pronounced because of the relatively small primary capacitor value (32nF). To overcome this problem, an additional electronic stabilizer circuit was constructed. It brought the relative amplitude stability of the charging voltage ΔUch/Uch within 200ppm.
32.00 32.05 32.10 32.15 32.20 32.25 32.30 32.35 32.40
1 3 5 7 9 11 13 15 17 19
Pulses
Uch (kV)
302.0 302.5 303.0 303.5 304.0 304.5 305.0 305.5 306.0
Uout (kV)
Uch, kV Uout, kV
Fig. 8. Charging voltage Uch and output voltage Uout for 20 consequent pulses measured at 300kV.
Fig. 9. shows the output voltage jitter histogram, for a series of 100 pulses, with the stabilizing system on. The relative amplitude jitter ΔUGp/UGp is 480ppm rms. This is the second component of the acceleration voltage jitter.
0 0.2 0.4 0.6 0.8 1 1.2
400.2 400.4
400.6 400.8
401.0 401.2
401.4 401.6
401.8 402.0
402.2 Usec (kV)
Normalized frequency
Output voltage Gaussian fit
Fig. 9. Output voltage histogram of 100 pulses with stabilization on.
To estimate the overall acceleration voltage jitter ΔUG/UG it is necessary to add the two components - the time driven jitter ΔUGt/UGt and amplitude jitter ΔUGp/UGp. Since the two instabilities are not correlated the total relative jitter is expressed as follows:
693
2 2
⎟ =
⎟
⎠
⎞
⎜⎜
⎝
⎛Δ
⎟⎟ +
⎠
⎜⎜ ⎞
⎝
⎛Δ Δ =
Gp Gp Gt
Gt G
G
U U U
U U
U ppm (6)
This value is half the required jitter performance defined in section IV.
VII. PRESENT TASKS
There are ongoing HG tests in order to get a stable HG diode configuration. Different electrode materials and different surface treatments are examined. Up to now hand polished electrodes have given the best gradient of 130MV/m.
The extracted charge with laser illumination is ~12pC, with quantum efficiency in the order of 1x10-5.
VIII. SUMMARY
At the end of 2007, the 500kV pulser was successfully commissioned. It was capable of delivering pulses with amplitude up to 500kV and duration of 250ns FWHM to HG diode. Simulated output waveforms are in a good agreement with the measured ones.
A detailed jitter analysis was done to derive the amplitude stability requirements using the time acceptance of the RF cavity. Based on the measured amplitude and time jitter of the pulser, the estimated jitter of the electron bunch arrival time at the entrance of the cavity is 90fs rms. This satisfies the design requirement of 200fs rms
During the HG tests, two types of sparks were observed.
The “soft” ones do not destroy the electrode surface and increase the held gradient. The “hard” sparks create rough craters on the surface. After such an event, the holding capability of the electrodes is reduced. A scintillator based spark detection system was constructed to protect HG diode in case of continual breakdowns.
The pulser is being successfully used to test electrode surfaces capable of holding gradients above 100MV/m.
ACKNOWLEDGMENT
The authors acknowledge the help provided by the mechanical design and construction department of PSI and especially want to thank W. Pfister for his dedication to the project.
REFERENCES
[1] R. Abela, R. Bakker, M. Chergui, L. Rivkin, J Friso van der Veen, A.
Wrulich, “Ultrafast X-ray Science with a Free Electron Laser at PSI”, Paul Scherrer Institut, Villigen, Switzerland, 2006, Online available:
http://fel.web.psi.ch/public/info/FEL-ETH-Application%20.pdf [2] M. Paraliev, C. Gough, S. Ivkovic, “Tesla Coil Design for Electron
Gun Application”, 15th IEEE International Pulsed Power Conference, Monterey, CA USA, 2005, p.1085-1088
[3] René Bakker at al., LEG: Low Emittance Gun Project X-ray Free- Electron Laser
http://fel.web.psi.ch/public/publications/2005/Anual2005-LEG.pdf [4] M. Paraliev, “Fast high voltage signals generator for low emittance
electron gun”, XLII International scientific conference on information, communication and energy systems and technologies, Ohrid, Macedonia, 2007, Vol. II, p. 663-6
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